Chapter 19

Chemical ThermodynamicsHW: 13 23 39 43 49 57 67 73 77

19.1 - Spontaneous Processes

• Spontaneous processes are those that can proceed without any outside intervention.

• The gas in vessel B will spontaneously effuse into vessel A, but once the gas is in both vessels, it will not spontaneously effuse

Spontaneous Processes

Processes that are spontaneous in one direction are nonspontaneous in the reverse direction.

Spontaneous Processes• Processes that are spontaneous at one temperature

may be nonspontaneous at other temperatures.• Above 0C it is spontaneous for ice to melt.• Below 0C the reverse process is spontaneous.

Enthalpy-based Spontaneity

• OFTEN, spontaneous reactions are exothermic (DH is -)

• NOT ALWAYS, though!• Ex – Ice melting– It is ENDOTHERMIC, but occurs spontaneously

when T>0oC at standard pressure

19.2 - Entropy-based Spontaneity

• Entropy (S) is a term coined by Rudolph Clausius in the 19th century.

Entropy

• Entropy can be thought of as a measure of the randomness of a system.

• It is related to the various modes of motion in molecules.

Entropy• Like total energy (E) and enthalpy (H)

entropy is a state function.• Therefore,

S = Sfinal Sinitial

• The more disorder the higher the S value.• If S is +, the reaction increases in disorder• If S is -, the reaction decreases in disorder

Solutions

Generally, when a solid is dissolved in a solvent, entropy increases.

Entropy Changes

• In general, entropy increases when– Gases are formed from

liquids and solids.– Liquids or solutions are

formed from solids.– The number of gas

molecules increases.– The number of moles

increases.

Second Law of Thermodynamics

The second law of thermodynamics states that the entropy of the universe increases for spontaneous processes, and the entropy of the universe does not change for reversible processes.

Second Law of Thermodynamics

In other words:For equilibrium processes:

Suniv = Ssystem + Ssurroundings = 0

For spontaneous processes:

Suniv = Ssystem + Ssurroundings > 0

Second Law of Thermodynamics

These last truths mean that as a result of all spontaneous processes the entropy of the universe increases.

19.4 - Standard Entropies

• These are molar entropy values of substances in their standard states.

• Standard entropies tend to increase with increasing molar mass.

• Appendix in the back of the book.

Standard Entropies

Larger and more complex molecules have greater entropies.

Entropy ChangesEntropy changes for a reaction can be estimated in a manner analogous to that by which H is estimated:

S° = nS°(products) - mS°(reactants)

where n and m are the coefficients in the balanced chemical equation.

Example:

3 O2 (g) -> 2 O3 (g)

Does this make sense??

Third Law of ThermodynamicsThe entropy of a pure crystalline substance at absolute zero is 0.

19.5 - Gibbs Free Energy

• Gibbs free energy, G.• G is negative, a process is spontaneous.• G is positive, it is not spontaneous (it is

spontaneous in the reverse direction)• G = 0, it is at equilibrium

Standard Free Energy ChangesStandard free energies of formation, G is the free energy change when 1 mole of compound is synthe

f

DG = SnDG(products) SmG(reactants)f f

where n and m are the stoichiometric coefficients.

19.6 - Free Energy Changes

At temperatures other than 25°C,

DG° = DH TS

How does G change with temperature?

Free Energy and Temperature

• There are two parts to the free energy equation:– H— the enthalpy term– TS — the entropy term

• The temperature dependence of free energy, then comes from the entropy term.

Free Energy and Temperature

Temperature and Chemical Reactions

• It is possible to calculate the temperature at which a reaction becomes favored.

CaCO3(s) CaO(s) + CO2(g)

DH = +177.8 kJDS = +160.5 J/KAt what temp does the reaction become

favored???

Favored or not favored with respect to…?

During Phase Changes

DG = 0 = DH-TDSSo…DS = +DH / T where DH is the molar heat of

vaporization / fusion / etc.

Before / After phase changes, heat is found using q = msDT

19.7 - Free Energy and Equilibrium

Under any conditions, standard or nonstandard, the free energy change can be found this way:

G = G + RT lnQ

(Under standard conditions, all concentrations are 1 M, so Q = 1 and lnQ = 0; the last term drops out.)

Free Energy and Equilibrium

• At equilibrium, Q = K, and G = 0.• The equation becomes

0 = G + RT lnK• Rearranging, this becomes

G = RT lnKor,

K = eG/RT

Free Energy and EquilibriumK ln K DG Favored species

>1 + - Products

=1 0 0 Both

<1 - + Reactants