1332 -CHAPTER 17 Sample Problems
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1 1332 -CHAPTER 17 Sample Problems Important Data Compound ΔH f S ΔG f kj/mol J/mol K kj/mol SO 2 (g) -301 SO 3 (g) -372 NO(g) NO 2 (g) NOCl(g) N 2 O(g) Cr (s) O 2 (g) Cr 2 O CH H 2 O CO H C 3 H Predicting Entropy 1. Which change is likely to be accompanied by the greatest increase in entropy? N 2 (g) + 3H 2 (g) 2NH 3 (g) Ag + (aq) + Cl (aq) AgCl(s) CO 2 (s) CO 2 (g) H 2 O(g) H 2 O(l) 2. Under which conditions does nitrogen have the largest entropy per mole? N 2 (s) at 50 K and l atm N 2 (l) at 70 K and l atm N 2 (g) at 80 K and 1 atm N 2 (g) at 80 K and 0.5 atm 3. In which process is entropy decreased? dissolving sugar in water expanding a gas evaporating a liquid freezing water 4. Which reaction has the largest positive entropy change per mole of product formed? S(s) + 3F 2 (g) SF 6 (g) SO 2 (g) + Na 2 O(s) Na 2 SO 3 (s) Fe 3+ (aq) + SCN - (aq) FeSCN 2+ (aq) H 2 O(l) H 2 O(g) 5. Which pair of the following has the member with the greater molar entropy listed first? CO(g), CO 2 (g) NaCl(s), NaCl(aq) H 2 S(g), H 2 S(aq) Li(s), Pb(s) 1
2 6. For each pair of substances, at the same temperature, choose the one with the HIGHER entropy I. (i) I 2 (l) (ii) I 2 (g) II. (i) PCl 3 (g) (ii) PCl 5 (g) III. (i) MgF 2 (s) (ii) BaI 2 (s) IV. (i) 2 moles of NO 2 (g) (ii) 1 mole of N 2 O 4 (g) (ii) (ii) (ii) (i) (i) (ii) (ii) (ii) (ii) (ii) (ii) (ii) (i) (ii) (i) (ii) (E) (ii) (i) (ii) (i) 7. Dissolving crystalline ammonium chloride in water lowers the temperature of the solution, so for this process: ΔH is negative and ΔS is positive ΔH is positive and ΔS is negative ΔH is positive and ΔS is positive ΔH is negative and ΔS is negative (E) cannot be determined Entropy Calculations 8. The value of ΔS for the following reaction 2 NO(g) + Cl 2 (g) 2 NOCl(g) is 117 J/K at 25 C. What is the entropy of Cl 2 at this temperature? Calculate ΔS (in J/K) for the reaction, 4 Cr + 3 O 2 2 Cr 2 O General ΔG 10. Vaporization of a liquid is an example of a process for which ΔH, ΔS, and ΔG are positive at all temperatures. ΔH and ΔS are positive. ΔG is negative at low temperatures, positive at high temperatures. ΔH = ΔS 11. A chemical reaction has a negative ΔH and negative ΔS. Which statement is correct? The reaction is spontaneous at all temperatures. The reaction is nonspontaneous at all temperatures. The reaction becomes spontaneous as temperature increases. The reaction becomes spontaneous as temperature decreases. 12. A reaction is spontaneous at all temperatures if ΔH and ΔS are both positive. ΔH and ΔS are both negative. ΔH is positive and ΔS is negative. ΔH is negative and ΔS is positive. 13. Which one of the following statements is true about the equilibrium constant for a reaction if ΔG 0 for the reaction is negative? K = 0 K = 1 K < 1 K > 1 2
3 14. If K is much less than 1 for a given reaction then: ΔG will have the same number as K ΔG is a large negative number ΔG is a large positive number ΔG is a small negative number (E) ΔG is a small positive number 15. Which of the following is NOT true about ΔG 0? (E) If it is negative for a reaction, the reaction is spontaneous. If negative, it represents the maximum work available from a reaction It cannot be measured directly in the lab. If it is large and negative, K for the reaction is large and positive If it is negative, ΔH has to be negative also. 16. Which of the following is false? (E) The first law of thermodynamics relates to the conservation of energy The second law of thermodynamics states that a spontaneous process MUST be accompanied by an increase in the entropy of the universe. The third law of thermodynamics states that only a perfect crystal at 0 K can have zero entropy The standard state of a species is a defined quantity Complex processes violate the laws of thermodynamics 17. Which of the following statements are TRUE? I. ΔG is less than zero for a spontaneous process II. ΔG is equal to ΔH - TΔS III. ΔG is the maximum amount of work that can be obtained from a reaction IV. There are no instruments that can measure ΔG directly. III I and II all of them II and III (E) I and III 18. A spontaneous reaction is one that starts right away always an exothermic reaction always very fast none of the above (E) one that proceeds without continuous input of energy 19. Consider the endothermic reaction C 2 H 4 (g) + H 2 O(g) C 2 H 5 OH(g). When would you expect this reaction to be spontaneous? At all temperatures Above a certain temperature below a certain temperature At no temperature (E) Not enough information given ΔG Calculations 20. Calculate ΔG for the reaction, MnO CO(g) Mn(s) + 2 CO 2 (g) given the following values of ΔG f (kj/mol): MnO 2 (s), ; CO(g), ; CO 2 (g), kj kj kj kj (E) kj 3
4 21. What is ΔG 0 for this reaction 2SO 2 (g) + O 2 (g) 2SO 3 (g) at 298 K? 142 kj 200 kj 744 kj 142 kj 22. Determine the value of ΔG 0 (in kj) for the reaction 2 NO(g) + O 2 (g) 2 NO 2 (g) at 25 C: What is the value of ΔH 0 (in kj) for the reaction N 2 O + NO 2 3 NO at 25 C? For the combustion of one mole of liquid methanol, ΔH = -726 kj. Which answer is closest to ΔG for this combustion at 2500 C? Molecule CH 3 OH O 2 CO 2 H 2 O S (J/K mol) kj -726 kj -291 kj 291 kj (E) 1161 kj 25. At what temperature (in K) will the reaction of methane with water to give hydrogen gas and carbon dioxide become spontaneous? Calculate the temperature at which the following reaction becomes spontaneous: H 2 (g) + I 2 (g) 2 HI given that ΔS = J/K and ΔH = 51.8 kj. 0 K never 298 K 312 K (E) 3.25 K 27. Iron was originally smelted from ore by heating it with charcoal. Calculate the lowest temperature at which this process could be carried out. 2 Fe 2 O 3 (s) + 3 C(s) 4 Fe(s) + 3 CO 2 (g) ΔH f (CO 2 ) = kj/mol; ΔH f (Fe 2 O 3 ) = kj/mol S = 5.7 J/K mol; S (Fe) = 27.3 J/K mol; S (CO 2 ) = J/K mol; S (Fe 2 O 3 ) = 87.4J/K mol C C 2646 C C (E) 0.84 C 28. Coupling of the two reactions below affords the overall reaction: Fe 3 O 4 (s) + 2 C 3 Fe(s) + 2 CO 2 (g). What is the value of ΔG 0 for this reaction? 1. Fe 3 O 4 (s) 3 Fe(s) + 2 O 2 (g) ΔG 0 = kj 2. O 2 (s) + C CO 2 (g) ΔG 0 = kj
5 29. Given that ΔG 0 for Ba 2+ (aq) is 560.7, for CO 3 2- (aq) is and for BaCO 3 (s) is 1139 (all in kj), calculate Ksp for BaCO 3 at 25 C x x x Calculate the value of ΔG for a reaction for which K c = 5.0 x 10 8 at 25 C: -50 kj 22 kj -4.2 kj -25 kj (E) 49,500 kj 31. Given that ΔG f for IBr is 1.3 kj/mol, calculate K for the reaction at 25 C: I 2 + Br 2 2 IBr x 10-6 (E) A particular reaction has an equilibrium constant of 0.48 at 25 C. What is the value of ΔG 0 (in kj)? Calculate ΔG 0 (in kj) for the combustion of propane (C 3 H 8 ) ΔH 0 = kj and ΔG 0 = 91.2 kj for the reaction C(s) + H 2 O(g) CO 2 (g) + H 2 (g). Calculate ΔS 0 in J/K for this reaction at 25 C ANSWERS: 1. C 2. D 3. D 4. D 5. C 6. A 7. C 8. C 9. A 10. B 11. D 12. D 13. D 14. C 15. E 16. E 17. C 18. E 19. D 20 B 21. A 22. A 23. A 24. A 25. C 26. D 27. D 28. C 29. C 30. A 31. E 32. A 33. C 34. D 5
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