1. Thermodynamics Chapter 20

2. Thermodynamics Prediction of whether change will occur No indication of timeframe Spontaneous: occurs without external intervention Nonspontaneous: requires outside “push”

4. Entropy and Spontaneity Driving force for a spontaneous change is an increase in entropy of the universe Entropy, S:measure of disorder Spontaneous change implies: more order  less order fewer ways of arranging particles  more

5. Second Law of Thermodynamics In any spontaneous change, there is always an increase in entropy of the universe. Units: J K

6. Entropy 1877 Ludwig Boltzmann: k = Boltzmann constant, R/N A W = no. of possible arrangements Third Law of Thermodynamics: The entropy of a perfect crystal at 0 K is zero.

7. Positional Entropy Why does a gas expand into a vacuum? Expanded state has highest positional probability of states available.

10. Other factors in entropy Size: increase in S with increasing size (mass) Molecular complexity: increase in S with increasing complexity Generally effect of physical state >> complexity

12. Reactions For a spontaneous reaction: NaOH (s) + CO 2(g)  Na 2 CO 3(s) + H 2 O (l) NaOH (s) + CO 2(g)  Na 2 CO 3(s) + H 2 O (l) S 0 64.45 213.7 139 69.94 J/K Is the reaction spontaneous as written?

13. Spontaneity and S Spontaneous:  S univ > 0 Nonspontaneous:  S univ < 0 At equilibrium:  S univ = 0  S sys can be positive if  S surr increases enough

15. Surroundings and S univ Surroundings add or remove heat Exothermic:  q sys < 0  q surr > 0 so  S surr > 0 Endothermic:  q sys > 0  q surr < 0 so  S surr < 0

16.  S surr and  S sys  S surr :  S surr  -  q sys  S surr  1/T  S surr  1/T At constant pressure:

17. The Math Given: @constant P: Multiply by  T: Result:

18. Reactions and  G  G 0 :Standard Free Energy Reactants in standard states are converted to products in standard states

19. Gibb’s Free Energy Overall criterion for spontaneity from the standpoint of the system A process at constant temp. and pressure is spontaneous in the direction  G decreases

20.  G =  H - T  S HHHH SSSS GGGGSpontaneous? “Good”:  H < 0 “Good”:  S > 0 “Good”:  G < 0 -+- At all temperatures --? At low temperatures ++? At high temperatures +-+ Not at any temperature

22. Summary  G < 0 Spontaneous as written  G > 0 Not spontaneous as written Reverse process spontaneous  G = 0 At equilibrium

23. A Closer Look… T  S: energy not avail. for doing work  G: E avail. as heat – E not avail. for work E avail. as heat – E not avail. for work  max. work available (constant T and P) Amount of work actually obtained depends on path

24.  G and Work  G Spontaneousmax. work obtainable Nonspontaneousmin. work required Work and path-dependence w max (w min )process performed reversibly theoretical w actual < w max performed irreversibly real world

25. Reversible vs. 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.

26. Free Energy and Pressure Q:reaction quotient from mass action law

27. Free Energy and Equilibrium K:equilibrium constant At equilibrium:  G = 0 K = Q K = Q

28. A B

29.  G and Extent of Reaction A B  G 0 B <  G 0 A Spontaneous C D  G 0 D >  G 0 C Nonspontaneous

30. Temperature Dependence of K Plot lnK vs. 1/T slope = -  H 0 /Rintercept =  S 0 /R *assumes  H 0,  S 0 relatively T independent