Thermodynamics Chapter 19. First Law of Thermodynamics You will recall from Chapter 5 that energy cannot be created or destroyed. Therefore, the total

ThermodynamicsThermodynamics

Chapter 19

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En

erg

y

Reaction progress

Reactants

Products

Domain of kinetics(the reaction pathway)

Domain ofthermodynamics

(the initial andfinal states)

First Law of Thermodynamics

First Law of Thermodynamics

You will recall from Chapter 5 that energy cannot be created or destroyed.

Therefore, the total energy of the universe is a constant.

Energy can, however, be converted from one form to another or transferred from a system to the surroundings or vice versa.

© 2012 Pearson Education, Inc.

Spontaneous Processes and Entropy

Spontaneous Processes and Entropy

Thermodynamics lets us predict whether a process will occur but gives no information about the amount of time required for the process.

A spontaneous process is one that occurs without outside intervention.

Spontaneous ProcessesSpontaneous ProcessesSpontaneous 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 return to vessel B.


Spontaneous ProcessesSpontaneous Processes

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


Spontaneous ProcessesSpontaneous ProcessesProcesses that are spontaneous at one temperature may be

nonspontaneous at other temperatures.

Above 0 C, it is spontaneous for ice to melt.

Below 0 C, the reverse process is spontaneous.


Reversible v. Irreversible Processes

Reversible v. Irreversible Processes

Reversible: The universe is exactly the same as it was before the cyclic process.

Irreversible: The universe is different after the cyclic process.

All real processes are irreversible -- (some work is changed to heat).

The Second Law of Thermodynamics

The Second Law of Thermodynamics

. . . in any spontaneous process there is always an increase in the entropy of the universe.

Suniv > 0

for a spontaneous process.

Second Law of Thermodynamics

Second Law of Thermodynamics

In other words:For reversible processes:

Suniv = Ssystem + Ssurroundings = 0

For irreversible processes:

Suniv = Ssystem + Ssurroundings > 0


EntropyEntropy

The driving force for a spontaneous process is an increase in the entropy of the universe.

Entropy, S, can be viewed as a measure of randomness, or disorder.

EntropyEntropy

Like total energy, E, and enthalpy, H, entropy is a state function.

Therefore,

S = Sfinal Sinitial


Entropy on the Molecular Scale

Entropy on the Molecular Scale

The number of microstates and, therefore, the entropy, tends to increase with increases in

– Temperature

– Volume

– The number of independently moving molecules.


SolutionsSolutions

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


Positional EntropyPositional Entropy

A gas expands into a vacuum because the expanded state has the highest positional probability of states available to the system.

Therefore,

Ssolid < Sliquid << Sgas

Entropy and Physical StatesEntropy and Physical States

Entropy increases with the freedom of motion of molecules.

Therefore,

S(g) > S(l) > S(s)


Third Law of ThermodynamicsThird Law of Thermodynamics

The entropy of a pure crystalline substance at absolute zero is 0.


EntropyEntropy

Standard EntropiesStandard Entropies

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

Standard entropies tend to increase with increasing molar mass.


Standard EntropiesStandard Entropies

Larger and more complex molecules have greater entropies.


Entropy ChangesEntropy 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.


Entropy Changes in Surroundings

Entropy Changes in Surroundings

Heat that flows into or out of the system changes the entropy of the surroundings.

For an isothermal process:Ssurr =

qsys

T

• At constant pressure, qsys is simply H for the system.


Entropy Change in the Universe

Entropy Change in the Universe

The universe is composed of the system and the surroundings.

Therefore,

Suniverse = Ssystem + Ssurroundings

For spontaneous processes

Suniverse > 0


Free EnergyFree Energy

G = H TS (from the standpoint of the system)

A process (at constant T, P) is spontaneous in the direction in which free energy decreases:

G means +Suniv

Gibbs Free EnergyGibbs Free Energy

1. If DG is negative, the forward reaction is spontaneous.

2. If DG is 0, the system is at equilibrium.

3. If G is positive, the reaction is spontaneous in the reverse direction.


Free Energy and 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 theentropy term.


Effect of H and S on Spontaneity

Effect of H and S on Spontaneity

H S Result

+ spontaneous at all temps

+ + spontaneous at high temps

spontaneous at low temps

+ not spontaneous at any temp

Free Energy Change and Chemical Reactions

Free Energy Change and Chemical Reactions

G = standard free energy change that occurs if reactants in their standard state are converted to products in their standard state.

G = npGf(products) nrGf(reactants)

Free Energy and Equilibrium


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

G = G + RT ln Q

(Under standard conditions, all concentrations are 1 M, so Q = 1 and ln Q = 0; the last term drops out.)© 2012 Pearson Education, Inc.

Free Energy and PressureFree Energy and Pressure

G = G + RT ln(Q)

Q = reaction quotient from the law of mass action.



G = RT ln(K)

K = equilibrium constant

This is so because G = 0 and Q = K at equilibrium.

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A

B

A

B

(a) (b)

C

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0Fraction of A reacted

0.5 1.0

G

Equilibriumoccurs here


0.5 1.0

G



0.5 1.0


(a) (b) (c)

G

Temperature Dependence of K

Temperature Dependence of K

y = mx + b

(H and S independent of temperature over a small temperature range)

ln(K) = -Ho/R*(1/T) + So/R

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Thermodynamics Chapter 19. First Law of Thermodynamics You will recall from Chapter 5 that energy cannot be created or destroyed. Therefore, the total