copyright 1999, prentice hallchapter 51 thermochemistry david p. white university of north carolina,...

Copyright 1999, PRENTICE HALL Chapter 5 1

ThermochemistryThermochemistry

Chapter 5Chapter 5

David P. WhiteDavid P. White

University of North Carolina, WilmingtonUniversity of North Carolina, Wilmington


The Nature of EnergyThe Nature of Energy

Kinetic and Potential EnergyKinetic and Potential EnergyFrom Physics:• Force is a push or pull on an object.• Work is the product of force applied to an object over

a distance:w = F d

• Energy is the work done to move an object against a force.

• Kinetic energy is the energy of motion:

221 mvEk



Kinetic and Potential EnergyKinetic and Potential Energy• Potential energy is the energy an

object possesses by virtue of its position.

• Potential energy can be converted into kinetic energy. Example: a ball of clay dropping off a building.



Energy UnitsEnergy UnitsSI Unit for energy is the joule, J:

We sometimes use the calorie instead of the joule:1 cal = 4.184 J (exactly)

A nutritional Calorie:1 Cal = 1000 cal = 1 kcal

J 1

s/m kg 1

m/s 1kg 2

22

2212

21

mvEk



Systems and SurroundingsSystems and SurroundingsSystem: part of the universe we are interested in.Surroundings: the rest of the universe.


First Law of ThermodynamicsFirst Law of Thermodynamics

Internal EnergyInternal Energy• Internal Energy:

total energy of a system.

• Cannot measure absolute internal energy.

• Change in internal energy, E = Efinal - Einitial


Relating Relating EE to Heat and Work to Heat and WorkEnergy cannot be created or destroyed.Energy of (system + surroundings) is constant.Any energy transferred from a system must be transferred to the surroundings (and vice versa).From the first law of thermodynamics:

when a system undergoes a physical or chemical change, the when a system undergoes a physical or chemical change, the change in internal energy is given by the heat added to or change in internal energy is given by the heat added to or absorbed by the system plus the work done on or by the absorbed by the system plus the work done on or by the system:system:

EE = = qq + + ww



Relating Relating EE to Heat and Work to Heat and Work



Endothermic and Exothermic ProcessesEndothermic and Exothermic ProcessesEndothermic: absorbs heat from the surroundings.Exothermic: transfers heat to the surroundings.An endothermic reaction feels cold.An exothermic reaction feels hot.



State FunctionsState FunctionsState function: depends only on the initial and final states of system, not on how the internal energy is used.



State FunctionsState Functions



Enthalpy, H: Heat transferred between the system and surroundings carried out under constant pressure.Can only measure the change in enthalpy:

H = Hfinal - Hinitial = qP

EnthalpyEnthalpy


For a reaction

Hrxn = H(products) - H (reactants)

Enthalpy is an extensive property (magnitude H is

directly proportional to amount):

CH4(g) + 2O2(g) CO2(g) + 2H2O(g) H = -802 kJ

2CH4(g) + 4O2(g) 2CO2(g) + 4H2O(g) H = -1604 kJ

When we reverse a reaction, we change the sign of H:

CO2(g) + 2H2O(g) CH4(g) + 2O2(g) H = +802 kJ

Change in enthalpy depends on state:

H2O(g) H2O(l) H = -88 kJ

Enthalpies of Reaction Enthalpies of Reaction


Heat Capacity and Specific HeatHeat Capacity and Specific HeatCalorimetry = measurement of heat flow.Calorimeter = apparatus that measures heat flow.Heat capacity = the amount of energy required to raise the temperature of an object (by one degree).Molar heat capacity = heat capacity of 1 mol of a substance.Specific heat = specific heat capacity = heat capacity of 1 g of a substance.

q = (specific heat) (grams of substance) T.Be careful of the sign of q.

Calorimetry Calorimetry


Heat Capacity and Specific HeatHeat Capacity and Specific HeatCalorimetry Calorimetry


Constant-Pressure Constant-Pressure CalorimetryCalorimetryAtmospheric pressure is constant!

H = qP

qrxn = -qsoln = -(specific heat of solution) (grams of solution)

T.



Bomb Calorimetry (Constant-Volume Bomb Calorimetry (Constant-Volume Calorimetry)Calorimetry)Reaction carried out under constant volume.Use a bomb calorimeter.Usually study combustion.



qrxn = -CcalorimeterT.

Bomb Calorimetry (Constant-Volume Bomb Calorimetry (Constant-Volume Calorimetry)Calorimetry)



• Hess’s lawHess’s law: if a reaction is carried out in a number of steps, H for the overall reaction is the sum of H for each individual step.

• For example:

CH4(g) + 2O2(g) CO2(g) + 2H2O(g) H = -802 kJ

2H2O(g) 2H2O(l) H = -88 kJ

CH4(g) + 2O2(g) CO2(g) + 2H2O(l) H = -890 kJ

Hess’s Law Hess’s Law


In the above enthalpy diagram note that H1 = H2 + H3

Hess’s Law Hess’s Law


Enthalpies of FormationEnthalpies of Formation

• If 1 mol of compound is formed from its constituent elements, then the enthalpy change for the reaction is called the enthalpy of formation, Ho

f .

• Standard conditions (standard state): 1 atm and 25 oC (298 K).

• Standard enthalpy, Ho, is the enthalpy measured when everything is in its standard state.

• Standard enthalpy of formation: 1 mol of compound is formed from substances in their standard states.

• If there is more than one state for a substance under standard conditions, the more stable one is used.


Enthalpies of FormationEnthalpies of Formation• Standard enthalpy of formation of the most stable

form of an element is zero.


Hrxn = H1 + H2 + H3


Using Enthalpies of Formation to Calculate Using Enthalpies of Formation to Calculate Enthalpies of ReactionEnthalpies of ReactionWe use Hess’ Law to calculate enthalpies of a reaction from enthalpies of formation.


reactants products rxnf fH m H n H


Using Enthalpies of Formation to Calculate Using Enthalpies of Formation to Calculate Enthalpies of ReactionEnthalpies of ReactionFor a reaction:


Foods and FuelsFoods and Fuels

FoodsFoods •Fuel value = energy released when 1 g of substance is burned.•1 nutritional Calorie, 1 Cal = 1000 cal = 1 kcal.•Energy in our bodies comes from carbohydrates and fats (mostly).•Intestines: carbohydrates converted into glucose:

C6H12O6 + 6O2 6CO2 + 6H2O, H = -2816 kJ

•Fats break down as follows:2C57H110O6 + 163O2 114CO2 + 110H2O, H = -75,520 kJ

•Fats: contain more energy; are not water soluble, so are good for energy storage.


Foods and FuelsFoods and Fuels

FoodsFoods


Foods and FuelsFoods and FuelsFuelsFuels U.S.: 1.0 x 106 kJ of fuel per day.Most from petroleum and natural gas.Remainder from coal, nuclear, and hydroelectric.Fossil fuels are not renewable.


Foods and FuelsFoods and FuelsFuelsFuels Fuel value = energy released when 1 g of substance is burned.Hydrogen has great potential as a fuel with a fuel value of 142 kJ/g.


End of Chapter 5End of Chapter 5

ThermochemistryThermochemistry

copyright 1999, prentice hallchapter 51 thermochemistry david p. white university of north carolina,...

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