Thermodynamics Chapter 19. First Law of Thermodynamics You will recall from Chapter 5 that energy...

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.

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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.

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Spontaneous ProcessesSpontaneous Processes

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

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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.

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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

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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

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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.

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SolutionsSolutions

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

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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)

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Third Law of ThermodynamicsThird Law of Thermodynamics

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

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EntropyEntropy

Standard EntropiesStandard Entropies

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

Standard entropies tend to increase with increasing molar mass.

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Standard EntropiesStandard Entropies

Larger and more complex molecules have greater entropies.

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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.

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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.

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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

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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.

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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.

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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

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.

Free Energy and Equilibrium

Free Energy and Equilibrium

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

0Fraction of A reacted

0.5 1.0

G

Equilibriumoccurs here

0Fraction of A reacted

0.5 1.0

Equilibriumoccurs here

(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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