Chapter 17: Thermodynamics: Entropy, Free Energy, and Equilibrium

1. Chapter 17: Thermodynamics: Entropy, Free Energy, and Equilibrium

2. Chapter 17 Outline Spontaneous Processes Entropy Second Law of Thermodynamics Molecular Interpretation Chemical Reactions Gibbs Free Energy Temperature Equilibrium Constant

3. Thermochemistry Thermochemistry is the correlation of chemical processes and energy changes. Thermodynamics: Can a reaction occur?

5. Spontaneous Processes Any process that occurs without outside intervention is spontaneous. When two eggs are dropped they spontaneously break. The reverse reaction is not spontaneous. We can conclude that a spontaneous process has a direction.

6. Spontaneous Processes Some classes of spontaneous processes: Phase transitions (melting, freezing) Mixing Expansion Heat transfer Movement towards chemical equilibrium

7. Spontaneous Processes

8. Spontaneous Processes Which of the following are spontaneous? Spreading of the fragrance of perfume through a room Formation of CH4 and O2 from CO2 and H20 at room temp and 1 atm. Dissolution of sugar in a cup of hot coffee

9. Spontaneous Processes Reversible and Irreversible� A reversible process is one that can go back and forth between states along the same path Chemical systems in equilibrium are reversible In any spontaneous process, the path between reactants and products is irreversible. Thermodynamics gives us the direction of a process. It cannot predict the speed at which the process will occur.

10. Spontaneous Processes Some reactions are spontaneous, others never occur. Why? A system tries to minimize its ENERGY A system tries to maximize its ENTROPY How can we predict whether a reaction will occur over time? Thermodynamics!

11. Exothermic Spontaneous Processes In general, product-favored reactions are exothermic and spontaneous E.g. thermite reaction Fe2O3(s) + 2 Al(s) ? 2 Fe(s) + Al2O3(s) ?H = - 848 kJ

12. Non-Exothermic Spontaneous Processes But many spontaneous reactions or processes are endothermic . . .

14. Expansion of a gas When there are many molecules, it is much more probable that the molecules will distribute among to the two flasks than all remain in only one flask.

15. Entropy Entropy, S, is a measure of the disorder of a system. Spontaneous reactions proceed to lower energy or higher entropy. In ice, the molecules are very well ordered because of the H-bonds. Therefore, ice has a low entropy.

17. Expansion of a gas

29. Conceptual Question Which of the following processes produces a decrease in the entropy of the system? A. Boiling water to form steam B. Dissolution of solid KCl in water C. Mixing of two gases into one container D. Freezing water to form ice E. Melting ice to form water

30. Conceptual Question Consider a pure crystalline solid that is heated from absolute zero to a temperature above the boiling point of the liquid. Which of the following processes produces the greatest increase in the entropy of the substance? A. melting the solid B. heating the liquid C. heating the gas D. heating the solid E. vaporizing the liquid

31. Recap� Do you agree with the following statements? A. Spontaneous reactions are always fast. B. In any spontaneous process, the entropy of the system always increases. C. An endothermic reaction is always non-spontaneous.

35. 2nd Law Example ?Suniv = ?Ssys + ?Ssurr Is the decomposition of calcium carbonate spontaneous under standard conditions? Calculate ?Suniv to support your answer. CaCO3 (s) ? CaO (s) + CO2 (g)

36. Entropy Changes in Chemical Reactions Used calorimetry to measure ?H� for chemical reactions No comparable method for measuring S� (variation of heat capacity at different temperatures) Standard molar entropy, S?: entropy of a substance in its standard state. Similar in concept to ?H?. Units: J/mol-K. See Appendix C

39. Conceptual Question Which substances in each of the following pairs would you expect to have the higher standard molar entropy? Why? C2H2 (g) or C2H6 (g) b. CO2 (g) or CO (g) c. I2 (s) or I2 (g) d. CH3OH (g) or CH3OH (l)

40. Entropy Changes in Chemical Reactions Can Calculate ?S� for chemical reactions Calculate ?S? for the dissolution of ammonium nitrate, given the following entropy values: NH4NO3 (s) NH4+ (aq) + NO3- (aq) 151.04 J/mol*K 112.8 146.4

41. Problem Solving Calculate the standard entropy change for the following reaction: Al2O3 (s) + 3H2 (g) 2Al (s) + 3 H2O (g)

42. 2nd Law Example ?Suniv = ?Ssys + ?Ssurr Is the decomposition of calcium carbonate spontaneous under standard conditions? Calculate ?Suniv to support your answer. CaCO3 (s) ? CaO (s) + CO2 (g)

43. Entropy Changes in the Surroundings We can calculate Entropy changes in the system, what about the surroundings? ?Ssurr = -?H/T

44. 2nd Law Example ?Suniv = ?Ssys + ?Ssurr ?Ssurr = - ?H/T Is the decomposition of calcium carbonate spontaneous under standard conditions? Calculate ?Suniv to support your answer. CaCO3 (s) ? CaO (s) + CO2 (g)

45. You Try! Is the decomposition of Be(OH)2 spontaneous at 25 �C? Be(OH)2 (s) ? BeO (s) + H2O (g)

47. What is �Free� about Free Energy? The change in free energy for a process equals the maximum useful work that can be done by the system on the surroundings in a spontaneous process occurring at constant temperature and pressure. It is the portion of the energy change of a spontaneous process that is free to do useful work. For a process that is not spontaneous, the free energy change is the measure of the minimum amount of work that must be done to cause the process to occur.

50. Problem Solving N2 (g) + 2O2(g) ? 2NO2 (g)

53. Problem Solving Sulfur dioxide in the effluent gases from coal-burning electric power plants is one of the principal causes of acid rain. One method for reducing SO2 emissions involves partial reduction of SO2 to H2S, followed by catalytic conversion of the H2S and the remaining SO2 to elemental sulfur: 2H2S (g) + SO2 (g) ? 3S (s) + 2H2O (g) a. Using the charts, calculate ?G�. Is this reaction spontaneous at 25�C? b. Estimate the temperature at which the reaction becomes non-spontaneous.

58. Problem Solving Calculate the free energy change for ammonia synthesis at 25�C given the following sets of partial pressures. N2 (g) + 3H2 (g) ? 2NH3 (g) ?Gf = -16.5 kJ/mol 1.0 atm N2, 3.0 amt H2, 0.020 atm NH3 0.010 atm N2, 0.030 amt H2, 2.0 atm NH3

60. Problem Solving Use standard free energies of formation to calculate Keq at 25?C for the reaction below: N2 (g) + 3H2 (g) 2NH3 (g)

61. Problem Solving A. Write the equation that represents the normal boiling point of liquid CCl4. B. What is the value of ?G? for CCl4 in equilibrium with its vapor at the normal boiling point? C. Estimate the normal boiling point of CCl4.

62. Biological Energetics of Life Read Handout Discuss with your group Write one �group� statement answering the question and turn it in with all your names. Yes, it is worth points!

63. Problem Solving Calculate ?G at 298 K for a reaction mixture that consists of 1.0 atm N2, 3.0 atm H2, and 0.50 atm NH3. N2 (g) + 3H2 (g) 2NH3 (g)