Thermochemistry

Have you ever wondered how your body regulates its own temperature

The human body has the ability to maintain a constant temperature within the range 965-99degF (358-372degC)

This is controlled by your own thermostat the hypothalamus gland

This gland controls the rate at which the body metabolizes glucose and other heat related functions

C6H12O6(s) + 6O2(g) rarr 6CO2(g) + 6H2O(l) ΔH = -2803 kJmol

Thermochemistry

C6H12O6(s) + 6O2(g) rarr 6CO2(g) + 6H2O(l) ΔH = -2803 kJmol

bull For every mole of glucose you burn you are burning _____ calories

bull This process releases energy therefore it is exothermic

bull The hypothalamus gland controls other mechanisms in the body to help retain or release heat

bull Thermochemistry at work

Thermochemistry

Chapter 5Thermochemistry

John D Bookstaver

St Charles Community College

St Peters MO

2006 Prentice Hall Inc

Chemistry The Central Science 10th editionTheodore L Brown H Eugene LeMay Jr

and Bruce E Bursten

Hmwk pg 188-196 7 12 15 19 21 23 29 33 41 45 51 53 59 61 67 71 77 83 97 103

Thermochemistry

Energy

bull The ability to do work or transfer heatWork Energy used to cause an object that

has mass to moveHeat Energy used to cause the

temperature of an object to rise

Thermochemistry

Potential Energy

Energy an object possesses by virtue of its position or chemical composition

(Chemicals have potential within the bonds)

Thermochemistry

Kinetic Energy

Energy an object possesses by virtue of its motion

12

KE = mv2

Thermochemistry

Units of Energy

bull The SI unit of energy is the joule (J)

bull An older non-SI unit is still in widespread use The calorie (cal)

1 cal = 4184 Jbull Calories that we eat are actually kilocalories are

abbreviated as Cal

1 J = 1 kg m2

s2

Thermochemistry

System and Surroundings

bull The system includes the molecules we want to study (here the hydrogen and oxygen molecules)

bull The surroundings are everything else (here the cylinder and piston)

Thermochemistry

Work

bull Energy used to move an object over some distance

bull w = F d

where w is work F is the force and d is the distance over which the force is exerted

Thermochemistry

Heat

bull Energy can also be transferred as heat

bull Heat flows from warmer objects to cooler objects

bull Recall cold is simply a lack of heat

Thermochemistry

Transferal of Energy

a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall

Thermochemistry

Transferal of Energy

a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall

b) As the ball falls its potential energy is converted to kinetic energy

Thermochemistry

Transferal of Energy

a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall

b) As the ball falls its potential energy is converted to kinetic energy

c) When it hits the ground its kinetic energy falls to zero (since it is no longer moving) some of the energy does work on the ball the rest is dissipated as heat

Thermochemistry

First Law of Thermodynamicsbull Energy is neither created nor destroyedbull In other words the total energy of the universe is

a constant if the system loses energy it must be gained by the surroundings and vice versa

Use Fig 55

Thermochemistry

Internal EnergyThe internal energy of a system is the sum of all kinetic and potential energies of all components of the system we call it E

Use Fig 55

Thermochemistry

Internal EnergyBy definition the change in internal energy E is the final energy of the system minus the initial energy of the system

E = Efinal minus Einitial

Use Fig 55

Thermochemistry

Changes in Internal Energy

bull If E gt 0 Efinal gt Einitial

Therefore the system absorbed energy from the surroundings

This energy change is called endergonic

Thermochemistry

Changes in Internal Energy

bull If E lt 0 Efinal lt Einitial

Therefore the system released energy to the surroundings

This energy change is called exergonic

Thermochemistry

Changes in Internal Energy

bull When energy is exchanged between the system and the surroundings it is exchanged as either heat (q) or work (w)

bull That is E = q + w

Thermochemistry

E q w and Their Signs

Thermochemistry

Exchange of Heat between System and Surroundings

bull When heat is absorbed by the system from the surroundings the process is endothermic

Thermochemistry

Exchange of Heat between System and Surroundings

When heat is released by the system to the surroundings the process is exothermic

Thermochemistry

State Functions

Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem

Thermochemistry

State Functionsbull However we do know that the internal energy

of a system is independent of the path by which the system achieved that state In the system below the water could have reached

room temperature from either direction

Thermochemistry

State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the

system not on the path by which the system arrived at that state

bull And so E depends only on Einitial and Efinal

Thermochemistry

State Functions

bull However q and w are not state functions

bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different

in the two cases

Thermochemistry

State Functions

Yes

Your bank account

(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)

No

The distance traveled from Penncrest to

K o P mall The distance will depend on the route taken

Thermochemistry

Work

When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)

Thermochemistry

WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston

w = minusPV

Thermochemistry

Enthalpy

bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system

bull Enthalpy is the internal energy plus the product of pressure and volume

H = E + PV

Thermochemistry

Enthalpy

bull When the system changes at constant pressure the change in enthalpy H is

H = (E + PV)

bull This can be written

H = E + PV

Thermochemistry

Enthalpy

bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression

H = E + PV

H = (q+w) minus w

H = q

bull So at constant pressure the change in enthalpy is the heat gained or lost

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic then when H is positive

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic when H is positive

bull A process is exothermic when H is negative

Thermochemistry

Endo or Exo State whether ΔH is positive or negative

bull The combustion of fuel in a car engine

bull Salting winter roads

bull Making snow on the ski trails

bull Skiing down the trails

bull Snow-covered trails melt in the spring time

bull Placing an ice pack on an injury

Thermochemistry

bull The combustion of fuel in a car engine

Heat is leaving ΔH is negative exothermicbull Salting winter roads

Energy between NaCl molecules will leave ΔH is negative exothermic

bull Making snow on the ski trails

Heat is leaving ΔH is negative exothermicbull Skiing down the trails

Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic

bull Snow covered trails melt in the spring time

Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury

Heat released by injury is absorbed ΔH is positive endothermic

Thermochemistry

Demonstration

Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O

Thermochemistry

Enthalpies of Reaction

The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants

H = Hproducts minus Hreactants

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

C6H12O6(s) + 6O2(g) rarr 6CO2(g) + 6H2O(l) ΔH = -2803 kJmol

bull For every mole of glucose you burn you are burning _____ calories

bull This process releases energy therefore it is exothermic

bull The hypothalamus gland controls other mechanisms in the body to help retain or release heat

bull Thermochemistry at work

Thermochemistry

Chapter 5Thermochemistry

John D Bookstaver

St Charles Community College

St Peters MO

2006 Prentice Hall Inc

Chemistry The Central Science 10th editionTheodore L Brown H Eugene LeMay Jr

and Bruce E Bursten

Hmwk pg 188-196 7 12 15 19 21 23 29 33 41 45 51 53 59 61 67 71 77 83 97 103

Thermochemistry

Energy

bull The ability to do work or transfer heatWork Energy used to cause an object that

has mass to moveHeat Energy used to cause the

temperature of an object to rise

Thermochemistry

Potential Energy

Energy an object possesses by virtue of its position or chemical composition

(Chemicals have potential within the bonds)

Thermochemistry

Kinetic Energy

Energy an object possesses by virtue of its motion

12

KE = mv2

Thermochemistry

Units of Energy

bull The SI unit of energy is the joule (J)

bull An older non-SI unit is still in widespread use The calorie (cal)

1 cal = 4184 Jbull Calories that we eat are actually kilocalories are

abbreviated as Cal

1 J = 1 kg m2

s2

Thermochemistry

System and Surroundings

bull The system includes the molecules we want to study (here the hydrogen and oxygen molecules)

bull The surroundings are everything else (here the cylinder and piston)

Thermochemistry

Work

bull Energy used to move an object over some distance

bull w = F d

where w is work F is the force and d is the distance over which the force is exerted

Thermochemistry

Heat

bull Energy can also be transferred as heat

bull Heat flows from warmer objects to cooler objects

bull Recall cold is simply a lack of heat

Thermochemistry

Transferal of Energy

a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall

Thermochemistry

Transferal of Energy

a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall

b) As the ball falls its potential energy is converted to kinetic energy

Thermochemistry

Transferal of Energy

a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall

b) As the ball falls its potential energy is converted to kinetic energy

c) When it hits the ground its kinetic energy falls to zero (since it is no longer moving) some of the energy does work on the ball the rest is dissipated as heat

Thermochemistry

First Law of Thermodynamicsbull Energy is neither created nor destroyedbull In other words the total energy of the universe is

a constant if the system loses energy it must be gained by the surroundings and vice versa

Use Fig 55

Thermochemistry

Internal EnergyThe internal energy of a system is the sum of all kinetic and potential energies of all components of the system we call it E

Use Fig 55

Thermochemistry

Internal EnergyBy definition the change in internal energy E is the final energy of the system minus the initial energy of the system

E = Efinal minus Einitial

Use Fig 55

Thermochemistry

Changes in Internal Energy

bull If E gt 0 Efinal gt Einitial

Therefore the system absorbed energy from the surroundings

This energy change is called endergonic

Thermochemistry

Changes in Internal Energy

bull If E lt 0 Efinal lt Einitial

Therefore the system released energy to the surroundings

This energy change is called exergonic

Thermochemistry

Changes in Internal Energy

bull When energy is exchanged between the system and the surroundings it is exchanged as either heat (q) or work (w)

bull That is E = q + w

Thermochemistry

E q w and Their Signs

Thermochemistry

Exchange of Heat between System and Surroundings

bull When heat is absorbed by the system from the surroundings the process is endothermic

Thermochemistry

Exchange of Heat between System and Surroundings

When heat is released by the system to the surroundings the process is exothermic

Thermochemistry

State Functions

Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem

Thermochemistry

State Functionsbull However we do know that the internal energy

of a system is independent of the path by which the system achieved that state In the system below the water could have reached

room temperature from either direction

Thermochemistry

State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the

system not on the path by which the system arrived at that state

bull And so E depends only on Einitial and Efinal

Thermochemistry

State Functions

bull However q and w are not state functions

bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different

in the two cases

Thermochemistry

State Functions

Yes

Your bank account

(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)

No

The distance traveled from Penncrest to

K o P mall The distance will depend on the route taken

Thermochemistry

Work

When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)

Thermochemistry

WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston

w = minusPV

Thermochemistry

Enthalpy

bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system

bull Enthalpy is the internal energy plus the product of pressure and volume

H = E + PV

Thermochemistry

Enthalpy

bull When the system changes at constant pressure the change in enthalpy H is

H = (E + PV)

bull This can be written

H = E + PV

Thermochemistry

Enthalpy

bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression

H = E + PV

H = (q+w) minus w

H = q

bull So at constant pressure the change in enthalpy is the heat gained or lost

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic then when H is positive

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic when H is positive

bull A process is exothermic when H is negative

Thermochemistry

Endo or Exo State whether ΔH is positive or negative

bull The combustion of fuel in a car engine

bull Salting winter roads

bull Making snow on the ski trails

bull Skiing down the trails

bull Snow-covered trails melt in the spring time

bull Placing an ice pack on an injury

Thermochemistry

bull The combustion of fuel in a car engine

Heat is leaving ΔH is negative exothermicbull Salting winter roads

Energy between NaCl molecules will leave ΔH is negative exothermic

bull Making snow on the ski trails

Heat is leaving ΔH is negative exothermicbull Skiing down the trails

Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic

bull Snow covered trails melt in the spring time

Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury

Heat released by injury is absorbed ΔH is positive endothermic

Thermochemistry

Demonstration

Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O

Thermochemistry

Enthalpies of Reaction

The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants

H = Hproducts minus Hreactants

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Chapter 5Thermochemistry

John D Bookstaver

St Charles Community College

St Peters MO

2006 Prentice Hall Inc

Chemistry The Central Science 10th editionTheodore L Brown H Eugene LeMay Jr

and Bruce E Bursten

Hmwk pg 188-196 7 12 15 19 21 23 29 33 41 45 51 53 59 61 67 71 77 83 97 103

Thermochemistry

Energy

bull The ability to do work or transfer heatWork Energy used to cause an object that

has mass to moveHeat Energy used to cause the

temperature of an object to rise

Thermochemistry

Potential Energy

Energy an object possesses by virtue of its position or chemical composition

(Chemicals have potential within the bonds)

Thermochemistry

Kinetic Energy

Energy an object possesses by virtue of its motion

12

KE = mv2

Thermochemistry

Units of Energy

bull The SI unit of energy is the joule (J)

bull An older non-SI unit is still in widespread use The calorie (cal)

1 cal = 4184 Jbull Calories that we eat are actually kilocalories are

abbreviated as Cal

1 J = 1 kg m2

s2

Thermochemistry

System and Surroundings

bull The system includes the molecules we want to study (here the hydrogen and oxygen molecules)

bull The surroundings are everything else (here the cylinder and piston)

Thermochemistry

Work

bull Energy used to move an object over some distance

bull w = F d

where w is work F is the force and d is the distance over which the force is exerted

Thermochemistry

Heat

bull Energy can also be transferred as heat

bull Heat flows from warmer objects to cooler objects

bull Recall cold is simply a lack of heat

Thermochemistry

Transferal of Energy

a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall

Thermochemistry

Transferal of Energy

a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall

b) As the ball falls its potential energy is converted to kinetic energy

Thermochemistry

Transferal of Energy

a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall

b) As the ball falls its potential energy is converted to kinetic energy

c) When it hits the ground its kinetic energy falls to zero (since it is no longer moving) some of the energy does work on the ball the rest is dissipated as heat

Thermochemistry

First Law of Thermodynamicsbull Energy is neither created nor destroyedbull In other words the total energy of the universe is

a constant if the system loses energy it must be gained by the surroundings and vice versa

Use Fig 55

Thermochemistry

Internal EnergyThe internal energy of a system is the sum of all kinetic and potential energies of all components of the system we call it E

Use Fig 55

Thermochemistry

Internal EnergyBy definition the change in internal energy E is the final energy of the system minus the initial energy of the system

E = Efinal minus Einitial

Use Fig 55

Thermochemistry

Changes in Internal Energy

bull If E gt 0 Efinal gt Einitial

Therefore the system absorbed energy from the surroundings

This energy change is called endergonic

Thermochemistry

Changes in Internal Energy

bull If E lt 0 Efinal lt Einitial

Therefore the system released energy to the surroundings

This energy change is called exergonic

Thermochemistry

Changes in Internal Energy

bull When energy is exchanged between the system and the surroundings it is exchanged as either heat (q) or work (w)

bull That is E = q + w

Thermochemistry

E q w and Their Signs

Thermochemistry

Exchange of Heat between System and Surroundings

bull When heat is absorbed by the system from the surroundings the process is endothermic

Thermochemistry

Exchange of Heat between System and Surroundings

When heat is released by the system to the surroundings the process is exothermic

Thermochemistry

State Functions

Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem

Thermochemistry

State Functionsbull However we do know that the internal energy

of a system is independent of the path by which the system achieved that state In the system below the water could have reached

room temperature from either direction

Thermochemistry

State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the

system not on the path by which the system arrived at that state

bull And so E depends only on Einitial and Efinal

Thermochemistry

State Functions

bull However q and w are not state functions

bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different

in the two cases

Thermochemistry

State Functions

Yes

Your bank account

(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)

No

The distance traveled from Penncrest to

K o P mall The distance will depend on the route taken

Thermochemistry

Work

When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)

Thermochemistry

WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston

w = minusPV

Thermochemistry

Enthalpy

bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system

bull Enthalpy is the internal energy plus the product of pressure and volume

H = E + PV

Thermochemistry

Enthalpy

bull When the system changes at constant pressure the change in enthalpy H is

H = (E + PV)

bull This can be written

H = E + PV

Thermochemistry

Enthalpy

bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression

H = E + PV

H = (q+w) minus w

H = q

bull So at constant pressure the change in enthalpy is the heat gained or lost

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic then when H is positive

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic when H is positive

bull A process is exothermic when H is negative

Thermochemistry

Endo or Exo State whether ΔH is positive or negative

bull The combustion of fuel in a car engine

bull Salting winter roads

bull Making snow on the ski trails

bull Skiing down the trails

bull Snow-covered trails melt in the spring time

bull Placing an ice pack on an injury

Thermochemistry

bull The combustion of fuel in a car engine

Heat is leaving ΔH is negative exothermicbull Salting winter roads

Energy between NaCl molecules will leave ΔH is negative exothermic

bull Making snow on the ski trails

Heat is leaving ΔH is negative exothermicbull Skiing down the trails

Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic

bull Snow covered trails melt in the spring time

Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury

Heat released by injury is absorbed ΔH is positive endothermic

Thermochemistry

Demonstration

Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O

Thermochemistry

Enthalpies of Reaction

The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants

H = Hproducts minus Hreactants

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Energy

bull The ability to do work or transfer heatWork Energy used to cause an object that

has mass to moveHeat Energy used to cause the

temperature of an object to rise

Thermochemistry

Potential Energy

Energy an object possesses by virtue of its position or chemical composition

(Chemicals have potential within the bonds)

Thermochemistry

Kinetic Energy

Energy an object possesses by virtue of its motion

12

KE = mv2

Thermochemistry

Units of Energy

bull The SI unit of energy is the joule (J)

bull An older non-SI unit is still in widespread use The calorie (cal)

1 cal = 4184 Jbull Calories that we eat are actually kilocalories are

abbreviated as Cal

1 J = 1 kg m2

s2

Thermochemistry

System and Surroundings

bull The system includes the molecules we want to study (here the hydrogen and oxygen molecules)

bull The surroundings are everything else (here the cylinder and piston)

Thermochemistry

Work

bull Energy used to move an object over some distance

bull w = F d

where w is work F is the force and d is the distance over which the force is exerted

Thermochemistry

Heat

bull Energy can also be transferred as heat

bull Heat flows from warmer objects to cooler objects

bull Recall cold is simply a lack of heat

Thermochemistry

Transferal of Energy

a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall

Thermochemistry

Transferal of Energy

a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall

b) As the ball falls its potential energy is converted to kinetic energy

Thermochemistry

Transferal of Energy

a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall

b) As the ball falls its potential energy is converted to kinetic energy

c) When it hits the ground its kinetic energy falls to zero (since it is no longer moving) some of the energy does work on the ball the rest is dissipated as heat

Thermochemistry

First Law of Thermodynamicsbull Energy is neither created nor destroyedbull In other words the total energy of the universe is

a constant if the system loses energy it must be gained by the surroundings and vice versa

Use Fig 55

Thermochemistry

Internal EnergyThe internal energy of a system is the sum of all kinetic and potential energies of all components of the system we call it E

Use Fig 55

Thermochemistry

Internal EnergyBy definition the change in internal energy E is the final energy of the system minus the initial energy of the system

E = Efinal minus Einitial

Use Fig 55

Thermochemistry

Changes in Internal Energy

bull If E gt 0 Efinal gt Einitial

Therefore the system absorbed energy from the surroundings

This energy change is called endergonic

Thermochemistry

Changes in Internal Energy

bull If E lt 0 Efinal lt Einitial

Therefore the system released energy to the surroundings

This energy change is called exergonic

Thermochemistry

Changes in Internal Energy

bull When energy is exchanged between the system and the surroundings it is exchanged as either heat (q) or work (w)

bull That is E = q + w

Thermochemistry

E q w and Their Signs

Thermochemistry

Exchange of Heat between System and Surroundings

bull When heat is absorbed by the system from the surroundings the process is endothermic

Thermochemistry

Exchange of Heat between System and Surroundings

When heat is released by the system to the surroundings the process is exothermic

Thermochemistry

State Functions

Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem

Thermochemistry

State Functionsbull However we do know that the internal energy

of a system is independent of the path by which the system achieved that state In the system below the water could have reached

room temperature from either direction

Thermochemistry

State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the

system not on the path by which the system arrived at that state

bull And so E depends only on Einitial and Efinal

Thermochemistry

State Functions

bull However q and w are not state functions

bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different

in the two cases

Thermochemistry

State Functions

Yes

Your bank account

(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)

No

The distance traveled from Penncrest to

K o P mall The distance will depend on the route taken

Thermochemistry

Work

When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)

Thermochemistry

WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston

w = minusPV

Thermochemistry

Enthalpy

bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system

bull Enthalpy is the internal energy plus the product of pressure and volume

H = E + PV

Thermochemistry

Enthalpy

bull When the system changes at constant pressure the change in enthalpy H is

H = (E + PV)

bull This can be written

H = E + PV

Thermochemistry

Enthalpy

bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression

H = E + PV

H = (q+w) minus w

H = q

bull So at constant pressure the change in enthalpy is the heat gained or lost

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic then when H is positive

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic when H is positive

bull A process is exothermic when H is negative

Thermochemistry

Endo or Exo State whether ΔH is positive or negative

bull The combustion of fuel in a car engine

bull Salting winter roads

bull Making snow on the ski trails

bull Skiing down the trails

bull Snow-covered trails melt in the spring time

bull Placing an ice pack on an injury

Thermochemistry

bull The combustion of fuel in a car engine

Heat is leaving ΔH is negative exothermicbull Salting winter roads

Energy between NaCl molecules will leave ΔH is negative exothermic

bull Making snow on the ski trails

Heat is leaving ΔH is negative exothermicbull Skiing down the trails

Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic

bull Snow covered trails melt in the spring time

Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury

Heat released by injury is absorbed ΔH is positive endothermic

Thermochemistry

Demonstration

Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O

Thermochemistry

Enthalpies of Reaction

The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants

H = Hproducts minus Hreactants

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Potential Energy

Energy an object possesses by virtue of its position or chemical composition

(Chemicals have potential within the bonds)

Thermochemistry

Kinetic Energy

Energy an object possesses by virtue of its motion

12

KE = mv2

Thermochemistry

Units of Energy

bull The SI unit of energy is the joule (J)

bull An older non-SI unit is still in widespread use The calorie (cal)

1 cal = 4184 Jbull Calories that we eat are actually kilocalories are

abbreviated as Cal

1 J = 1 kg m2

s2

Thermochemistry

System and Surroundings

bull The system includes the molecules we want to study (here the hydrogen and oxygen molecules)

bull The surroundings are everything else (here the cylinder and piston)

Thermochemistry

Work

bull Energy used to move an object over some distance

bull w = F d

where w is work F is the force and d is the distance over which the force is exerted

Thermochemistry

Heat

bull Energy can also be transferred as heat

bull Heat flows from warmer objects to cooler objects

bull Recall cold is simply a lack of heat

Thermochemistry

Transferal of Energy

a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall

Thermochemistry

Transferal of Energy

a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall

b) As the ball falls its potential energy is converted to kinetic energy

Thermochemistry

Transferal of Energy

a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall

b) As the ball falls its potential energy is converted to kinetic energy

c) When it hits the ground its kinetic energy falls to zero (since it is no longer moving) some of the energy does work on the ball the rest is dissipated as heat

Thermochemistry

First Law of Thermodynamicsbull Energy is neither created nor destroyedbull In other words the total energy of the universe is

a constant if the system loses energy it must be gained by the surroundings and vice versa

Use Fig 55

Thermochemistry

Internal EnergyThe internal energy of a system is the sum of all kinetic and potential energies of all components of the system we call it E

Use Fig 55

Thermochemistry

Internal EnergyBy definition the change in internal energy E is the final energy of the system minus the initial energy of the system

E = Efinal minus Einitial

Use Fig 55

Thermochemistry

Changes in Internal Energy

bull If E gt 0 Efinal gt Einitial

Therefore the system absorbed energy from the surroundings

This energy change is called endergonic

Thermochemistry

Changes in Internal Energy

bull If E lt 0 Efinal lt Einitial

Therefore the system released energy to the surroundings

This energy change is called exergonic

Thermochemistry

Changes in Internal Energy

bull When energy is exchanged between the system and the surroundings it is exchanged as either heat (q) or work (w)

bull That is E = q + w

Thermochemistry

E q w and Their Signs

Thermochemistry

Exchange of Heat between System and Surroundings

bull When heat is absorbed by the system from the surroundings the process is endothermic

Thermochemistry

Exchange of Heat between System and Surroundings

When heat is released by the system to the surroundings the process is exothermic

Thermochemistry

State Functions

Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem

Thermochemistry

State Functionsbull However we do know that the internal energy

of a system is independent of the path by which the system achieved that state In the system below the water could have reached

room temperature from either direction

Thermochemistry

State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the

system not on the path by which the system arrived at that state

bull And so E depends only on Einitial and Efinal

Thermochemistry

State Functions

bull However q and w are not state functions

bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different

in the two cases

Thermochemistry

State Functions

Yes

Your bank account

(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)

No

The distance traveled from Penncrest to

K o P mall The distance will depend on the route taken

Thermochemistry

Work

When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)

Thermochemistry

WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston

w = minusPV

Thermochemistry

Enthalpy

bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system

bull Enthalpy is the internal energy plus the product of pressure and volume

H = E + PV

Thermochemistry

Enthalpy

bull When the system changes at constant pressure the change in enthalpy H is

H = (E + PV)

bull This can be written

H = E + PV

Thermochemistry

Enthalpy

bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression

H = E + PV

H = (q+w) minus w

H = q

bull So at constant pressure the change in enthalpy is the heat gained or lost

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic then when H is positive

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic when H is positive

bull A process is exothermic when H is negative

Thermochemistry

Endo or Exo State whether ΔH is positive or negative

bull The combustion of fuel in a car engine

bull Salting winter roads

bull Making snow on the ski trails

bull Skiing down the trails

bull Snow-covered trails melt in the spring time

bull Placing an ice pack on an injury

Thermochemistry

bull The combustion of fuel in a car engine

Heat is leaving ΔH is negative exothermicbull Salting winter roads

Energy between NaCl molecules will leave ΔH is negative exothermic

bull Making snow on the ski trails

Heat is leaving ΔH is negative exothermicbull Skiing down the trails

Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic

bull Snow covered trails melt in the spring time

Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury

Heat released by injury is absorbed ΔH is positive endothermic

Thermochemistry

Demonstration

Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O

Thermochemistry

Enthalpies of Reaction

The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants

H = Hproducts minus Hreactants

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Kinetic Energy

Energy an object possesses by virtue of its motion

12

KE = mv2

Thermochemistry

Units of Energy

bull The SI unit of energy is the joule (J)

bull An older non-SI unit is still in widespread use The calorie (cal)

1 cal = 4184 Jbull Calories that we eat are actually kilocalories are

abbreviated as Cal

1 J = 1 kg m2

s2

Thermochemistry

System and Surroundings

bull The system includes the molecules we want to study (here the hydrogen and oxygen molecules)

bull The surroundings are everything else (here the cylinder and piston)

Thermochemistry

Work

bull Energy used to move an object over some distance

bull w = F d

where w is work F is the force and d is the distance over which the force is exerted

Thermochemistry

Heat

bull Energy can also be transferred as heat

bull Heat flows from warmer objects to cooler objects

bull Recall cold is simply a lack of heat

Thermochemistry

Transferal of Energy

a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall

Thermochemistry

Transferal of Energy

a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall

b) As the ball falls its potential energy is converted to kinetic energy

Thermochemistry

Transferal of Energy

a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall

b) As the ball falls its potential energy is converted to kinetic energy

c) When it hits the ground its kinetic energy falls to zero (since it is no longer moving) some of the energy does work on the ball the rest is dissipated as heat

Thermochemistry

First Law of Thermodynamicsbull Energy is neither created nor destroyedbull In other words the total energy of the universe is

a constant if the system loses energy it must be gained by the surroundings and vice versa

Use Fig 55

Thermochemistry

Internal EnergyThe internal energy of a system is the sum of all kinetic and potential energies of all components of the system we call it E

Use Fig 55

Thermochemistry

Internal EnergyBy definition the change in internal energy E is the final energy of the system minus the initial energy of the system

E = Efinal minus Einitial

Use Fig 55

Thermochemistry

Changes in Internal Energy

bull If E gt 0 Efinal gt Einitial

Therefore the system absorbed energy from the surroundings

This energy change is called endergonic

Thermochemistry

Changes in Internal Energy

bull If E lt 0 Efinal lt Einitial

Therefore the system released energy to the surroundings

This energy change is called exergonic

Thermochemistry

Changes in Internal Energy

bull When energy is exchanged between the system and the surroundings it is exchanged as either heat (q) or work (w)

bull That is E = q + w

Thermochemistry

E q w and Their Signs

Thermochemistry

Exchange of Heat between System and Surroundings

bull When heat is absorbed by the system from the surroundings the process is endothermic

Thermochemistry

Exchange of Heat between System and Surroundings

When heat is released by the system to the surroundings the process is exothermic

Thermochemistry

State Functions

Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem

Thermochemistry

State Functionsbull However we do know that the internal energy

of a system is independent of the path by which the system achieved that state In the system below the water could have reached

room temperature from either direction

Thermochemistry

State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the

system not on the path by which the system arrived at that state

bull And so E depends only on Einitial and Efinal

Thermochemistry

State Functions

bull However q and w are not state functions

bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different

in the two cases

Thermochemistry

State Functions

Yes

Your bank account

(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)

No

The distance traveled from Penncrest to

K o P mall The distance will depend on the route taken

Thermochemistry

Work

When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)

Thermochemistry

WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston

w = minusPV

Thermochemistry

Enthalpy

bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system

bull Enthalpy is the internal energy plus the product of pressure and volume

H = E + PV

Thermochemistry

Enthalpy

bull When the system changes at constant pressure the change in enthalpy H is

H = (E + PV)

bull This can be written

H = E + PV

Thermochemistry

Enthalpy

bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression

H = E + PV

H = (q+w) minus w

H = q

bull So at constant pressure the change in enthalpy is the heat gained or lost

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic then when H is positive

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic when H is positive

bull A process is exothermic when H is negative

Thermochemistry

Endo or Exo State whether ΔH is positive or negative

bull The combustion of fuel in a car engine

bull Salting winter roads

bull Making snow on the ski trails

bull Skiing down the trails

bull Snow-covered trails melt in the spring time

bull Placing an ice pack on an injury

Thermochemistry

bull The combustion of fuel in a car engine

Heat is leaving ΔH is negative exothermicbull Salting winter roads

Energy between NaCl molecules will leave ΔH is negative exothermic

bull Making snow on the ski trails

Heat is leaving ΔH is negative exothermicbull Skiing down the trails

Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic

bull Snow covered trails melt in the spring time

Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury

Heat released by injury is absorbed ΔH is positive endothermic

Thermochemistry

Demonstration

Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O

Thermochemistry

Enthalpies of Reaction

The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants

H = Hproducts minus Hreactants

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Units of Energy

bull The SI unit of energy is the joule (J)

bull An older non-SI unit is still in widespread use The calorie (cal)

1 cal = 4184 Jbull Calories that we eat are actually kilocalories are

abbreviated as Cal

1 J = 1 kg m2

s2

Thermochemistry

System and Surroundings

bull The system includes the molecules we want to study (here the hydrogen and oxygen molecules)

bull The surroundings are everything else (here the cylinder and piston)

Thermochemistry

Work

bull Energy used to move an object over some distance

bull w = F d

where w is work F is the force and d is the distance over which the force is exerted

Thermochemistry

Heat

bull Energy can also be transferred as heat

bull Heat flows from warmer objects to cooler objects

bull Recall cold is simply a lack of heat

Thermochemistry

Transferal of Energy

a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall

Thermochemistry

Transferal of Energy

a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall

b) As the ball falls its potential energy is converted to kinetic energy

Thermochemistry

Transferal of Energy

a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall

b) As the ball falls its potential energy is converted to kinetic energy

c) When it hits the ground its kinetic energy falls to zero (since it is no longer moving) some of the energy does work on the ball the rest is dissipated as heat

Thermochemistry

First Law of Thermodynamicsbull Energy is neither created nor destroyedbull In other words the total energy of the universe is

a constant if the system loses energy it must be gained by the surroundings and vice versa

Use Fig 55

Thermochemistry

Internal EnergyThe internal energy of a system is the sum of all kinetic and potential energies of all components of the system we call it E

Use Fig 55

Thermochemistry

Internal EnergyBy definition the change in internal energy E is the final energy of the system minus the initial energy of the system

E = Efinal minus Einitial

Use Fig 55

Thermochemistry

Changes in Internal Energy

bull If E gt 0 Efinal gt Einitial

Therefore the system absorbed energy from the surroundings

This energy change is called endergonic

Thermochemistry

Changes in Internal Energy

bull If E lt 0 Efinal lt Einitial

Therefore the system released energy to the surroundings

This energy change is called exergonic

Thermochemistry

Changes in Internal Energy

bull When energy is exchanged between the system and the surroundings it is exchanged as either heat (q) or work (w)

bull That is E = q + w

Thermochemistry

E q w and Their Signs

Thermochemistry

Exchange of Heat between System and Surroundings

bull When heat is absorbed by the system from the surroundings the process is endothermic

Thermochemistry

Exchange of Heat between System and Surroundings

When heat is released by the system to the surroundings the process is exothermic

Thermochemistry

State Functions

Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem

Thermochemistry

State Functionsbull However we do know that the internal energy

of a system is independent of the path by which the system achieved that state In the system below the water could have reached

room temperature from either direction

Thermochemistry

State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the

system not on the path by which the system arrived at that state

bull And so E depends only on Einitial and Efinal

Thermochemistry

State Functions

bull However q and w are not state functions

bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different

in the two cases

Thermochemistry

State Functions

Yes

Your bank account

(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)

No

The distance traveled from Penncrest to

K o P mall The distance will depend on the route taken

Thermochemistry

Work

When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)

Thermochemistry

WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston

w = minusPV

Thermochemistry

Enthalpy

bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system

bull Enthalpy is the internal energy plus the product of pressure and volume

H = E + PV

Thermochemistry

Enthalpy

bull When the system changes at constant pressure the change in enthalpy H is

H = (E + PV)

bull This can be written

H = E + PV

Thermochemistry

Enthalpy

bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression

H = E + PV

H = (q+w) minus w

H = q

bull So at constant pressure the change in enthalpy is the heat gained or lost

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic then when H is positive

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic when H is positive

bull A process is exothermic when H is negative

Thermochemistry

Endo or Exo State whether ΔH is positive or negative

bull The combustion of fuel in a car engine

bull Salting winter roads

bull Making snow on the ski trails

bull Skiing down the trails

bull Snow-covered trails melt in the spring time

bull Placing an ice pack on an injury

Thermochemistry

bull The combustion of fuel in a car engine

Heat is leaving ΔH is negative exothermicbull Salting winter roads

Energy between NaCl molecules will leave ΔH is negative exothermic

bull Making snow on the ski trails

Heat is leaving ΔH is negative exothermicbull Skiing down the trails

Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic

bull Snow covered trails melt in the spring time

Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury

Heat released by injury is absorbed ΔH is positive endothermic

Thermochemistry

Demonstration

Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O

Thermochemistry

Enthalpies of Reaction

The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants

H = Hproducts minus Hreactants

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

System and Surroundings

bull The system includes the molecules we want to study (here the hydrogen and oxygen molecules)

bull The surroundings are everything else (here the cylinder and piston)

Thermochemistry

Work

bull Energy used to move an object over some distance

bull w = F d

where w is work F is the force and d is the distance over which the force is exerted

Thermochemistry

Heat

bull Energy can also be transferred as heat

bull Heat flows from warmer objects to cooler objects

bull Recall cold is simply a lack of heat

Thermochemistry

Transferal of Energy

a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall

Thermochemistry

Transferal of Energy

a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall

b) As the ball falls its potential energy is converted to kinetic energy

Thermochemistry

Transferal of Energy

a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall

b) As the ball falls its potential energy is converted to kinetic energy

c) When it hits the ground its kinetic energy falls to zero (since it is no longer moving) some of the energy does work on the ball the rest is dissipated as heat

Thermochemistry

First Law of Thermodynamicsbull Energy is neither created nor destroyedbull In other words the total energy of the universe is

a constant if the system loses energy it must be gained by the surroundings and vice versa

Use Fig 55

Thermochemistry

Internal EnergyThe internal energy of a system is the sum of all kinetic and potential energies of all components of the system we call it E

Use Fig 55

Thermochemistry

Internal EnergyBy definition the change in internal energy E is the final energy of the system minus the initial energy of the system

E = Efinal minus Einitial

Use Fig 55

Thermochemistry

Changes in Internal Energy

bull If E gt 0 Efinal gt Einitial

Therefore the system absorbed energy from the surroundings

This energy change is called endergonic

Thermochemistry

Changes in Internal Energy

bull If E lt 0 Efinal lt Einitial

Therefore the system released energy to the surroundings

This energy change is called exergonic

Thermochemistry

Changes in Internal Energy

bull When energy is exchanged between the system and the surroundings it is exchanged as either heat (q) or work (w)

bull That is E = q + w

Thermochemistry

E q w and Their Signs

Thermochemistry

Exchange of Heat between System and Surroundings

bull When heat is absorbed by the system from the surroundings the process is endothermic

Thermochemistry

Exchange of Heat between System and Surroundings

When heat is released by the system to the surroundings the process is exothermic

Thermochemistry

State Functions

Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem

Thermochemistry

State Functionsbull However we do know that the internal energy

of a system is independent of the path by which the system achieved that state In the system below the water could have reached

room temperature from either direction

Thermochemistry

State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the

system not on the path by which the system arrived at that state

bull And so E depends only on Einitial and Efinal

Thermochemistry

State Functions

bull However q and w are not state functions

bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different

in the two cases

Thermochemistry

State Functions

Yes

Your bank account

(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)

No

The distance traveled from Penncrest to

K o P mall The distance will depend on the route taken

Thermochemistry

Work

When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)

Thermochemistry

WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston

w = minusPV

Thermochemistry

Enthalpy

bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system

bull Enthalpy is the internal energy plus the product of pressure and volume

H = E + PV

Thermochemistry

Enthalpy

bull When the system changes at constant pressure the change in enthalpy H is

H = (E + PV)

bull This can be written

H = E + PV

Thermochemistry

Enthalpy

bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression

H = E + PV

H = (q+w) minus w

H = q

bull So at constant pressure the change in enthalpy is the heat gained or lost

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic then when H is positive

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic when H is positive

bull A process is exothermic when H is negative

Thermochemistry

Endo or Exo State whether ΔH is positive or negative

bull The combustion of fuel in a car engine

bull Salting winter roads

bull Making snow on the ski trails

bull Skiing down the trails

bull Snow-covered trails melt in the spring time

bull Placing an ice pack on an injury

Thermochemistry

bull The combustion of fuel in a car engine

Heat is leaving ΔH is negative exothermicbull Salting winter roads

Energy between NaCl molecules will leave ΔH is negative exothermic

bull Making snow on the ski trails

Heat is leaving ΔH is negative exothermicbull Skiing down the trails

Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic

bull Snow covered trails melt in the spring time

Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury

Heat released by injury is absorbed ΔH is positive endothermic

Thermochemistry

Demonstration

Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O

Thermochemistry

Enthalpies of Reaction

The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants

H = Hproducts minus Hreactants

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Work

bull Energy used to move an object over some distance

bull w = F d

where w is work F is the force and d is the distance over which the force is exerted

Thermochemistry

Heat

bull Energy can also be transferred as heat

bull Heat flows from warmer objects to cooler objects

bull Recall cold is simply a lack of heat

Thermochemistry

Transferal of Energy

a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall

Thermochemistry

Transferal of Energy

a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall

b) As the ball falls its potential energy is converted to kinetic energy

Thermochemistry

Transferal of Energy

a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall

b) As the ball falls its potential energy is converted to kinetic energy

c) When it hits the ground its kinetic energy falls to zero (since it is no longer moving) some of the energy does work on the ball the rest is dissipated as heat

Thermochemistry

First Law of Thermodynamicsbull Energy is neither created nor destroyedbull In other words the total energy of the universe is

a constant if the system loses energy it must be gained by the surroundings and vice versa

Use Fig 55

Thermochemistry

Internal EnergyThe internal energy of a system is the sum of all kinetic and potential energies of all components of the system we call it E

Use Fig 55

Thermochemistry

Internal EnergyBy definition the change in internal energy E is the final energy of the system minus the initial energy of the system

E = Efinal minus Einitial

Use Fig 55

Thermochemistry

Changes in Internal Energy

bull If E gt 0 Efinal gt Einitial

Therefore the system absorbed energy from the surroundings

This energy change is called endergonic

Thermochemistry

Changes in Internal Energy

bull If E lt 0 Efinal lt Einitial

Therefore the system released energy to the surroundings

This energy change is called exergonic

Thermochemistry

Changes in Internal Energy

bull When energy is exchanged between the system and the surroundings it is exchanged as either heat (q) or work (w)

bull That is E = q + w

Thermochemistry

E q w and Their Signs

Thermochemistry

Exchange of Heat between System and Surroundings

bull When heat is absorbed by the system from the surroundings the process is endothermic

Thermochemistry

Exchange of Heat between System and Surroundings

When heat is released by the system to the surroundings the process is exothermic

Thermochemistry

State Functions

Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem

Thermochemistry

State Functionsbull However we do know that the internal energy

of a system is independent of the path by which the system achieved that state In the system below the water could have reached

room temperature from either direction

Thermochemistry

State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the

system not on the path by which the system arrived at that state

bull And so E depends only on Einitial and Efinal

Thermochemistry

State Functions

bull However q and w are not state functions

bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different

in the two cases

Thermochemistry

State Functions

Yes

Your bank account

(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)

No

The distance traveled from Penncrest to

K o P mall The distance will depend on the route taken

Thermochemistry

Work

When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)

Thermochemistry

WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston

w = minusPV

Thermochemistry

Enthalpy

bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system

bull Enthalpy is the internal energy plus the product of pressure and volume

H = E + PV

Thermochemistry

Enthalpy

bull When the system changes at constant pressure the change in enthalpy H is

H = (E + PV)

bull This can be written

H = E + PV

Thermochemistry

Enthalpy

bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression

H = E + PV

H = (q+w) minus w

H = q

bull So at constant pressure the change in enthalpy is the heat gained or lost

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic then when H is positive

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic when H is positive

bull A process is exothermic when H is negative

Thermochemistry

Endo or Exo State whether ΔH is positive or negative

bull The combustion of fuel in a car engine

bull Salting winter roads

bull Making snow on the ski trails

bull Skiing down the trails

bull Snow-covered trails melt in the spring time

bull Placing an ice pack on an injury

Thermochemistry

bull The combustion of fuel in a car engine

Heat is leaving ΔH is negative exothermicbull Salting winter roads

Energy between NaCl molecules will leave ΔH is negative exothermic

bull Making snow on the ski trails

Heat is leaving ΔH is negative exothermicbull Skiing down the trails

Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic

bull Snow covered trails melt in the spring time

Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury

Heat released by injury is absorbed ΔH is positive endothermic

Thermochemistry

Demonstration

Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O

Thermochemistry

Enthalpies of Reaction

The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants

H = Hproducts minus Hreactants

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Heat

bull Energy can also be transferred as heat

bull Heat flows from warmer objects to cooler objects

bull Recall cold is simply a lack of heat

Thermochemistry

Transferal of Energy

a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall

Thermochemistry

Transferal of Energy

a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall

b) As the ball falls its potential energy is converted to kinetic energy

Thermochemistry

Transferal of Energy

a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall

b) As the ball falls its potential energy is converted to kinetic energy

c) When it hits the ground its kinetic energy falls to zero (since it is no longer moving) some of the energy does work on the ball the rest is dissipated as heat

Thermochemistry

First Law of Thermodynamicsbull Energy is neither created nor destroyedbull In other words the total energy of the universe is

a constant if the system loses energy it must be gained by the surroundings and vice versa

Use Fig 55

Thermochemistry

Internal EnergyThe internal energy of a system is the sum of all kinetic and potential energies of all components of the system we call it E

Use Fig 55

Thermochemistry

Internal EnergyBy definition the change in internal energy E is the final energy of the system minus the initial energy of the system

E = Efinal minus Einitial

Use Fig 55

Thermochemistry

Changes in Internal Energy

bull If E gt 0 Efinal gt Einitial

Therefore the system absorbed energy from the surroundings

This energy change is called endergonic

Thermochemistry

Changes in Internal Energy

bull If E lt 0 Efinal lt Einitial

Therefore the system released energy to the surroundings

This energy change is called exergonic

Thermochemistry

Changes in Internal Energy

bull When energy is exchanged between the system and the surroundings it is exchanged as either heat (q) or work (w)

bull That is E = q + w

Thermochemistry

E q w and Their Signs

Thermochemistry

Exchange of Heat between System and Surroundings

bull When heat is absorbed by the system from the surroundings the process is endothermic

Thermochemistry

Exchange of Heat between System and Surroundings

When heat is released by the system to the surroundings the process is exothermic

Thermochemistry

State Functions

Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem

Thermochemistry

State Functionsbull However we do know that the internal energy

of a system is independent of the path by which the system achieved that state In the system below the water could have reached

room temperature from either direction

Thermochemistry

State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the

system not on the path by which the system arrived at that state

bull And so E depends only on Einitial and Efinal

Thermochemistry

State Functions

bull However q and w are not state functions

bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different

in the two cases

Thermochemistry

State Functions

Yes

Your bank account

(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)

No

The distance traveled from Penncrest to

K o P mall The distance will depend on the route taken

Thermochemistry

Work

When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)

Thermochemistry

WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston

w = minusPV

Thermochemistry

Enthalpy

bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system

bull Enthalpy is the internal energy plus the product of pressure and volume

H = E + PV

Thermochemistry

Enthalpy

bull When the system changes at constant pressure the change in enthalpy H is

H = (E + PV)

bull This can be written

H = E + PV

Thermochemistry

Enthalpy

bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression

H = E + PV

H = (q+w) minus w

H = q

bull So at constant pressure the change in enthalpy is the heat gained or lost

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic then when H is positive

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic when H is positive

bull A process is exothermic when H is negative

Thermochemistry

Endo or Exo State whether ΔH is positive or negative

bull The combustion of fuel in a car engine

bull Salting winter roads

bull Making snow on the ski trails

bull Skiing down the trails

bull Snow-covered trails melt in the spring time

bull Placing an ice pack on an injury

Thermochemistry

bull The combustion of fuel in a car engine

Heat is leaving ΔH is negative exothermicbull Salting winter roads

Energy between NaCl molecules will leave ΔH is negative exothermic

bull Making snow on the ski trails

Heat is leaving ΔH is negative exothermicbull Skiing down the trails

Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic

bull Snow covered trails melt in the spring time

Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury

Heat released by injury is absorbed ΔH is positive endothermic

Thermochemistry

Demonstration

Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O

Thermochemistry

Enthalpies of Reaction

The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants

H = Hproducts minus Hreactants

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Transferal of Energy

a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall

Thermochemistry

Transferal of Energy

a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall

b) As the ball falls its potential energy is converted to kinetic energy

Thermochemistry

Transferal of Energy

a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall

b) As the ball falls its potential energy is converted to kinetic energy

c) When it hits the ground its kinetic energy falls to zero (since it is no longer moving) some of the energy does work on the ball the rest is dissipated as heat

Thermochemistry

First Law of Thermodynamicsbull Energy is neither created nor destroyedbull In other words the total energy of the universe is

a constant if the system loses energy it must be gained by the surroundings and vice versa

Use Fig 55

Thermochemistry

Internal EnergyThe internal energy of a system is the sum of all kinetic and potential energies of all components of the system we call it E

Use Fig 55

Thermochemistry

Internal EnergyBy definition the change in internal energy E is the final energy of the system minus the initial energy of the system

E = Efinal minus Einitial

Use Fig 55

Thermochemistry

Changes in Internal Energy

bull If E gt 0 Efinal gt Einitial

Therefore the system absorbed energy from the surroundings

This energy change is called endergonic

Thermochemistry

Changes in Internal Energy

bull If E lt 0 Efinal lt Einitial

Therefore the system released energy to the surroundings

This energy change is called exergonic

Thermochemistry

Changes in Internal Energy

bull When energy is exchanged between the system and the surroundings it is exchanged as either heat (q) or work (w)

bull That is E = q + w

Thermochemistry

E q w and Their Signs

Thermochemistry

Exchange of Heat between System and Surroundings

bull When heat is absorbed by the system from the surroundings the process is endothermic

Thermochemistry

Exchange of Heat between System and Surroundings

When heat is released by the system to the surroundings the process is exothermic

Thermochemistry

State Functions

Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem

Thermochemistry

State Functionsbull However we do know that the internal energy

of a system is independent of the path by which the system achieved that state In the system below the water could have reached

room temperature from either direction

Thermochemistry

State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the

system not on the path by which the system arrived at that state

bull And so E depends only on Einitial and Efinal

Thermochemistry

State Functions

bull However q and w are not state functions

bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different

in the two cases

Thermochemistry

State Functions

Yes

Your bank account

(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)

No

The distance traveled from Penncrest to

K o P mall The distance will depend on the route taken

Thermochemistry

Work

When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)

Thermochemistry

WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston

w = minusPV

Thermochemistry

Enthalpy

bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system

bull Enthalpy is the internal energy plus the product of pressure and volume

H = E + PV

Thermochemistry

Enthalpy

bull When the system changes at constant pressure the change in enthalpy H is

H = (E + PV)

bull This can be written

H = E + PV

Thermochemistry

Enthalpy

bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression

H = E + PV

H = (q+w) minus w

H = q

bull So at constant pressure the change in enthalpy is the heat gained or lost

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic then when H is positive

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic when H is positive

bull A process is exothermic when H is negative

Thermochemistry

Endo or Exo State whether ΔH is positive or negative

bull The combustion of fuel in a car engine

bull Salting winter roads

bull Making snow on the ski trails

bull Skiing down the trails

bull Snow-covered trails melt in the spring time

bull Placing an ice pack on an injury

Thermochemistry

bull The combustion of fuel in a car engine

Heat is leaving ΔH is negative exothermicbull Salting winter roads

Energy between NaCl molecules will leave ΔH is negative exothermic

bull Making snow on the ski trails

Heat is leaving ΔH is negative exothermicbull Skiing down the trails

Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic

bull Snow covered trails melt in the spring time

Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury

Heat released by injury is absorbed ΔH is positive endothermic

Thermochemistry

Demonstration

Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O

Thermochemistry

Enthalpies of Reaction

The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants

H = Hproducts minus Hreactants

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Transferal of Energy

a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall

b) As the ball falls its potential energy is converted to kinetic energy

Thermochemistry

Transferal of Energy

a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall

b) As the ball falls its potential energy is converted to kinetic energy

c) When it hits the ground its kinetic energy falls to zero (since it is no longer moving) some of the energy does work on the ball the rest is dissipated as heat

Thermochemistry

First Law of Thermodynamicsbull Energy is neither created nor destroyedbull In other words the total energy of the universe is

a constant if the system loses energy it must be gained by the surroundings and vice versa

Use Fig 55

Thermochemistry

Internal EnergyThe internal energy of a system is the sum of all kinetic and potential energies of all components of the system we call it E

Use Fig 55

Thermochemistry

Internal EnergyBy definition the change in internal energy E is the final energy of the system minus the initial energy of the system

E = Efinal minus Einitial

Use Fig 55

Thermochemistry

Changes in Internal Energy

bull If E gt 0 Efinal gt Einitial

Therefore the system absorbed energy from the surroundings

This energy change is called endergonic

Thermochemistry

Changes in Internal Energy

bull If E lt 0 Efinal lt Einitial

Therefore the system released energy to the surroundings

This energy change is called exergonic

Thermochemistry

Changes in Internal Energy

bull When energy is exchanged between the system and the surroundings it is exchanged as either heat (q) or work (w)

bull That is E = q + w

Thermochemistry

E q w and Their Signs

Thermochemistry

Exchange of Heat between System and Surroundings

bull When heat is absorbed by the system from the surroundings the process is endothermic

Thermochemistry

Exchange of Heat between System and Surroundings

When heat is released by the system to the surroundings the process is exothermic

Thermochemistry

State Functions

Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem

Thermochemistry

State Functionsbull However we do know that the internal energy

of a system is independent of the path by which the system achieved that state In the system below the water could have reached

room temperature from either direction

Thermochemistry

State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the

system not on the path by which the system arrived at that state

bull And so E depends only on Einitial and Efinal

Thermochemistry

State Functions

bull However q and w are not state functions

bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different

in the two cases

Thermochemistry

State Functions

Yes

Your bank account

(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)

No

The distance traveled from Penncrest to

K o P mall The distance will depend on the route taken

Thermochemistry

Work

When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)

Thermochemistry

WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston

w = minusPV

Thermochemistry

Enthalpy

bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system

bull Enthalpy is the internal energy plus the product of pressure and volume

H = E + PV

Thermochemistry

Enthalpy

bull When the system changes at constant pressure the change in enthalpy H is

H = (E + PV)

bull This can be written

H = E + PV

Thermochemistry

Enthalpy

bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression

H = E + PV

H = (q+w) minus w

H = q

bull So at constant pressure the change in enthalpy is the heat gained or lost

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic then when H is positive

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic when H is positive

bull A process is exothermic when H is negative

Thermochemistry

Endo or Exo State whether ΔH is positive or negative

bull The combustion of fuel in a car engine

bull Salting winter roads

bull Making snow on the ski trails

bull Skiing down the trails

bull Snow-covered trails melt in the spring time

bull Placing an ice pack on an injury

Thermochemistry

bull The combustion of fuel in a car engine

Heat is leaving ΔH is negative exothermicbull Salting winter roads

Energy between NaCl molecules will leave ΔH is negative exothermic

bull Making snow on the ski trails

Heat is leaving ΔH is negative exothermicbull Skiing down the trails

Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic

bull Snow covered trails melt in the spring time

Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury

Heat released by injury is absorbed ΔH is positive endothermic

Thermochemistry

Demonstration

Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O

Thermochemistry

Enthalpies of Reaction

The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants

H = Hproducts minus Hreactants

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Transferal of Energy

a) The potential energy of this ball of clay is increased when it is moved from the ground to the top of the wall

b) As the ball falls its potential energy is converted to kinetic energy

c) When it hits the ground its kinetic energy falls to zero (since it is no longer moving) some of the energy does work on the ball the rest is dissipated as heat

Thermochemistry

First Law of Thermodynamicsbull Energy is neither created nor destroyedbull In other words the total energy of the universe is

a constant if the system loses energy it must be gained by the surroundings and vice versa

Use Fig 55

Thermochemistry

Internal EnergyThe internal energy of a system is the sum of all kinetic and potential energies of all components of the system we call it E

Use Fig 55

Thermochemistry

Internal EnergyBy definition the change in internal energy E is the final energy of the system minus the initial energy of the system

E = Efinal minus Einitial

Use Fig 55

Thermochemistry

Changes in Internal Energy

bull If E gt 0 Efinal gt Einitial

Therefore the system absorbed energy from the surroundings

This energy change is called endergonic

Thermochemistry

Changes in Internal Energy

bull If E lt 0 Efinal lt Einitial

Therefore the system released energy to the surroundings

This energy change is called exergonic

Thermochemistry

Changes in Internal Energy

bull When energy is exchanged between the system and the surroundings it is exchanged as either heat (q) or work (w)

bull That is E = q + w

Thermochemistry

E q w and Their Signs

Thermochemistry

Exchange of Heat between System and Surroundings

bull When heat is absorbed by the system from the surroundings the process is endothermic

Thermochemistry

Exchange of Heat between System and Surroundings

When heat is released by the system to the surroundings the process is exothermic

Thermochemistry

State Functions

Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem

Thermochemistry

State Functionsbull However we do know that the internal energy

of a system is independent of the path by which the system achieved that state In the system below the water could have reached

room temperature from either direction

Thermochemistry

State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the

system not on the path by which the system arrived at that state

bull And so E depends only on Einitial and Efinal

Thermochemistry

State Functions

bull However q and w are not state functions

bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different

in the two cases

Thermochemistry

State Functions

Yes

Your bank account

(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)

No

The distance traveled from Penncrest to

K o P mall The distance will depend on the route taken

Thermochemistry

Work

When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)

Thermochemistry

WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston

w = minusPV

Thermochemistry

Enthalpy

bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system

bull Enthalpy is the internal energy plus the product of pressure and volume

H = E + PV

Thermochemistry

Enthalpy

bull When the system changes at constant pressure the change in enthalpy H is

H = (E + PV)

bull This can be written

H = E + PV

Thermochemistry

Enthalpy

bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression

H = E + PV

H = (q+w) minus w

H = q

bull So at constant pressure the change in enthalpy is the heat gained or lost

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic then when H is positive

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic when H is positive

bull A process is exothermic when H is negative

Thermochemistry

Endo or Exo State whether ΔH is positive or negative

bull The combustion of fuel in a car engine

bull Salting winter roads

bull Making snow on the ski trails

bull Skiing down the trails

bull Snow-covered trails melt in the spring time

bull Placing an ice pack on an injury

Thermochemistry

bull The combustion of fuel in a car engine

Heat is leaving ΔH is negative exothermicbull Salting winter roads

Energy between NaCl molecules will leave ΔH is negative exothermic

bull Making snow on the ski trails

Heat is leaving ΔH is negative exothermicbull Skiing down the trails

Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic

bull Snow covered trails melt in the spring time

Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury

Heat released by injury is absorbed ΔH is positive endothermic

Thermochemistry

Demonstration

Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O

Thermochemistry

Enthalpies of Reaction

The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants

H = Hproducts minus Hreactants

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

First Law of Thermodynamicsbull Energy is neither created nor destroyedbull In other words the total energy of the universe is

a constant if the system loses energy it must be gained by the surroundings and vice versa

Use Fig 55

Thermochemistry

Internal EnergyThe internal energy of a system is the sum of all kinetic and potential energies of all components of the system we call it E

Use Fig 55

Thermochemistry

Internal EnergyBy definition the change in internal energy E is the final energy of the system minus the initial energy of the system

E = Efinal minus Einitial

Use Fig 55

Thermochemistry

Changes in Internal Energy

bull If E gt 0 Efinal gt Einitial

Therefore the system absorbed energy from the surroundings

This energy change is called endergonic

Thermochemistry

Changes in Internal Energy

bull If E lt 0 Efinal lt Einitial

Therefore the system released energy to the surroundings

This energy change is called exergonic

Thermochemistry

Changes in Internal Energy

bull When energy is exchanged between the system and the surroundings it is exchanged as either heat (q) or work (w)

bull That is E = q + w

Thermochemistry

E q w and Their Signs

Thermochemistry

Exchange of Heat between System and Surroundings

bull When heat is absorbed by the system from the surroundings the process is endothermic

Thermochemistry

Exchange of Heat between System and Surroundings

When heat is released by the system to the surroundings the process is exothermic

Thermochemistry

State Functions

Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem

Thermochemistry

State Functionsbull However we do know that the internal energy

of a system is independent of the path by which the system achieved that state In the system below the water could have reached

room temperature from either direction

Thermochemistry

State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the

system not on the path by which the system arrived at that state

bull And so E depends only on Einitial and Efinal

Thermochemistry

State Functions

bull However q and w are not state functions

bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different

in the two cases

Thermochemistry

State Functions

Yes

Your bank account

(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)

No

The distance traveled from Penncrest to

K o P mall The distance will depend on the route taken

Thermochemistry

Work

When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)

Thermochemistry

WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston

w = minusPV

Thermochemistry

Enthalpy

bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system

bull Enthalpy is the internal energy plus the product of pressure and volume

H = E + PV

Thermochemistry

Enthalpy

bull When the system changes at constant pressure the change in enthalpy H is

H = (E + PV)

bull This can be written

H = E + PV

Thermochemistry

Enthalpy

bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression

H = E + PV

H = (q+w) minus w

H = q

bull So at constant pressure the change in enthalpy is the heat gained or lost

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic then when H is positive

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic when H is positive

bull A process is exothermic when H is negative

Thermochemistry

Endo or Exo State whether ΔH is positive or negative

bull The combustion of fuel in a car engine

bull Salting winter roads

bull Making snow on the ski trails

bull Skiing down the trails

bull Snow-covered trails melt in the spring time

bull Placing an ice pack on an injury

Thermochemistry

bull The combustion of fuel in a car engine

Heat is leaving ΔH is negative exothermicbull Salting winter roads

Energy between NaCl molecules will leave ΔH is negative exothermic

bull Making snow on the ski trails

Heat is leaving ΔH is negative exothermicbull Skiing down the trails

Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic

bull Snow covered trails melt in the spring time

Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury

Heat released by injury is absorbed ΔH is positive endothermic

Thermochemistry

Demonstration

Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O

Thermochemistry

Enthalpies of Reaction

The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants

H = Hproducts minus Hreactants

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Internal EnergyThe internal energy of a system is the sum of all kinetic and potential energies of all components of the system we call it E

Use Fig 55

Thermochemistry

Internal EnergyBy definition the change in internal energy E is the final energy of the system minus the initial energy of the system

E = Efinal minus Einitial

Use Fig 55

Thermochemistry

Changes in Internal Energy

bull If E gt 0 Efinal gt Einitial

Therefore the system absorbed energy from the surroundings

This energy change is called endergonic

Thermochemistry

Changes in Internal Energy

bull If E lt 0 Efinal lt Einitial

Therefore the system released energy to the surroundings

This energy change is called exergonic

Thermochemistry

Changes in Internal Energy

bull When energy is exchanged between the system and the surroundings it is exchanged as either heat (q) or work (w)

bull That is E = q + w

Thermochemistry

E q w and Their Signs

Thermochemistry

Exchange of Heat between System and Surroundings

bull When heat is absorbed by the system from the surroundings the process is endothermic

Thermochemistry

Exchange of Heat between System and Surroundings

When heat is released by the system to the surroundings the process is exothermic

Thermochemistry

State Functions

Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem

Thermochemistry

State Functionsbull However we do know that the internal energy

of a system is independent of the path by which the system achieved that state In the system below the water could have reached

room temperature from either direction

Thermochemistry

State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the

system not on the path by which the system arrived at that state

bull And so E depends only on Einitial and Efinal

Thermochemistry

State Functions

bull However q and w are not state functions

bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different

in the two cases

Thermochemistry

State Functions

Yes

Your bank account

(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)

No

The distance traveled from Penncrest to

K o P mall The distance will depend on the route taken

Thermochemistry

Work

When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)

Thermochemistry

WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston

w = minusPV

Thermochemistry

Enthalpy

bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system

bull Enthalpy is the internal energy plus the product of pressure and volume

H = E + PV

Thermochemistry

Enthalpy

bull When the system changes at constant pressure the change in enthalpy H is

H = (E + PV)

bull This can be written

H = E + PV

Thermochemistry

Enthalpy

bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression

H = E + PV

H = (q+w) minus w

H = q

bull So at constant pressure the change in enthalpy is the heat gained or lost

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic then when H is positive

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic when H is positive

bull A process is exothermic when H is negative

Thermochemistry

Endo or Exo State whether ΔH is positive or negative

bull The combustion of fuel in a car engine

bull Salting winter roads

bull Making snow on the ski trails

bull Skiing down the trails

bull Snow-covered trails melt in the spring time

bull Placing an ice pack on an injury

Thermochemistry

bull The combustion of fuel in a car engine

Heat is leaving ΔH is negative exothermicbull Salting winter roads

Energy between NaCl molecules will leave ΔH is negative exothermic

bull Making snow on the ski trails

Heat is leaving ΔH is negative exothermicbull Skiing down the trails

Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic

bull Snow covered trails melt in the spring time

Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury

Heat released by injury is absorbed ΔH is positive endothermic

Thermochemistry

Demonstration

Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O

Thermochemistry

Enthalpies of Reaction

The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants

H = Hproducts minus Hreactants

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Internal EnergyBy definition the change in internal energy E is the final energy of the system minus the initial energy of the system

E = Efinal minus Einitial

Use Fig 55

Thermochemistry

Changes in Internal Energy

bull If E gt 0 Efinal gt Einitial

Therefore the system absorbed energy from the surroundings

This energy change is called endergonic

Thermochemistry

Changes in Internal Energy

bull If E lt 0 Efinal lt Einitial

Therefore the system released energy to the surroundings

This energy change is called exergonic

Thermochemistry

Changes in Internal Energy

bull When energy is exchanged between the system and the surroundings it is exchanged as either heat (q) or work (w)

bull That is E = q + w

Thermochemistry

E q w and Their Signs

Thermochemistry

Exchange of Heat between System and Surroundings

bull When heat is absorbed by the system from the surroundings the process is endothermic

Thermochemistry

Exchange of Heat between System and Surroundings

When heat is released by the system to the surroundings the process is exothermic

Thermochemistry

State Functions

Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem

Thermochemistry

State Functionsbull However we do know that the internal energy

of a system is independent of the path by which the system achieved that state In the system below the water could have reached

room temperature from either direction

Thermochemistry

State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the

system not on the path by which the system arrived at that state

bull And so E depends only on Einitial and Efinal

Thermochemistry

State Functions

bull However q and w are not state functions

bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different

in the two cases

Thermochemistry

State Functions

Yes

Your bank account

(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)

No

The distance traveled from Penncrest to

K o P mall The distance will depend on the route taken

Thermochemistry

Work

When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)

Thermochemistry

WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston

w = minusPV

Thermochemistry

Enthalpy

bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system

bull Enthalpy is the internal energy plus the product of pressure and volume

H = E + PV

Thermochemistry

Enthalpy

bull When the system changes at constant pressure the change in enthalpy H is

H = (E + PV)

bull This can be written

H = E + PV

Thermochemistry

Enthalpy

bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression

H = E + PV

H = (q+w) minus w

H = q

bull So at constant pressure the change in enthalpy is the heat gained or lost

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic then when H is positive

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic when H is positive

bull A process is exothermic when H is negative

Thermochemistry

Endo or Exo State whether ΔH is positive or negative

bull The combustion of fuel in a car engine

bull Salting winter roads

bull Making snow on the ski trails

bull Skiing down the trails

bull Snow-covered trails melt in the spring time

bull Placing an ice pack on an injury

Thermochemistry

bull The combustion of fuel in a car engine

Heat is leaving ΔH is negative exothermicbull Salting winter roads

Energy between NaCl molecules will leave ΔH is negative exothermic

bull Making snow on the ski trails

Heat is leaving ΔH is negative exothermicbull Skiing down the trails

Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic

bull Snow covered trails melt in the spring time

Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury

Heat released by injury is absorbed ΔH is positive endothermic

Thermochemistry

Demonstration

Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O

Thermochemistry

Enthalpies of Reaction

The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants

H = Hproducts minus Hreactants

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Changes in Internal Energy

bull If E gt 0 Efinal gt Einitial

Therefore the system absorbed energy from the surroundings

This energy change is called endergonic

Thermochemistry

Changes in Internal Energy

bull If E lt 0 Efinal lt Einitial

Therefore the system released energy to the surroundings

This energy change is called exergonic

Thermochemistry

Changes in Internal Energy

bull When energy is exchanged between the system and the surroundings it is exchanged as either heat (q) or work (w)

bull That is E = q + w

Thermochemistry

E q w and Their Signs

Thermochemistry

Exchange of Heat between System and Surroundings

bull When heat is absorbed by the system from the surroundings the process is endothermic

Thermochemistry

Exchange of Heat between System and Surroundings

When heat is released by the system to the surroundings the process is exothermic

Thermochemistry

State Functions

Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem

Thermochemistry

State Functionsbull However we do know that the internal energy

of a system is independent of the path by which the system achieved that state In the system below the water could have reached

room temperature from either direction

Thermochemistry

State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the

system not on the path by which the system arrived at that state

bull And so E depends only on Einitial and Efinal

Thermochemistry

State Functions

bull However q and w are not state functions

bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different

in the two cases

Thermochemistry

State Functions

Yes

Your bank account

(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)

No

The distance traveled from Penncrest to

K o P mall The distance will depend on the route taken

Thermochemistry

Work

When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)

Thermochemistry

WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston

w = minusPV

Thermochemistry

Enthalpy

bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system

bull Enthalpy is the internal energy plus the product of pressure and volume

H = E + PV

Thermochemistry

Enthalpy

bull When the system changes at constant pressure the change in enthalpy H is

H = (E + PV)

bull This can be written

H = E + PV

Thermochemistry

Enthalpy

bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression

H = E + PV

H = (q+w) minus w

H = q

bull So at constant pressure the change in enthalpy is the heat gained or lost

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic then when H is positive

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic when H is positive

bull A process is exothermic when H is negative

Thermochemistry

Endo or Exo State whether ΔH is positive or negative

bull The combustion of fuel in a car engine

bull Salting winter roads

bull Making snow on the ski trails

bull Skiing down the trails

bull Snow-covered trails melt in the spring time

bull Placing an ice pack on an injury

Thermochemistry

bull The combustion of fuel in a car engine

Heat is leaving ΔH is negative exothermicbull Salting winter roads

Energy between NaCl molecules will leave ΔH is negative exothermic

bull Making snow on the ski trails

Heat is leaving ΔH is negative exothermicbull Skiing down the trails

Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic

bull Snow covered trails melt in the spring time

Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury

Heat released by injury is absorbed ΔH is positive endothermic

Thermochemistry

Demonstration

Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O

Thermochemistry

Enthalpies of Reaction

The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants

H = Hproducts minus Hreactants

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Changes in Internal Energy

bull If E lt 0 Efinal lt Einitial

Therefore the system released energy to the surroundings

This energy change is called exergonic

Thermochemistry

Changes in Internal Energy

bull When energy is exchanged between the system and the surroundings it is exchanged as either heat (q) or work (w)

bull That is E = q + w

Thermochemistry

E q w and Their Signs

Thermochemistry

Exchange of Heat between System and Surroundings

bull When heat is absorbed by the system from the surroundings the process is endothermic

Thermochemistry

Exchange of Heat between System and Surroundings

When heat is released by the system to the surroundings the process is exothermic

Thermochemistry

State Functions

Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem

Thermochemistry

State Functionsbull However we do know that the internal energy

of a system is independent of the path by which the system achieved that state In the system below the water could have reached

room temperature from either direction

Thermochemistry

State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the

system not on the path by which the system arrived at that state

bull And so E depends only on Einitial and Efinal

Thermochemistry

State Functions

bull However q and w are not state functions

bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different

in the two cases

Thermochemistry

State Functions

Yes

Your bank account

(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)

No

The distance traveled from Penncrest to

K o P mall The distance will depend on the route taken

Thermochemistry

Work

When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)

Thermochemistry

WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston

w = minusPV

Thermochemistry

Enthalpy

bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system

bull Enthalpy is the internal energy plus the product of pressure and volume

H = E + PV

Thermochemistry

Enthalpy

bull When the system changes at constant pressure the change in enthalpy H is

H = (E + PV)

bull This can be written

H = E + PV

Thermochemistry

Enthalpy

bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression

H = E + PV

H = (q+w) minus w

H = q

bull So at constant pressure the change in enthalpy is the heat gained or lost

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic then when H is positive

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic when H is positive

bull A process is exothermic when H is negative

Thermochemistry

Endo or Exo State whether ΔH is positive or negative

bull The combustion of fuel in a car engine

bull Salting winter roads

bull Making snow on the ski trails

bull Skiing down the trails

bull Snow-covered trails melt in the spring time

bull Placing an ice pack on an injury

Thermochemistry

bull The combustion of fuel in a car engine

Heat is leaving ΔH is negative exothermicbull Salting winter roads

Energy between NaCl molecules will leave ΔH is negative exothermic

bull Making snow on the ski trails

Heat is leaving ΔH is negative exothermicbull Skiing down the trails

Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic

bull Snow covered trails melt in the spring time

Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury

Heat released by injury is absorbed ΔH is positive endothermic

Thermochemistry

Demonstration

Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O

Thermochemistry

Enthalpies of Reaction

The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants

H = Hproducts minus Hreactants

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Changes in Internal Energy

bull When energy is exchanged between the system and the surroundings it is exchanged as either heat (q) or work (w)

bull That is E = q + w

Thermochemistry

E q w and Their Signs

Thermochemistry

Exchange of Heat between System and Surroundings

bull When heat is absorbed by the system from the surroundings the process is endothermic

Thermochemistry

Exchange of Heat between System and Surroundings

When heat is released by the system to the surroundings the process is exothermic

Thermochemistry

State Functions

Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem

Thermochemistry

State Functionsbull However we do know that the internal energy

of a system is independent of the path by which the system achieved that state In the system below the water could have reached

room temperature from either direction

Thermochemistry

State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the

system not on the path by which the system arrived at that state

bull And so E depends only on Einitial and Efinal

Thermochemistry

State Functions

bull However q and w are not state functions

bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different

in the two cases

Thermochemistry

State Functions

Yes

Your bank account

(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)

No

The distance traveled from Penncrest to

K o P mall The distance will depend on the route taken

Thermochemistry

Work

When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)

Thermochemistry

WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston

w = minusPV

Thermochemistry

Enthalpy

bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system

bull Enthalpy is the internal energy plus the product of pressure and volume

H = E + PV

Thermochemistry

Enthalpy

bull When the system changes at constant pressure the change in enthalpy H is

H = (E + PV)

bull This can be written

H = E + PV

Thermochemistry

Enthalpy

bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression

H = E + PV

H = (q+w) minus w

H = q

bull So at constant pressure the change in enthalpy is the heat gained or lost

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic then when H is positive

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic when H is positive

bull A process is exothermic when H is negative

Thermochemistry

Endo or Exo State whether ΔH is positive or negative

bull The combustion of fuel in a car engine

bull Salting winter roads

bull Making snow on the ski trails

bull Skiing down the trails

bull Snow-covered trails melt in the spring time

bull Placing an ice pack on an injury

Thermochemistry

bull The combustion of fuel in a car engine

Heat is leaving ΔH is negative exothermicbull Salting winter roads

Energy between NaCl molecules will leave ΔH is negative exothermic

bull Making snow on the ski trails

Heat is leaving ΔH is negative exothermicbull Skiing down the trails

Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic

bull Snow covered trails melt in the spring time

Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury

Heat released by injury is absorbed ΔH is positive endothermic

Thermochemistry

Demonstration

Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O

Thermochemistry

Enthalpies of Reaction

The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants

H = Hproducts minus Hreactants

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

E q w and Their Signs

Thermochemistry

Exchange of Heat between System and Surroundings

bull When heat is absorbed by the system from the surroundings the process is endothermic

Thermochemistry

Exchange of Heat between System and Surroundings

When heat is released by the system to the surroundings the process is exothermic

Thermochemistry

State Functions

Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem

Thermochemistry

State Functionsbull However we do know that the internal energy

of a system is independent of the path by which the system achieved that state In the system below the water could have reached

room temperature from either direction

Thermochemistry

State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the

system not on the path by which the system arrived at that state

bull And so E depends only on Einitial and Efinal

Thermochemistry

State Functions

bull However q and w are not state functions

bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different

in the two cases

Thermochemistry

State Functions

Yes

Your bank account

(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)

No

The distance traveled from Penncrest to

K o P mall The distance will depend on the route taken

Thermochemistry

Work

When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)

Thermochemistry

WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston

w = minusPV

Thermochemistry

Enthalpy

bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system

bull Enthalpy is the internal energy plus the product of pressure and volume

H = E + PV

Thermochemistry

Enthalpy

bull When the system changes at constant pressure the change in enthalpy H is

H = (E + PV)

bull This can be written

H = E + PV

Thermochemistry

Enthalpy

bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression

H = E + PV

H = (q+w) minus w

H = q

bull So at constant pressure the change in enthalpy is the heat gained or lost

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic then when H is positive

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic when H is positive

bull A process is exothermic when H is negative

Thermochemistry

Endo or Exo State whether ΔH is positive or negative

bull The combustion of fuel in a car engine

bull Salting winter roads

bull Making snow on the ski trails

bull Skiing down the trails

bull Snow-covered trails melt in the spring time

bull Placing an ice pack on an injury

Thermochemistry

bull The combustion of fuel in a car engine

Heat is leaving ΔH is negative exothermicbull Salting winter roads

Energy between NaCl molecules will leave ΔH is negative exothermic

bull Making snow on the ski trails

Heat is leaving ΔH is negative exothermicbull Skiing down the trails

Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic

bull Snow covered trails melt in the spring time

Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury

Heat released by injury is absorbed ΔH is positive endothermic

Thermochemistry

Demonstration

Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O

Thermochemistry

Enthalpies of Reaction

The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants

H = Hproducts minus Hreactants

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Exchange of Heat between System and Surroundings

bull When heat is absorbed by the system from the surroundings the process is endothermic

Thermochemistry

Exchange of Heat between System and Surroundings

When heat is released by the system to the surroundings the process is exothermic

Thermochemistry

State Functions

Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem

Thermochemistry

State Functionsbull However we do know that the internal energy

of a system is independent of the path by which the system achieved that state In the system below the water could have reached

room temperature from either direction

Thermochemistry

State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the

system not on the path by which the system arrived at that state

bull And so E depends only on Einitial and Efinal

Thermochemistry

State Functions

bull However q and w are not state functions

bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different

in the two cases

Thermochemistry

State Functions

Yes

Your bank account

(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)

No

The distance traveled from Penncrest to

K o P mall The distance will depend on the route taken

Thermochemistry

Work

When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)

Thermochemistry

WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston

w = minusPV

Thermochemistry

Enthalpy

bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system

bull Enthalpy is the internal energy plus the product of pressure and volume

H = E + PV

Thermochemistry

Enthalpy

bull When the system changes at constant pressure the change in enthalpy H is

H = (E + PV)

bull This can be written

H = E + PV

Thermochemistry

Enthalpy

bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression

H = E + PV

H = (q+w) minus w

H = q

bull So at constant pressure the change in enthalpy is the heat gained or lost

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic then when H is positive

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic when H is positive

bull A process is exothermic when H is negative

Thermochemistry

Endo or Exo State whether ΔH is positive or negative

bull The combustion of fuel in a car engine

bull Salting winter roads

bull Making snow on the ski trails

bull Skiing down the trails

bull Snow-covered trails melt in the spring time

bull Placing an ice pack on an injury

Thermochemistry

bull The combustion of fuel in a car engine

Heat is leaving ΔH is negative exothermicbull Salting winter roads

Energy between NaCl molecules will leave ΔH is negative exothermic

bull Making snow on the ski trails

Heat is leaving ΔH is negative exothermicbull Skiing down the trails

Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic

bull Snow covered trails melt in the spring time

Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury

Heat released by injury is absorbed ΔH is positive endothermic

Thermochemistry

Demonstration

Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O

Thermochemistry

Enthalpies of Reaction

The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants

H = Hproducts minus Hreactants

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Exchange of Heat between System and Surroundings

When heat is released by the system to the surroundings the process is exothermic

Thermochemistry

State Functions

Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem

Thermochemistry

State Functionsbull However we do know that the internal energy

of a system is independent of the path by which the system achieved that state In the system below the water could have reached

room temperature from either direction

Thermochemistry

State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the

system not on the path by which the system arrived at that state

bull And so E depends only on Einitial and Efinal

Thermochemistry

State Functions

bull However q and w are not state functions

bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different

in the two cases

Thermochemistry

State Functions

Yes

Your bank account

(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)

No

The distance traveled from Penncrest to

K o P mall The distance will depend on the route taken

Thermochemistry

Work

When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)

Thermochemistry

WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston

w = minusPV

Thermochemistry

Enthalpy

bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system

bull Enthalpy is the internal energy plus the product of pressure and volume

H = E + PV

Thermochemistry

Enthalpy

bull When the system changes at constant pressure the change in enthalpy H is

H = (E + PV)

bull This can be written

H = E + PV

Thermochemistry

Enthalpy

bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression

H = E + PV

H = (q+w) minus w

H = q

bull So at constant pressure the change in enthalpy is the heat gained or lost

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic then when H is positive

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic when H is positive

bull A process is exothermic when H is negative

Thermochemistry

Endo or Exo State whether ΔH is positive or negative

bull The combustion of fuel in a car engine

bull Salting winter roads

bull Making snow on the ski trails

bull Skiing down the trails

bull Snow-covered trails melt in the spring time

bull Placing an ice pack on an injury

Thermochemistry

bull The combustion of fuel in a car engine

Heat is leaving ΔH is negative exothermicbull Salting winter roads

Energy between NaCl molecules will leave ΔH is negative exothermic

bull Making snow on the ski trails

Heat is leaving ΔH is negative exothermicbull Skiing down the trails

Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic

bull Snow covered trails melt in the spring time

Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury

Heat released by injury is absorbed ΔH is positive endothermic

Thermochemistry

Demonstration

Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O

Thermochemistry

Enthalpies of Reaction

The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants

H = Hproducts minus Hreactants

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

State Functions

Usually we have no way of knowing the internal energy of a system finding that value is simply too complex a problem

Thermochemistry

State Functionsbull However we do know that the internal energy

of a system is independent of the path by which the system achieved that state In the system below the water could have reached

room temperature from either direction

Thermochemistry

State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the

system not on the path by which the system arrived at that state

bull And so E depends only on Einitial and Efinal

Thermochemistry

State Functions

bull However q and w are not state functions

bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different

in the two cases

Thermochemistry

State Functions

Yes

Your bank account

(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)

No

The distance traveled from Penncrest to

K o P mall The distance will depend on the route taken

Thermochemistry

Work

When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)

Thermochemistry

WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston

w = minusPV

Thermochemistry

Enthalpy

bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system

bull Enthalpy is the internal energy plus the product of pressure and volume

H = E + PV

Thermochemistry

Enthalpy

bull When the system changes at constant pressure the change in enthalpy H is

H = (E + PV)

bull This can be written

H = E + PV

Thermochemistry

Enthalpy

bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression

H = E + PV

H = (q+w) minus w

H = q

bull So at constant pressure the change in enthalpy is the heat gained or lost

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic then when H is positive

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic when H is positive

bull A process is exothermic when H is negative

Thermochemistry

Endo or Exo State whether ΔH is positive or negative

bull The combustion of fuel in a car engine

bull Salting winter roads

bull Making snow on the ski trails

bull Skiing down the trails

bull Snow-covered trails melt in the spring time

bull Placing an ice pack on an injury

Thermochemistry

bull The combustion of fuel in a car engine

Heat is leaving ΔH is negative exothermicbull Salting winter roads

Energy between NaCl molecules will leave ΔH is negative exothermic

bull Making snow on the ski trails

Heat is leaving ΔH is negative exothermicbull Skiing down the trails

Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic

bull Snow covered trails melt in the spring time

Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury

Heat released by injury is absorbed ΔH is positive endothermic

Thermochemistry

Demonstration

Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O

Thermochemistry

Enthalpies of Reaction

The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants

H = Hproducts minus Hreactants

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

State Functionsbull However we do know that the internal energy

of a system is independent of the path by which the system achieved that state In the system below the water could have reached

room temperature from either direction

Thermochemistry

State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the

system not on the path by which the system arrived at that state

bull And so E depends only on Einitial and Efinal

Thermochemistry

State Functions

bull However q and w are not state functions

bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different

in the two cases

Thermochemistry

State Functions

Yes

Your bank account

(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)

No

The distance traveled from Penncrest to

K o P mall The distance will depend on the route taken

Thermochemistry

Work

When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)

Thermochemistry

WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston

w = minusPV

Thermochemistry

Enthalpy

bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system

bull Enthalpy is the internal energy plus the product of pressure and volume

H = E + PV

Thermochemistry

Enthalpy

bull When the system changes at constant pressure the change in enthalpy H is

H = (E + PV)

bull This can be written

H = E + PV

Thermochemistry

Enthalpy

bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression

H = E + PV

H = (q+w) minus w

H = q

bull So at constant pressure the change in enthalpy is the heat gained or lost

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic then when H is positive

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic when H is positive

bull A process is exothermic when H is negative

Thermochemistry

Endo or Exo State whether ΔH is positive or negative

bull The combustion of fuel in a car engine

bull Salting winter roads

bull Making snow on the ski trails

bull Skiing down the trails

bull Snow-covered trails melt in the spring time

bull Placing an ice pack on an injury

Thermochemistry

bull The combustion of fuel in a car engine

Heat is leaving ΔH is negative exothermicbull Salting winter roads

Energy between NaCl molecules will leave ΔH is negative exothermic

bull Making snow on the ski trails

Heat is leaving ΔH is negative exothermicbull Skiing down the trails

Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic

bull Snow covered trails melt in the spring time

Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury

Heat released by injury is absorbed ΔH is positive endothermic

Thermochemistry

Demonstration

Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O

Thermochemistry

Enthalpies of Reaction

The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants

H = Hproducts minus Hreactants

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

State Functionsbull Therefore internal energy is a state functionbull It depends only on the present state of the

system not on the path by which the system arrived at that state

bull And so E depends only on Einitial and Efinal

Thermochemistry

State Functions

bull However q and w are not state functions

bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different

in the two cases

Thermochemistry

State Functions

Yes

Your bank account

(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)

No

The distance traveled from Penncrest to

K o P mall The distance will depend on the route taken

Thermochemistry

Work

When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)

Thermochemistry

WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston

w = minusPV

Thermochemistry

Enthalpy

bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system

bull Enthalpy is the internal energy plus the product of pressure and volume

H = E + PV

Thermochemistry

Enthalpy

bull When the system changes at constant pressure the change in enthalpy H is

H = (E + PV)

bull This can be written

H = E + PV

Thermochemistry

Enthalpy

bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression

H = E + PV

H = (q+w) minus w

H = q

bull So at constant pressure the change in enthalpy is the heat gained or lost

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic then when H is positive

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic when H is positive

bull A process is exothermic when H is negative

Thermochemistry

Endo or Exo State whether ΔH is positive or negative

bull The combustion of fuel in a car engine

bull Salting winter roads

bull Making snow on the ski trails

bull Skiing down the trails

bull Snow-covered trails melt in the spring time

bull Placing an ice pack on an injury

Thermochemistry

bull The combustion of fuel in a car engine

Heat is leaving ΔH is negative exothermicbull Salting winter roads

Energy between NaCl molecules will leave ΔH is negative exothermic

bull Making snow on the ski trails

Heat is leaving ΔH is negative exothermicbull Skiing down the trails

Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic

bull Snow covered trails melt in the spring time

Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury

Heat released by injury is absorbed ΔH is positive endothermic

Thermochemistry

Demonstration

Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O

Thermochemistry

Enthalpies of Reaction

The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants

H = Hproducts minus Hreactants

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

State Functions

bull However q and w are not state functions

bull Whether the battery is shorted out or is discharged by running the fan its E is the sameBut q and w are different

in the two cases

Thermochemistry

State Functions

Yes

Your bank account

(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)

No

The distance traveled from Penncrest to

K o P mall The distance will depend on the route taken

Thermochemistry

Work

When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)

Thermochemistry

WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston

w = minusPV

Thermochemistry

Enthalpy

bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system

bull Enthalpy is the internal energy plus the product of pressure and volume

H = E + PV

Thermochemistry

Enthalpy

bull When the system changes at constant pressure the change in enthalpy H is

H = (E + PV)

bull This can be written

H = E + PV

Thermochemistry

Enthalpy

bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression

H = E + PV

H = (q+w) minus w

H = q

bull So at constant pressure the change in enthalpy is the heat gained or lost

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic then when H is positive

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic when H is positive

bull A process is exothermic when H is negative

Thermochemistry

Endo or Exo State whether ΔH is positive or negative

bull The combustion of fuel in a car engine

bull Salting winter roads

bull Making snow on the ski trails

bull Skiing down the trails

bull Snow-covered trails melt in the spring time

bull Placing an ice pack on an injury

Thermochemistry

bull The combustion of fuel in a car engine

Heat is leaving ΔH is negative exothermicbull Salting winter roads

Energy between NaCl molecules will leave ΔH is negative exothermic

bull Making snow on the ski trails

Heat is leaving ΔH is negative exothermicbull Skiing down the trails

Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic

bull Snow covered trails melt in the spring time

Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury

Heat released by injury is absorbed ΔH is positive endothermic

Thermochemistry

Demonstration

Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O

Thermochemistry

Enthalpies of Reaction

The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants

H = Hproducts minus Hreactants

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

State Functions

Yes

Your bank account

(the balance is $25 whether you simply deposited $25 or deposited $100 and withdrew $75)

No

The distance traveled from Penncrest to

K o P mall The distance will depend on the route taken

Thermochemistry

Work

When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)

Thermochemistry

WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston

w = minusPV

Thermochemistry

Enthalpy

bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system

bull Enthalpy is the internal energy plus the product of pressure and volume

H = E + PV

Thermochemistry

Enthalpy

bull When the system changes at constant pressure the change in enthalpy H is

H = (E + PV)

bull This can be written

H = E + PV

Thermochemistry

Enthalpy

bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression

H = E + PV

H = (q+w) minus w

H = q

bull So at constant pressure the change in enthalpy is the heat gained or lost

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic then when H is positive

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic when H is positive

bull A process is exothermic when H is negative

Thermochemistry

Endo or Exo State whether ΔH is positive or negative

bull The combustion of fuel in a car engine

bull Salting winter roads

bull Making snow on the ski trails

bull Skiing down the trails

bull Snow-covered trails melt in the spring time

bull Placing an ice pack on an injury

Thermochemistry

bull The combustion of fuel in a car engine

Heat is leaving ΔH is negative exothermicbull Salting winter roads

Energy between NaCl molecules will leave ΔH is negative exothermic

bull Making snow on the ski trails

Heat is leaving ΔH is negative exothermicbull Skiing down the trails

Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic

bull Snow covered trails melt in the spring time

Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury

Heat released by injury is absorbed ΔH is positive endothermic

Thermochemistry

Demonstration

Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O

Thermochemistry

Enthalpies of Reaction

The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants

H = Hproducts minus Hreactants

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Work

When a process occurs in an open container commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings)

Thermochemistry

WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston

w = minusPV

Thermochemistry

Enthalpy

bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system

bull Enthalpy is the internal energy plus the product of pressure and volume

H = E + PV

Thermochemistry

Enthalpy

bull When the system changes at constant pressure the change in enthalpy H is

H = (E + PV)

bull This can be written

H = E + PV

Thermochemistry

Enthalpy

bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression

H = E + PV

H = (q+w) minus w

H = q

bull So at constant pressure the change in enthalpy is the heat gained or lost

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic then when H is positive

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic when H is positive

bull A process is exothermic when H is negative

Thermochemistry

Endo or Exo State whether ΔH is positive or negative

bull The combustion of fuel in a car engine

bull Salting winter roads

bull Making snow on the ski trails

bull Skiing down the trails

bull Snow-covered trails melt in the spring time

bull Placing an ice pack on an injury

Thermochemistry

bull The combustion of fuel in a car engine

Heat is leaving ΔH is negative exothermicbull Salting winter roads

Energy between NaCl molecules will leave ΔH is negative exothermic

bull Making snow on the ski trails

Heat is leaving ΔH is negative exothermicbull Skiing down the trails

Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic

bull Snow covered trails melt in the spring time

Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury

Heat released by injury is absorbed ΔH is positive endothermic

Thermochemistry

Demonstration

Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O

Thermochemistry

Enthalpies of Reaction

The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants

H = Hproducts minus Hreactants

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston

w = minusPV

Thermochemistry

Enthalpy

bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system

bull Enthalpy is the internal energy plus the product of pressure and volume

H = E + PV

Thermochemistry

Enthalpy

bull When the system changes at constant pressure the change in enthalpy H is

H = (E + PV)

bull This can be written

H = E + PV

Thermochemistry

Enthalpy

bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression

H = E + PV

H = (q+w) minus w

H = q

bull So at constant pressure the change in enthalpy is the heat gained or lost

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic then when H is positive

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic when H is positive

bull A process is exothermic when H is negative

Thermochemistry

Endo or Exo State whether ΔH is positive or negative

bull The combustion of fuel in a car engine

bull Salting winter roads

bull Making snow on the ski trails

bull Skiing down the trails

bull Snow-covered trails melt in the spring time

bull Placing an ice pack on an injury

Thermochemistry

bull The combustion of fuel in a car engine

Heat is leaving ΔH is negative exothermicbull Salting winter roads

Energy between NaCl molecules will leave ΔH is negative exothermic

bull Making snow on the ski trails

Heat is leaving ΔH is negative exothermicbull Skiing down the trails

Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic

bull Snow covered trails melt in the spring time

Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury

Heat released by injury is absorbed ΔH is positive endothermic

Thermochemistry

Demonstration

Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O

Thermochemistry

Enthalpies of Reaction

The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants

H = Hproducts minus Hreactants

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Enthalpy

bull If a process takes place at constant pressure (as the majority of processes we study do) and the only work done is this pressure-volume work we can account for heat flow during the process by measuring the enthalpy of the system

bull Enthalpy is the internal energy plus the product of pressure and volume

H = E + PV

Thermochemistry

Enthalpy

bull When the system changes at constant pressure the change in enthalpy H is

H = (E + PV)

bull This can be written

H = E + PV

Thermochemistry

Enthalpy

bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression

H = E + PV

H = (q+w) minus w

H = q

bull So at constant pressure the change in enthalpy is the heat gained or lost

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic then when H is positive

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic when H is positive

bull A process is exothermic when H is negative

Thermochemistry

Endo or Exo State whether ΔH is positive or negative

bull The combustion of fuel in a car engine

bull Salting winter roads

bull Making snow on the ski trails

bull Skiing down the trails

bull Snow-covered trails melt in the spring time

bull Placing an ice pack on an injury

Thermochemistry

bull The combustion of fuel in a car engine

Heat is leaving ΔH is negative exothermicbull Salting winter roads

Energy between NaCl molecules will leave ΔH is negative exothermic

bull Making snow on the ski trails

Heat is leaving ΔH is negative exothermicbull Skiing down the trails

Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic

bull Snow covered trails melt in the spring time

Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury

Heat released by injury is absorbed ΔH is positive endothermic

Thermochemistry

Demonstration

Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O

Thermochemistry

Enthalpies of Reaction

The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants

H = Hproducts minus Hreactants

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Enthalpy

bull When the system changes at constant pressure the change in enthalpy H is

H = (E + PV)

bull This can be written

H = E + PV

Thermochemistry

Enthalpy

bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression

H = E + PV

H = (q+w) minus w

H = q

bull So at constant pressure the change in enthalpy is the heat gained or lost

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic then when H is positive

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic when H is positive

bull A process is exothermic when H is negative

Thermochemistry

Endo or Exo State whether ΔH is positive or negative

bull The combustion of fuel in a car engine

bull Salting winter roads

bull Making snow on the ski trails

bull Skiing down the trails

bull Snow-covered trails melt in the spring time

bull Placing an ice pack on an injury

Thermochemistry

bull The combustion of fuel in a car engine

Heat is leaving ΔH is negative exothermicbull Salting winter roads

Energy between NaCl molecules will leave ΔH is negative exothermic

bull Making snow on the ski trails

Heat is leaving ΔH is negative exothermicbull Skiing down the trails

Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic

bull Snow covered trails melt in the spring time

Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury

Heat released by injury is absorbed ΔH is positive endothermic

Thermochemistry

Demonstration

Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O

Thermochemistry

Enthalpies of Reaction

The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants

H = Hproducts minus Hreactants

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Enthalpy

bull Since E = q + w and w = minusPV we can substitute these into the enthalpy expression

H = E + PV

H = (q+w) minus w

H = q

bull So at constant pressure the change in enthalpy is the heat gained or lost

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic then when H is positive

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic when H is positive

bull A process is exothermic when H is negative

Thermochemistry

Endo or Exo State whether ΔH is positive or negative

bull The combustion of fuel in a car engine

bull Salting winter roads

bull Making snow on the ski trails

bull Skiing down the trails

bull Snow-covered trails melt in the spring time

bull Placing an ice pack on an injury

Thermochemistry

bull The combustion of fuel in a car engine

Heat is leaving ΔH is negative exothermicbull Salting winter roads

Energy between NaCl molecules will leave ΔH is negative exothermic

bull Making snow on the ski trails

Heat is leaving ΔH is negative exothermicbull Skiing down the trails

Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic

bull Snow covered trails melt in the spring time

Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury

Heat released by injury is absorbed ΔH is positive endothermic

Thermochemistry

Demonstration

Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O

Thermochemistry

Enthalpies of Reaction

The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants

H = Hproducts minus Hreactants

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic then when H is positive

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic when H is positive

bull A process is exothermic when H is negative

Thermochemistry

Endo or Exo State whether ΔH is positive or negative

bull The combustion of fuel in a car engine

bull Salting winter roads

bull Making snow on the ski trails

bull Skiing down the trails

bull Snow-covered trails melt in the spring time

bull Placing an ice pack on an injury

Thermochemistry

bull The combustion of fuel in a car engine

Heat is leaving ΔH is negative exothermicbull Salting winter roads

Energy between NaCl molecules will leave ΔH is negative exothermic

bull Making snow on the ski trails

Heat is leaving ΔH is negative exothermicbull Skiing down the trails

Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic

bull Snow covered trails melt in the spring time

Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury

Heat released by injury is absorbed ΔH is positive endothermic

Thermochemistry

Demonstration

Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O

Thermochemistry

Enthalpies of Reaction

The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants

H = Hproducts minus Hreactants

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Endothermicity and Exothermicity

bull A process is endothermic when H is positive

bull A process is exothermic when H is negative

Thermochemistry

Endo or Exo State whether ΔH is positive or negative

bull The combustion of fuel in a car engine

bull Salting winter roads

bull Making snow on the ski trails

bull Skiing down the trails

bull Snow-covered trails melt in the spring time

bull Placing an ice pack on an injury

Thermochemistry

bull The combustion of fuel in a car engine

Heat is leaving ΔH is negative exothermicbull Salting winter roads

Energy between NaCl molecules will leave ΔH is negative exothermic

bull Making snow on the ski trails

Heat is leaving ΔH is negative exothermicbull Skiing down the trails

Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic

bull Snow covered trails melt in the spring time

Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury

Heat released by injury is absorbed ΔH is positive endothermic

Thermochemistry

Demonstration

Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O

Thermochemistry

Enthalpies of Reaction

The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants

H = Hproducts minus Hreactants

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Endo or Exo State whether ΔH is positive or negative

bull The combustion of fuel in a car engine

bull Salting winter roads

bull Making snow on the ski trails

bull Skiing down the trails

bull Snow-covered trails melt in the spring time

bull Placing an ice pack on an injury

Thermochemistry

bull The combustion of fuel in a car engine

Heat is leaving ΔH is negative exothermicbull Salting winter roads

Energy between NaCl molecules will leave ΔH is negative exothermic

bull Making snow on the ski trails

Heat is leaving ΔH is negative exothermicbull Skiing down the trails

Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic

bull Snow covered trails melt in the spring time

Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury

Heat released by injury is absorbed ΔH is positive endothermic

Thermochemistry

Demonstration

Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O

Thermochemistry

Enthalpies of Reaction

The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants

H = Hproducts minus Hreactants

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

bull The combustion of fuel in a car engine

Heat is leaving ΔH is negative exothermicbull Salting winter roads

Energy between NaCl molecules will leave ΔH is negative exothermic

bull Making snow on the ski trails

Heat is leaving ΔH is negative exothermicbull Skiing down the trails

Kinetic energy is dissipated in the snow under the skis in the form of heat ΔH is negative exothermic

bull Snow covered trails melt in the spring time

Heat is absorbed ΔH is positive endothermicbull Placing an ice pack on an injury

Heat released by injury is absorbed ΔH is positive endothermic

Thermochemistry

Demonstration

Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O

Thermochemistry

Enthalpies of Reaction

The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants

H = Hproducts minus Hreactants

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Demonstration

Ba(OH)28H2O + 2NH4SCN rarr Ba(SCN)2 + 2NH3 + 10H2O

Thermochemistry

Enthalpies of Reaction

The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants

H = Hproducts minus Hreactants

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Enthalpies of Reaction

The change in enthalpy H is the enthalpy of the products minus the enthalpy of the reactants

H = Hproducts minus Hreactants

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Enthalpies of Reaction

This quantity H is called the enthalpy of reaction or the heat of reaction

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

The Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Enthalpy Calculation

Sucrose is oxdized to carbon dioxide and water The enthalpy change can be measured in the laboratory as shownC12H22O11 + 12O2 rarr 12CO2 + 11H2O ΔH = -5645 kJ

What is the enthalpy change for the oxidation of 500 g of sugar

q = -825 kJ

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Calorimetry

Since we cannot know the exact enthalpy of the reactants and products we measure H through calorimetry the measurement of heat flow

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Heat Capacity and Specific Heat

bull The amount of energy required to raise the temperature of a substance by 1 K (1C) is its heat capacity

bull We define specific heat capacity (or simply specific heat) as the amount of energy required to raise the temperature of 1 g of a substance by 1 K

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Heat Capacity and Specific Heat

Specific heat then is

Specific heat =heat transferred

mass temperature change

s =q

m T

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Constant Pressure Calorimetry

Because the specific heat for water is well known (4184 Jg-K) we can measure H for the reaction with this equation

q = m s T

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Using Specific Heat Capacity

A 192-g piece of copper is heated to 1000 C in boiling water and then dropped into a beaker containing 751 g of water at 40 C What is the final temperature of the copper and water after thermal equilibrium is reached if the specific heat of copper is 0385 JgC

Tfinal = 62degC

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Same Type of Problemhellip

A 885-g piece of iron has a temperature of 788C is placed in a beaker of water containing 244 g of water at 188C When thermal equilibrium is reached what is the final termperature if the specific heat of iron is 045 JgC

Tfinal = 21degC

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Another Example

A 155-g piece of chromium heated to 1000degC is dropped into 555 g of water at 165degC The final temperature of the metal and water is 189degC What is the specific heat capacity of chromium (Assume no heat is lost to warm the container or the surroundings)

smetal = 044 JgmiddotK

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Bomb CalorimetryReactions can be carried out in a sealed ldquobombrdquo such as this one and measure the heat absorbed by the waterThe bomb and its contents are defined as the system

qrxn = -Ccal x ΔT

Ccal is the heat capacity of the bomb calorimeter

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Bomb Calorimetry

bull Because the volume in the bomb calorimeter is constant work cannot occur What is measured is really the change in internal energy E not H

bull For most reactions the difference is very small

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Using a Bomb Calorimeter

Methylhydrazine (CH6N2) is commonly used as a liquid rocket fuel The combustion of methylhydrazine is as follows

2CH6N2 + 5O2 rarr 2N2 + 2CO2 + 6H2OWhen 400 g of methylhydrazine is combusted in a

bomb calorimeter the temperature of the calorimeter increases from 2500degC to 3950degC In a separate experiment the heat capacity of the calorimeter is measured to be 7794 kJdegC What is the heat of reaction for the combustion of a mole of CH6N2 in this calorimeter Was the reaction endo- or exothermic

-130 x103 kJmol CH6N2

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Hessrsquos Law

H is well known for many reactions and it is inconvenient to measure H for every reaction in which we are interested

bull However we can estimate H using H values that are published and the properties of enthalpy

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

RecallhellipThe Truth about Enthalpy

1 Enthalpy is an extensive property

2 H for a reaction in the forward direction is equal in size but opposite in sign to H for the reverse reaction

3 H for a reaction depends on the state of the products and the state of the reactants

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Hessrsquos Law

Hessrsquos law states that ldquoIf a reaction is carried out in a series of steps H for the overall reaction will be equal to the sum of the enthalpy changes for the individual stepsrdquo

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Hessrsquos Law

Because H is a state function the total enthalpy change depends only on the initial state of the reactants and the final state of the products

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Example

Calculate ΔH for this reaction

2C (s) + H2 (g) rarr C2H2 (g)

Given the following reactions and their respective enthalpy changesC2H2 (g) + 52O2 (g) rarr 2CO2 (g) + H2O (l) ΔH = -12996 kJ

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJ

H2 (g) + 12O2 (g) rarr H2O (l) ΔH = -2858 kJ

2268 kJ

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

You Try This One

Suppose you want to know the enthalpy change for the formation of methane CH4 from solid carbon (as graphite) and hydrogen gas

C(s) + 2H2 (g) rarr CH4 (g) ΔH = The enthalpy change for this reaction cannot be

measured in the laboratory because the reaction is very slow We can however measure the enthalpy changes for the combustion of carbon hydrogen and methane

C (s) + O2 (g) rarr CO2 (g) ΔH = -3935 kJH2 (g) + frac12 O2 (g) rarr H2O (l) ΔH = -2858 kJ

CH4 (g) + 2O2 (g) rarr CO2 (g) + 2H2O (l) ΔH = -8903 kJ

Use these energies to obtain ΔH for the formation of methane

ΔH = -748 kJ

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Calculation of H

bull Imagine this as occurringin 3 steps

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

bull The sum of these equations is

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Enthalpies of Formation

An enthalpy of formation Hf is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Standard Enthalpies of FormationStandard enthalpies of formation Hf are measured under standard conditions (25degC and 100 atm pressure) and represent the change in enthalpy associated with the reaction that forms 1 mole of the compound from its elements with all substances in their standard states If the element exists in more than one form the most stable form of the element is used for the formation reaction

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

H = [3(-3935 kJ) + 4(-2858 kJ)] - [1(-10385 kJ) + 5(0 kJ)]

= [(-11805 kJ) + (-11432 kJ)] - [(-10385 kJ) + (0 kJ)]

= (-23237 kJ) - (-10385 kJ)

= -22199 kJ Notice that by definition the standard enthalpy of formation of the most stable form of any element is zero and therefore the standard enthalpy of O2 is zero

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Calculation of H

We can use Hessrsquos law in this way

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Standard Enthalpies of Formation

Example Write the equation corresponding to the standard enthalpy of formation of acetylene (C2H2) gas

C(graphite) + H2(g) rarr C2H2 (g)

Write the equation corresponding to the standard enthalpy of formation of solid silver chloride

Ag(s) + frac12Cl2(g) rarr AgCl(s)

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Use the Formula

Given the following standard enthalpy of reaction use the standard enthalpies of formation in the table to calculate the standard enthalpy of formation of CuO(s)

CuO(s) + H2(g) rarr Cu(s) + H2O(l) ΔHdeg = -1297 kJ

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fatsThe energy content in foods in reported in Calories and is determined with the use of a bomb calorimeter

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Fuels

The vast majority of the energy consumed in this country comes from fossil fuels

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Thermochemistry is Clearly Important to Our Everyday Lives

bull Personal Nutrition ndash nutritional value labels

bull Creation of alternative fuel sources

bull Addressing Global Warming

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions

Thermochemistry

Chapter Summarybull Thermodynamics is the study of energy and its transformationsbull All chemical changes involve a transfer of energy be it into the reaction

or out of the reactionbull Transformed energy in a chemical reaction comes from or forms

chemical bonds and is exchanged with the surroundings as heat andor work

bull When a gas is produced or consumed in a chemical reaction at constant pressure we call the energy change enthalpy All substances have a characteristic enthalpy

bull Calorimeters and bomb calorimeters are devices that allow us to measure the enthalpy change of a chemical reaction by measuring the temperature change associated with such a chemical reaction

bull Because enthalpy is a state function the enthalpy of a reaction depends only on the final and initial states of the system and therefore the enthalpy of a process is the same whether a reaction is carried out in one or in multi-steps Hessrsquos Law allows us the calculate the unknown enthalpy of a reaction from a series of know enthalpies of reactions