1. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 1 Chemistry FIFTH EDITION by Steven S. Zumdahl University of Illinois

2. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 2 Chemistry FIFTH EDITION Chapter 16 Spontaneity, Entropy, and Free Energy

3. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 3 Section 16.7 THE DEPENDENCE OF FREE ENERGY ON PRESSURE The free energy of a reaction system changes as the reaction proceeds because free energy is dependent on the pressure of a gas or on the concentration of species in solution. We will consider now the pressure dependence of free energy of an ideal gas.

4. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 4 Section 16.7 THE DEPENDENCE OF FREE ENERGY ON PRESSURE G = H –TS H, Enthalpy is not pressure dependent S, Entropy is pressure dependent A mole of gas at a given T has a greater positional entropy at larger Volume (or lower Pressure.)

5. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 5 G = G ° + RT ln (P) where G = free energy at pressure = P G ° = free energy at pressure = 1 atm. R = 8.3145 J/ K mole T = Temperature in Kelvin Pressure Dependence of the Free Energy of an Ideal Gas.

6. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 6 Free Energy and Pressure  G =  G  + RT ln(Q) Q = reaction quotient from the law of mass action. See pages 770 - 772. Read Sample Exercise 16.13 Change in Free Energy for a Reaction (for the specified pressures.)

7. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 7 Examples: (1) N 2 (g) + 3 H 2 (g)  2 NH 3 (g) Q = P (NH 3 ) 2 / P (N 2 ) P(H 2 ) 3 (2) CO (g) + 2H 2 (g)  CH 3 OH (l) Q = 1/ P(CO) P(H 2 ) 2 See Sample Exercise 16.13 on p. 772.

8. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 8  G is negative  Spontaneous Rxn? Tells us that reaction system favors the products. But it doesn’t mean the system will proceed to pure product.

9. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 9 System will spontaneously go to the equilibrium position -- which is the lowest free energy value available to a particular reaction system.

10. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 10 Figure 16.7 Schematic Representations of Balls Rolling Down into Two Types of Hills Ball will roll to point B.Ball will not get to point B because there is a lower potential energy at point C.

11. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 11 Section 16.8 EQUILIBRIUM Reaction proceeds to the minimum free energy (equilibrium) Which corresponds to the point where  G = 0 (  G = G products – G reactants ) Since  G =  G  + RT ln(Q)  =  G  + RT ln(K)  That is,  G  = - RT ln (K)

12. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 12 Free Energy and Equilibrium  G  =  RT ln(K) K = equilibrium constant This is so because  G = 0 and Q = K at equilibrium.

13. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 13 See Table 16.6 on page 775. Read textbook starting at Case 1 on the bottom of page 775. GKGK  G  = 0K = 1 Equilibrium when Press. of all gases = 1 atm  G  1 Press. of Prod. > 1 atm; Press. of react. < 1 atm  G  > 0K < 1 Press. of Prod. < 1 atm; Press. of react. > 1 atm

14. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 14. Let’s do # 57, 59, 65.

15. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 15 Temperature Dependence of K Since  G  =  H  - T  S  and  G  = - RT ln (K) Then -RT ln (K) =  H  - T  S  Rearrange Equation!

16. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 16 Temperature Dependence of K y = mx + b (  H  and S   independent of temperature over a small temperature range) Plot ln (K) versus 1/T Slope = -  H  /R Y-intercept =  S  /R R

17. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 17 Section 16.9 FREE ENERGY & WORK w max : maximum possible useful work w max =  G Maximum possible useful work obtainable from a process at constant T & P is equal to the change in free energy. Go to Page 778. Read!

18. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 18 FREE ENERGY & WORK Spontaneous process:  G is energy that is free to do useful work. Non-spontaneous process:  G is minimum work that must be expended to make the process occur. (Hence the name – free energy!)

19. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 19 The Amount of Work we actually obtain from a Spontaneous Process Is Always less than the maximum possible amount.

20. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 20 Consider a Car Battery: Chemical change in the battery can do work by sending current to a starter motor. Does all the energy do work? NO!! Current flowing through the wires Causes frictional heating This heat represents wasted energy!!

21. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 21 Figure 16.10 A Battery

22. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 22 Any real pathway wastes energy!!!!! Achieving the maximum work available from a spontaneous process can occur only via a hypothetical pathway.

23. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 23 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).

24. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 24 In any real cyclic process in the system, Work is changed to Heat in the surroundings, and the Entropy of the Universe increases. When energy is used to do work, the total energy of the universe remains constant, BUT it is rendered less useful. After being used to do work, concentrated energy is spread out in the surroundings as thermal energy, and this is the crux of our energy problem.