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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