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1332 -CHAPTER 17 Sample Problems Important Data Compound SO2(g) SO3(g) NO(g) NO2 (g) NOCl(g) N2O(g) Cr (s) O2 (g) Cr 2O3 CH4 H2O CO2 H2 C3H8 ΔHf kJ/mol S J/mol K -74.87 -241.8 -393.5 0 211 240 264 220 23.77 205.138 81.2 186.1 188.8 213.7 130.7 ΔGf kJ/mol -301 -372 86.7 51.8 66.3 103.6 -50.81 -228.6 -394.4 0 -24.5 Predicting Entropy 1. Which change is likely to be accompanied by the greatest increase in entropy? (A) (C) N2(g) + 3H2(g) → 2NH3(g) CO2(s) → CO2(g) 2. Under which conditions does nitrogen have the largest entropy per mole? (A) (C) N2(s) at 50 K and l atm N2(g) at 80 K and 1 atm 3. In which process is entropy decreased? (A) (C) dissolving sugar in water evaporating a liquid 4. Which reaction has the largest positive entropy change per mole of product formed? (A) (C) S(s) + 3F2(g) → SF6(g) Fe3+(aq) + SCN-(aq) → FeSCN2+(aq) 5. Which pair of the following has the member with the greater molar entropy listed first? (A) CO(g), CO2(g) (B) (B) (D) Ag+(aq) + Cl–(aq) → AgCl(s) H2O(g) → H2O(l) (B) (D) (B) (D) NaCl(s), NaCl(aq) N2(l) at 70 K and l atm N2(g) at 80 K and 0.5 atm expanding a gas freezing water (B) (D) (C) 1 SO2(g) + Na2O(s) → Na2SO3(s) H2O(l) → H2O(g) H2S(g), H2S(aq) (D) Li(s), Pb(s) 6. For each pair of substances, at the same temperature, choose the one with the HIGHER entropy I. III. (i) I2(l) (i) MgF2(s) (A) (D) (ii) (ii) (ii) (i) (i) (ii) (i) (ii) 7. Dissolving crystalline ammonium chloride in water lowers the temperature of the solution, so for this process: (A) (C) (E) ΔH is negative and ΔS is positive ΔH is positive and ΔS is positive cannot be determined (ii) I2(g) (ii) BaI2(s) (B) (E) II. IV. (i) PCl3(g) (ii) PCl5(g) (i) 2 moles of NO2(g) (ii) 1 mole of N2O4(g) (i) (ii) (ii) (ii) (ii) (i) (ii) (i) (B) (D) (C) (ii) (ii) (ii) (ii) ΔH is positive and ΔS is negative ΔH is negative and ΔS is negative Entropy Calculations 8. The value of ΔS for the following reaction 2 NO(g) + Cl2(g) → 2 NOCl(g) is –117 J/K at 25 °C. What is the entropy of Cl2 at this temperature? (A) 106 9. Calculate ΔS (in J/K) for the reaction, 4 Cr + 3 O2 → 2 Cr2O3 (A) -548.1 (B) (B) 11 -147.7 (C) (C) 223 147.7 (D) (D) -223 548.1 General ΔG 10. Vaporization of a liquid is an example of a process for which (A) (C) ΔH, ΔS, and ΔG are positive at all temperatures. (B) ΔG is negative at low temperatures, positive at high temperatures. (D) 11. A chemical reaction has a negative ΔH and negative ΔS. Which statement is correct? (A) (B) (C) (D) 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 (A) (C) ΔH and ΔS are both positive. ΔH is positive and ΔS is negative. 13. Which one of the following statements is true about the equilibrium constant for a reaction if ΔG0 for the reaction is negative? (A) K=0 (B) K=1 (B) (D) (C) ΔH and ΔS are positive. ΔH = ΔS ΔH and ΔS are both negative. ΔH is negative and ΔS is positive. K<1 2 (D) K>1 14. If K is much less than 1 for a given reaction then: (A) (C) (E) ΔG will have the same number as K ΔG is a large positive number ΔG is a small positive number 15. Which of the following is NOT true about ΔG0? (A) (B) (C) (D) (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? (A) (B) (C) (D) (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. III. IV. ΔG is less than zero for a spontaneous process II. ΔG is equal to ΔH - TΔS ΔG is the maximum amount of work that can be obtained from a reaction There are no instruments that can measure ΔG directly. (A) III 18. A spontaneous reaction is (A) (C) (E) one that starts right away (B) always very fast (D) one that proceeds without continuous input of energy 19. Consider the endothermic reaction C2H4(g) + H2O(g) → C2H5OH(g). When would you expect this reaction to be spontaneous? (B) I and II (A) At all temperatures (D) At no temperature (C) (B) (D) all of them ΔG is a large negative number ΔG is a small negative number (D) (B) Above a certain temperature (E) Not enough information given II and III (E) I and III always an exothermic reaction none of the above (C) below a certain temperature ΔG Calculations 20. Calculate ΔG° for the reaction, MnO2 + 2 CO(g) ⇔ Mn(s) + 2 CO2(g) given the following values of ΔGf° (kJ/mol): MnO2(s), -466.1; CO(g), -137.2; CO2(g), -394.4 (A) 209.1 kJ (B) -48.3 kJ (C) 185.5 kJ 3 (D) -185.5 kJ (E) -346.1 kJ 21. What is ΔG0 for this reaction 2SO2(g) + O2(g) → 2SO3(g) at 298 K? (A) –142 kJ 22. Determine the value of ΔG0 (in kJ) for the reaction 2 NO(g) + O2(g) → 2 NO2(g) at 25 °C: (A) -69.8 23. What is the value of ΔH0 (in kJ) for the reaction N2O + NO2 → 3 NO at 25 °C? (A) 156.2 24. For the combustion of one mole of liquid methanol, ΔH° = -726 kJ. Which answer is closest to ΔG° for this combustion at 2500 °C? (B) (B) (B) –200 kJ -34.9 (B) (C) 5.5 Molecule CH3OH S° (J/K mol) 126.8 (C) (C) O2 205.1 (D) 53.2 (D) CO2 213.7 H2O 188.8 -291 kJ (D) 142 kJ -104.7 109.0 -1161 kJ 25. At what temperature (in K) will the reaction of methane with water to give hydrogen gas and carbon dioxide become spontaneous? (A) 658 26. Calculate the temperature at which the following reaction becomes spontaneous: H2(g) + I2(g) ⇔ 2 HI given that ΔS = 165.9 J/K and ΔH = 51.8 kJ. (A) 0K 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 Fe2O3(s) + 3 C(s) → 4 Fe(s) + 3 CO2(g) (B) (C) 104.7 (D) (A) (B) -726 kJ –744 kJ 683 never (C) (C) 298 K 956 (D) (D) 312 K 291 kJ (E) 1161 kJ 1229 (E) 3.25 K ΔHf(CO2) = -393.5 kJ/mol; ΔHf (Fe2O3) = -825.5 kJ/mol S° (C) = 5.7 J/K mol; S° (Fe) = 27.3 J/K mol; S°(CO2) = 213.7 J/K mol; S° (Fe2O3) = 87.4J/K mol (A) 1061.2 °C 28. Coupling of the two reactions below affords the overall reaction: Fe3O4(s) + 2 C → 3 Fe(s) + 2 CO2 (g). What is the value of ΔG0 for this reaction? 1. 2. Fe3O4(s) → 3 Fe(s) + 2 O2 (g) O2(s) + C → CO2 (g) (A) +788.2 (B) (B) 842.6 °C -788.2 (C) 2646 °C (D) 569.5 °C ΔG0 = 1014.2 kJ ΔG0 = -394.4 kJ (C) +225.4 4 (D) +619.8 (E) 0.84 °C 29. Given that ΔG0 for Ba2+(aq) is –560.7, for CO32-(aq) is –528.1 and for BaCO3(s) is –1139 (all in kJ), calculate Ksp for BaCO3 at 25 °C. (A) 5.86 30. Calculate the value of ΔG° for a reaction for which Kc = 5.0 x 108 at 25 °C: (A) -50 kJ 31. Given that ΔGf for IBr is 1.3 kJ/mol, calculate K for the reaction at 25 °C: I2 + Br2 ⇔ 2 IBr (A) 1.00 32. A particular reaction has an equilibrium constant of 0.48 at 25 °C. What is the value of ΔG0 (in kJ)? (A) 1.8 33. Calculate ΔG0 (in kJ) for the combustion of propane (C3H8). (A) -598.5 34. ΔH0 = 131.4 kJ and ΔG0= 91.2 kJ for the reaction C(s) + H2O(g) → CO2(g) + H2(g). Calculate ΔS0 in J/K for this reaction at 25 °C. (A) -135 (B) (B) (B) 0.59 (B) (B) (B) 6.3 x 108 22 kJ (C) -4.2 598.5 1.6 (C) (C) 1.3 (D) (C) (C) (C) 1.59 x 10-9 -4.2 kJ 3.7 x 10-6 150 -2073.1 -1.6 (D) (D) (E) (D) (D) (D) 2.18 x 10-27 -25 kJ (E) 49,500 kJ 0.35 4.2 2073.1 135 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