1. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 1 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.

2. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 2 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.

3. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 3 Positional Entropy A gas expands into a vacuum because the expanded state has the highest positional probability of states available to the system. Therefore, S solid < S liquid << S gas

4. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 4 The Second Law of Thermodynamics...in any spontaneous process there is always an increase in the entropy of the universe.  S univ > 0 for a spontaneous process.

5. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 5 Free Energy  G =  H  T  S (from the standpoint of the system) A process (at constant T, P) is spontaneous in the direction in which free energy decreases:  G means +  S univ

6. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 6 Effect of  H and  S on Spontaneity

7. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 7 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.

8. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 8 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  =  n p  G f  (products)   n r  G f  (reactants)

9. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 9 Free Energy and Pressure  G =  G  + RT ln(Q) Q = reaction quotient from the law of mass action.

10. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 10 Free Energy and Equilibrium  G  =  RT ln(K) K = equilibrium constant This is so because  G = 0 and Q = K at equilibrium.

11. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 11 Temperature Dependence of K y = mx + b (  H  and S   independent of temperature over a small temperature range)

12. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 12 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).