Chapter 19 Chemical Thermodynamics

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1 Chapter 19. Chemical Thermodynamics Sample Exercise 19.2 (p. 819) Elemental mercury is a silver liquid at room temperature. Its normal freezing point is o C, and its molar enthalpy of fusion is H fusion = 2.29 kj/mol. What is the entropy change of the system when 50.0 g of Hg (l) freezes at the normal freezing point? ( S sys = J/K) Practice Exercise 1 (19.2) Do all exothermic phase changes have a negative value for the entropy change of the system? a) Yes, because the heat transferred from the system has a negative sign. b) Yes, because the temperature decreases during the phase transition. c) No, because the entropy change depends on the sign of the heat transferred to or from the system. d) No, because the heat transferred to the system has a positive sign. e) More than one of the previous answers is correct. Practice Exercise 2 (19.2) The normal boiling point of ethanol, C 2 H 5 OH, is 78.3 o C, and its molar enthalpy of vaporization is kj/mol. What is the change in entropy in the system when 68.3 g of C 2 H 5 OH (g) at 1 atm condenses to liquid at the normal boiling point? (-163 J/K) - 1 -

2 Sample Exercise 19.3 (p. 826) Predict whether S is positive or negative for each of the following processes, assuming each occurs at constant temperature: a) H 2 O (l) H 2 O (g) b) Ag + (aq) + Cl - (aq) AgCl (s) c) 4 Fe (s) + 3 O 2(g) 2 Fe 2 O 3(s) d) N 2(g) + O 2(g) 2 NO (g) Practice Exercise 1 (19.3) Indicate whether each process produces an increase or decrease in the entropy of the system: a) CO 2(s) CO 2(g) b) CaO (s) + CO 2(g) CaCO 3(s) c) HCl (g) + NH 3(g) NH 4 Cl (s) d) 2 SO 2(g) + O 2(g) 2 SO 3(g) Practice Exercise 2 (19.3) Since the entropy of the universe increases for spontaneous processes, does it mean that the entropy of the universe decreases for nonspontaneous processes? - 2 -

3 Sample Exercise 19.4 (p. 827) In each pair, choose the system that has greater entropy and explain your choice: a) 1 mol of NaCl (s) or 1 mol of HCl (g) at 25 o C. b) 2 mol of HCl (g) or 1 mol of HCl (g) at 25 o C c) 1 mol of HCl (g) or 1 mol of Ar (g) at 298 K. Which system has the greatest entropy? a) 1 mol of H 2(g) at STP b) 1 mol of H 2(g) at 100 o C and 0.5 atm c) 1 mol of H 2 O (s) at 0 o C d) 1 mol of H 2 O (l) at 25 o C Practice Exercise 1 (19.4) Practice Exercise 2 (19.4) Choose the substance with the greater entropy in each case: a) 1 mol of H 2(g) at STP or 1 mol of SO 2(g) at STP b) 1 mol of N 2 O 4(g) at STP or 2 mol of NO 2(g) at STP

4 Sample Exercise 19.5 (p. 829) Calculate the change in the standard entropy of the system, S o, for the synthesis of ammonia from N 2(g) and H 2(g) at 298 K. ( J/K) N 2(g) + 3 H 2(g) 2 NH 3(g) Practice Exercise 1 (19.5) Using the standard molar entropies in Appendix C, calculate the standard entropy change, S o, for the watersplitting reaction at 298 K: 2 H 2 O (l) 2 H 2(g) + O 2(g) a) J/K b) J/K c) J/K d) 88.5 J/K e) J/K Practice Exercise 2 (19.5) Using the standard entropies in Appendix C, calculate the standard entropy change, S o for the following reaction at 298 K: ( J/K) Al 2 O 3(s) + 3 H 2(g) 2 Al (s) + 3 H 2 O (g) Sample Exercise 19.6 (p. 833) Calculate the standard free energy change for the formation of NO (g) from N 2(g) and O 2(g) at 298 K: N 2(g) + O 2(g) 2 NO (g) Given that H o = kj and S o = 24.7 J/K. Is the reaction spontaneous under these circumstances? Which of the following statements is true? Practice Exercise 1 (19.6) a) All spontaneous reactions have a negative enthalpy change, b) All spontaneous reactions have a positive entropy change, c) All spontaneous reactions have a positive free-energy change, d) All spontaneous reactions have a negative free-energy change, e) All spontaneous reactions have a negative entropy change

5 Practice Exercise 2 (19.6) A particular reaction has H o = 24.6 kj and S o = 132 J/K at 298 K. Calculate G o. Is the reaction spontaneous under these conditions? Sample Exercise 19.7 (p. 834) a) Use data from Appendix C to calculate the standard free-energy change for the following reaction at 298 K: P 4(g) + 6 Cl 2(g) 4 PCl 3(g) ( kj) b) What is G o for the reverse of the above reaction? ( kj) Practice Exercise 1 (19.7 ) The following chemical equations describe the same chemical reaction. How do the free energies of these two chemical equations compare? (1) 2 H 2 O (l) 2 H 2(g) + O 2(g) (2) H 2 O (l) H 2(g) + ½ O 2(g) a) G 1 o = G 2 o b) G 1 o = 2 G 2 o c) 2 G 1 o = G 2 o d) none of the above Practice Exercise 2 (19.7 ) Using the data from Appendix C, calculate G o at 298 K for the combustion of methane: ( kj) CH 4(g) + 2 O 2(g) CO 2(g) + 2 H 2 O (g) - 5 -

6 Sample Exercise 19.8 (p. 835) In Section 5.7 we used Hess s law to calculate H o for the combustion of propane gas at 298 K: C 3 H 8(g) + 5 O 2(g) 3 CO 2(g) + 4 H 2 O (l) H o = kj a) Without using data from Appendix C, predict whether G o for this reaction is more negative or less negative than H o. b) Use data from Appendix C to calculate the standard free-energy change for the reaction at 298 K. Is your prediction from part (a) correct? (-2108 kj) Practice Exercise 1 (19.8) If a reaction is exothermic and its entropy change is positive, which statement is true? a) The reaction is spontaneous at all temperatures, b) The reaction if nonspontaneous at all temperatures, c) The reaction is spontaneous only at higher temperatures, d) The reaction is spontaneous only at lower temperatures. Practice Exercise 2 (19.8) Consider the combustion of propane to form CO 2(g) and H 2 O (g) at 298 K: C 3 H 8(g) + 5 O 2(g) 3 CO 2(g) + 4 H 2 O (g) H o = kj Would you expect G o to be more negative or less negative than H o? - 6 -

7 Sample Exercise 19.9 (p. 837) The Haber process for the production of ammonia involves the following equilibrium: N 2(g) + 3 H 2(g) 2 NH 3(g) Assume that H o and S o for this reaction do not change with temperature. a) Predict the direction in which G for this reaction changes with increasing temperature. b) Calculate G at 25 o C and 500 o C. (-33.3 kj, 61 kj) Practice Exercise 1 (19.9) What is the temperature above which the Haber ammonia process becomes nonspontaneous? a) 25 o C b) 47 o C c) 61 o C d) 193 o C e) 500 o C Practice Exercise 2 (19.9) a) Using standard enthalpies of formation and standard entropies in Appendix C, calculate H o and S o at 298 K for the following reaction: 2 SO 2(g) + O 2(g) 2 SO 3(g). ( H o = kj, S o = J/K) b) Use your values from part (a) to estimate G at 400 K. ( G o = kj) - 7 -

8 Sample Exercise (p. 839) As we saw in Section 11.5, the normal boiling point is the temperature at which a pure liquid is in equilibrium with its vapor at a pressure of 1 atm. a) Write the chemical equation that defines the normal boiling point of liquid carbon tetrachloride, CCl 4(l). b) What is the value of G o for the equilibrium in part (a)? c) Use data from Appendix C and G o = H o T S o to estimate the normal boiling point of CCl 4. (70 o C) Practice Exercise 1 (19.10) If the normal boiling point of a liquid is 67 o C, and the standard molar entropy change for the boiling process is +100 J/K, estimate the standard molar enthalpy change for the boiling process. a) J b) J c) +34,000 J d) -34,000 J Practice Exercise 2 (19.10) Use data in Appendix C to estimate the normal boiling point, in K, for elemental bromine, Br 2(l). (The experimental value is given in Figure 11.5). (330 K) - 8 -

9 Sample Exercise (p. 840) We will continue to explore the Haber process for the synthesis of ammonia: N 2(g) + 3 H 2(g) 2 NH 3(g) Calculate G at 298 K for a reaction mixture that consists of 1.0 atm N 2, 3.0 atm H 2, and 0.50 atm NH 3. (-44.9 kj/mol) Which of the following statements is true? Practice Exercise 1 (19.11) a) The larger the Q, the larger the G o. b) If Q = 0, the system is at equilibrium. c) If a reaction is spontaneous under standard conditions, it is spontaneous under all conditions. d) The free-energy change for a reaction is independent of temperature. e) If Q > 1, G > G o. Practice Exercise 2 (19.11) Calculate G at 298 K for the reaction of nitrogen and hydrogen to form ammonia if the reaction mixture consists of 0.50 atm N 2, 0.75 atm H 2, and 2.0 atm NH 3. (-26.0 kj/mol) - 9 -

10 Sample Exercise (p. 841) The standard free energy change for the Haber process at 25 o C was obtained in Sample Exercise 19.9 for the Haber reaction (-33.3 kj): N 2(g) + 3 H 2(g) 2 NH 3(g) Use this value of G o to calculate the equilibrium constant for the process at 25 o C. (7 x 10 5 ) Practice Exercise 1 (19.12) The K sp for a very insoluble salt is 4.2 x at 298 K. What is G o for the dissolution of the salt in water? a) -265 kj/mol b) -115 kj/mol c) kj/mol d) +115 kj/mol e) +265 kj/mol Practice Exercise 2 (19.12) Use data from Appendix C to calculate the standard free-energy change, G o, and the equilibrium constant, K, at 298 K for the following reaction: H 2(g) + Br 2(l) 2 HBr (g) ( kj/mol; 4 x )

11 Sample Integrative Exercise 19 (p. 843) Consider the simple salts NaCl (s) and AgCl (s). We will examine the equilibria in which these salts dissolve in water to form aqueous solutions of ions: NaCl (s) Na + (aq) + Cl - (aq) AgCl (s) Ag + (aq) + Cl - (aq) a) Calculate the value of G o at 298 K for each of the preceding reactions. b) The two values from part (a) are very different. Is this difference primarily due to the enthalpy term or the entropy term of the standard free-energy change? c) Use the values of G o to calculate K sp values for the two salts at 298 K. d) Sodium chloride is considered a soluble salt, whereas silver chloride is considered insoluble. Are these descriptions consistent with the answers to part (c)? e) How will G o for the solution process of these salts change with increasing T? What effect should this change have on the solubility of the salts?

Thermodynamics- Chapter 19 Schedule and Notes

Thermodynamics- Chapter 19 Schedule and Notes Date Topics Video cast DUE Assignment during class time One Review of thermodynamics ONE and TWO Review of thermo Wksheet Two 19.1-4; state function THREE