Chapter 15

Thermochemistry “heat changes”

Some Thermodynamic TermsSystem - substances involved in the chemical and

physical changes under investigation(for us this is what is happening inside the beaker)

Surroundings - rest of the universe(outside the beaker)

Universe - system plus surroundings

Thermodynamic State of a System - set of conditions that describe and define the system

Ex. number of moles of each substance; temperature; pressure; physical states of each substance

State Functions - properties of a system that depend only on the state of the system

Normally capital letters

Heat is a state function: it is a variable of a system that is independent of the path it took to get there.

Most thermodynamic quantities are state functions.

Properties that depend only on values of state functions are also state functions

Ex: T, P, V

Enthalpy (heat changes) H

Entropy (order and disorder) S

Gibbs Free energy (thermodynamically favored or not) G

We use thermo to control rxnsEx. raw food + heat cooked

Important Thermo terms

The total amount of energy in the universe is constant.ΔErxn = 0

Energy involved in a chemical rxn is neither created nor destroyed.

Energy can be converted from one form to another but cannot be created.

Also known as Law of Conservation of Energy

1st Law of Thermodynamics

1st Law of Thermodynamics 2 basic ideas of importance

I. systems tend toward a state of minimum potential energy

a. H O flows downhillb. objects fall when droppedc. E mgh

d. E mg h

2

potential

potential

1st Law of Thermodynamics

II. systems tend toward a state of maximum disorder

a. mirror shatters when droppedb. easy to scramble an eggc. food coloring disperses in water

What is energy? The capacity to do work or transfer heat;

hot stuff which changes other stuff.

Energy causes changes in stuff. Examples:

atomic rearrangement (rxns) C + O2 CO2

splitting/forming nuclei

changes to structure

nearly every kind of change

Potential – stored energy P.E. = mgh

Kinetic – energy of moving objects K.E. = ½ mv2

Energy depends on mass

Bond energy (B.E.) – the amount of energy needed to break one mole of bonds in a

covalent gas substance to form new gaseous products

Types of Energy

In gas phase reactions Ho values may be related to bond energies of all species in the reaction.

Use the bond energies listed in Table 15-2 & 15-3 to estimate the heat of reaction for

1) CCl2F2 + F2 CF4 + Cl2

2) CH4 + O2 CO2 + H2O

Bond Energies

A - B bond energy A + B

H - Cl H + Clg g g

gkJ

mol g g

432

H BE BE298o

reactants products

Use the bond energies listed in Table 15-2 & 15-3 to estimate the heat of reaction for

1) CCl2F2 + F2 CF4 + Cl2

Use the bond energies listed to estimate the heat of reaction for

2) CH4 + O2 CO2 + H2O

Reaction Diagrams – show amount of activation energy needed for the rxn to occur.

Reaction Diagrams

Exothermic reactions release specific amounts of heat as products

Potential energies of products are lower than potential energies of reactants.

Enthalpy Change, HChemistry is done at constant pressure

open beakers on a desk top are at atmospheric pressure

DH - enthalpy changechange in heat content at constant pressure

H = qp

DHrxn - heat of reactionHrxn = Hproducts - Hreactants

Hrxn = Hsubstances produced - Hsubstances consumed

Enthalpy Change, H

Change in enthalpy, DH, or heat of reaction is amount of heat absorbed or released when a

reaction occurs at constant pressure.

When:DH is > 0, the reaction is endothermic (heat is a

reactant)

DH is < 0, the reaction is exothermic (heat is a product)

Standard States & Standard Enthalpy Changes

Thermochemical standard state conditionsT = 298.15 K

P = 1.0000 atm

Thermochemical standard statespure substances in their liquid or solid phase - standard

state is the pure liquid or solidgases - standard state is the gas at 1.00 atm of pressure

gaseous mixtures - partial pressure must be 1.00 atmaqueous solutions - 1.00 M concentration

Standard Molar Enthalpies of Formation, Hf

o

Standard molar enthalpy of formationsymbol is Hf

o

defined as the enthalpy for the reaction in which one mole of a substance is formed from its

constituent elements

for example:

Mg Cl MgCl kJ

H kJ / mol

s 2 g 2 s

f MgClo

2 s

6418

6418

.

.

Standard molar enthalpies of formation have been determined for many substances and are tabulated in Table 15-1 and Appendix K in the

text.

Standard molar enthalpies of elements in their most stable forms at 298.15 K and 1.000 atm

are zero.

The standard molar enthalpy of formation for phosphoric acid is -1281 kJ/mol. Write the

equation for the reaction for which Horxn = -

1281 kJ. P in standard state is P4

32

142 1281

1281

H O P H PO kJ

H kJ / mol

2 g 2 g 4 s 3 4 s

f H PO o

3 4 s

Hess’s Law of heat summation – the overall enthalpy change in a rxn is equal to the sum

of the enthalpy changes for the individual steps in the process.

enthalpy change for a reaction is the same whether it occurs by one step or by any (hypothetical) series of steps ~ true because H is a state

function

Hess’s Law

3. Using Hess’s law find ΔHrxn for 2HCl(g) + F2(g) 2HF(l) + Cl2(g) from the following rxns

ΔH4HCl(g) + O2(g) 2H2O(l) + 2Cl2(g) -202.4

kJ/mol½ H2(g) + ½ F2(g) HF(l) -600.0

kJ/molH2(g) + ½ O2(g) H2O(l) -285.8 kJ/mol

4. ΔH for 4FeO(s) + O2(g) 2Fe2O3(s) is -560 kJ. Use the ΔH for the following two rxns to verify.

ΔH2Fe(s) + O2(g) 2FeO -544 kJ4Fe(s) + 3O2(g) 2Fe2O3(s) -1648 kJ

Hess’s LawHess’s Law in a more useful form

any chemical reaction at standard conditions, the standard enthalpy change is the sum of the standard molar enthalpies of formation of the products (each multiplied by its coefficient in the balanced chemical equation) minus the corresponding sum for the reactants

H H H298o

f productso

f reactantso n n

n n

2nd Law of Thermodynamics - The universe tends toward a state of greater disorder and low energy in a thermodynamically favored (spontaneous) reaction…

exothermic rxns tend to lower energy states and order requires more energy than disorder.

Spontaneous ~ continues on its own after given activation energy to start the rxn. Happens without

any continuing outside influences.

Spontaneous processes require:~ free energy change of system must be negative~ entropy of universe must increase

2nd Law of Thermodynamics

Spontaneity of Physical & Chemical Changesrusting of iron - thermodynamically favored so it

occurs spontaneouslyHave you ever seen rust turn into iron metal without help?

melting of ice at room temperature - thermodynamically favored, occurs spontaneously

Will water spontaneously freeze at room temperature?

* Exothermicity does not ensure spontaneityEx. Freezing of water

exothermic and thermodynamically favored (spontaneous) only below 0oC

* Increase in disorder of the system also does not ensure spontaneity

Entropy, SEntropy is a measure of the disorder or randomness of a

system. The entropy of the universe is increasing.

When:S is positive disorder increases (favors spontaneity)

S is negative disorder decreases (disfavors spontaneity)

From 2nd Law of Thermodynamics, for a spontaneous process

In general:Sgas> Sliquid > Ssolid

S S Suniverse system surroundings 0

3rd Law of Thermodynamics

3rd Law of ThermodynamicsThe entropy of a hypothetical pure, perfect, crystalline substance

at absolute zero temperature is zero.

allows us to measure absolute values of entropy for substancescool them down to 0 K, or as close as possible, then measure

entropy increase as substance warms up

Entropy changes for reactions can be determined similarly to H for reactions. As with H, entropies have been measured and

tabulated in Appendix K as So298. When:

S n S n S298o

productso

reactantso

Free Energy Change, G, and Spontaneity

J. Willard Gibbs determined the relationship of enthalpy and entropy that best describes the maximum useful

energy which can be obtained in the form of work from a process at constant Temperature & Pressure.

The relationship also describes the spontaneity of a system. Whether the reaction is thermodynamically

favored or not thermodynamically favored

This is also a new state function, G, the Gibbs Free Energy. G = H - T S (at constant T & P)

The change in the Gibbs Free Energy is a reliable indicator of spontaneity of a physical process or

chemical reaction.does not tell us the speed of the process (kinetics does -

Ch. 16)

When:G is > 0 reaction is not thermodynamically favored

(nonspontaneous) reactant favored

G is = 0 system is at equilibrium

G is < 0 reaction is thermodynamically favored (spontaneous) product favored

Changes in free energy obey the same type of relationship we have described for enthalpy and entropy changes. G = n G n G298

oproductso

reactantso

Enthalpy and entropy can sometimes reinforce each other – this makes the reaction really go or really not go.

Ex. Dynamite has a neg Δ H & a pos Δ S so the rxn really goes once started…

 If the signs don’t reinforce does a rxn occur? This is where

Gibbs Free energy addresses the spontaneity of rxns.Ex. Liquid water to water vapor Δ H is + and Δ S is +

 Δ G is -, rxn is thermodynamically favored (spontaneous) –

reaction will go on its own once started to make the products.

Ex. gas burning.

Δ G is +, rxn is not thermodynamically favored (nonspontaneous) – rxn won’t go on it’s own, wants to stay

as reactants. Ex. batter cake

State function + -Δ H Endothermic

(taking in heat)ice melting

Exothermic (giving off heat)

dynamite Δ S Towards disorder Towards orderΔ G not thermodynamically

favoredthermodynamically

favored

Must Memorize!

Enthalpy (heat changes) H

Entropy (order and disorder) S

Gibbs Free energy (spontaneity) G

We use thermo to control rxnsEx. raw food + heat cooked

Temperature Dependence of SpontaneityThe general relationship of G, H, and S is

Which gives us 4 possibilities among the signs

H S G Therefore - + - forward rxn spontaneous at all T’s - - ? forward rxn spontaneous at low T’s + + ? forward rxn spontaneous at high T’s + - + forward rxn nonspontaneous at all T’s

G = H - T S

5. Find the a) enthalpy, b) entropy, and c) Gibbs free energy for the following rxn using data from Appendix K. C2H5OH(l) + 3O2(g) 2CO2(g) + 3H2O(g)

Also, using the information for a: is the rxn exothermic or endothermic, for b: more or less ordered, and for c: thermodynamically favored, (spontaneous/product favored), or not thermodynamically favored, (nonspontaneous/reactant favored)?

 

6. Given that the ΔHfo for O2 = 0; ΔHf

o for SO2

= -296.8 kJ/mole; ΔHfo for H2O = -285.8

kJ/mole and the ΔHrxn for the following balanced equation is = -1124 kJ, what is the Δ Hf

o for H2S? 2H2S + 3O2 2SO2 + 2H2O

7. Calculate the enthalpy change for the reaction in which 15.0 g of aluminum reacts with oxygen to form Al2O3 at 25oC and one atmosphere. ΔH rxn is -3352 kJ/mole

 

8. Calculate ΔS298 for the combustion of propane. Δ H298= -2219.9 kJ, and ΔG298= -2108.5 kJ.

Calorimetry

coffee-cup calorimeter - used to measure the amount of heat produced (or absorbed) in a reaction at constant Pmeasures qP

Calorimetryexothermic reaction - heat evolved by reaction is

determined from the temperature rise of the solution

Amount of heat gained by calorimeter is the heat capacity of the calorimeter or calorimeter constantvalue determined by adding a specific amount of heat

to calorimeter and measuring T rise

Amount of heatreleased by reaction

Amount of heatgained by calorimeter

Amount of heatgained by solution

9. When 3.425 kJ of heat is added to a calorimeter containing 50.00 g of water the temperature rises from 24.000oC to 36.540oC. Calculate the heat capacity of the calorimeter in J/oC. The specific heat of water is 4.184 J/goC.

 

10. A coffee-cup calorimeter is used to determine the heat of reaction for the acid-base neutralization of acetic acid and sodium hydroxide. When we add 25.00 mL of 0.500 M NaOH at 23.0000C to 25.00 mL of 0.600 M CH3COOH already in the calorimeter at the same temperature, the resulting temperature is observed to be 25.9470C. The heat capacity of the calorimeter had previously been determined to be 27.8 J/0C. Assume that the specific heat of the mixture is the same as that of water, 4.18 J/g0C and that the density of the mixture is 1.02 g/mL.

A) Calculate the amount of heat given off in the reaction.

B) Determine ΔH for the reaction under the conditions of the experiment. (must determine the number of moles of reactants consumed; use limiting reactant)

When it rains an inch of rain, that means that if we built a one inch high wall around a piece of ground that the rain would completely fill this enclosed space to the top of the wall. Rain is water that has been evaporated from a lake, ocean, or river and then precipitated back onto the land. How much heat must the sun provide to evaporate enough water to rain 1.0 inch onto 1.0 acre of land?

1 acre = 43,460 ft2 vaporization of water = 44.0 kJ/mol

H