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## PowerPoint Slideshow about ' Thermodynamics' - vaughan-graves

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### Thermodynamics

Chapter 19

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.

Entropy

- 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.

Positional 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

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.

Free Energy

- G = HTS(from the standpoint of the system)
- A process (at constant T, P) is spontaneous in the direction in which free energy decreases:
- Gmeans+Suniv

Effect of H and S on Spontaneity

The Third Law of Thermodynamics

- . . . the entropy of a perfect crystal at 0 K is zero.
- Because S is explicitly known (= 0) at 0 K, S values at other temps can be calculated.

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 Pressure

- G = G + RT ln(Q)
- Q = reaction quotient from the law of mass action.

Free Energy and Equilibrium

- G = RT ln(K)
- K = equilibrium constant
- This is so because G = 0 and Q = K at equilibrium.

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

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).

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