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CH 1020: Mock Exam 3 - Spring 2015 1. Kc = 2 for the reaction X(g) + Y2(g) XY(g) + Y(g) Which of the reaction mixtures below represents a mixture at equilibrium? Assume that dark spheres represent Y atoms and light spheres represent X atoms. 731520685800 A. Mixture 1 C. Mixture 3 B. Mixture 2 D. None of the above 2. If Kc = 46.9 at 700 K for the reaction, PCl5(g) PCl3(g) + Cl2(g) what is Kc for the reverse reaction? 3. In which of the following reactions will Kc = Kp? A. H2(g) + I2(g) 2HI(g) B. CH4(g) + H2O(g) CO(g) + 3H2(g) C. N2O4(g) 2NO2(g) D. CO(g) + 2H2(g) CH3OH(g) 4. Consider the following reaction at 1700 C. 2CH4(g) C2H2(g) + 3H2(g) Initially, the reaction mixture contains only [CH4] = 0.320 M. At equilibrium, the mixture contains [C2H2] = 0.070 M. What is the equilibrium constant for this reaction at 1700 C? 5. If Kc = 1.70 x 103 for the reaction, N2(g) + O2(g) 2 NO(g) and the initial concentrations are [N2]= 0.515 M, [O2] = 0.175 M, and [NO] = 0.135 M, what is required for the reaction to reach equilibrium? A. Nothing, because the reaction is already at equilibrium. B. More NO must form. C. More O2 must form. D. A catalyst must be added. 6. In which of the following situations will Keq change? A. when reactant concentrations change B. when temperature is changed C. when product concentrations change D. when a catalyst is added 7. The diagram to the left represents an equilibrium mixture for the reaction shown. What can be said about the mixture to the right? A. The system is at equilibrium. B. The system is not at equilibrium the forward reaction predominates. C. The system is not at equilibrium, the reverse reaction predominates. 8. Consider the following reaction at 100 C. N2O4(g) 2NO2(g) Kc = 0.36 A reaction mixture at 100 C initially contains [NO2] = 0.100 M. What is the equilibrium concentration of NO2? 9. What effect will a decrease in volume have on the reaction below? C(s) + H2O(g) CO(g) + H2(g) A. K decreases. B. more CO(g) and H2(g) are produced. C. no change. D. more H2O(g) produced. E. K increases. 10. Consider the following reaction: CO(g) + 2H2(g) CH3OH(g) At room temperature, K is approximately 2 104, but at a higher temperature K is substantially smaller. Which of the following is true? A. The reaction is endothermic. B. The value of Kc for this reaction is smaller at all temperatures. C. At the higher temperature, more CH3OH(g) is produced. D. The reaction is exothermic. E. The reaction becomes spontaneous at higher temperatures. 11. Consider 0.10 M solutions of the following: HNO2, NaOH, NH3 and HI. One of the solutions was found to have a pH of 11.12 and another solution, a pH of 1.67. What are the identities of the two solutions? A. The solution with pH = 11.12 is NaOH and the solution with pH = 1.67 is HI. B. The solution with pH = 11.12 is NaOH and the solution with pH = 1.67 is HNO2. C. The solution with pH = 11.12 is HI and the solution with pH = 1.67 is NaOH. D. The solution with pH = 11.12 is NH3 and the solution with pH = 1.67 is HI. E. The solution with pH = 11.12 is NH3 and the solution with pH = 1.67 is HNO2. 123. Calculate the hydroxide ion concentration for an aqueous solution that has a pH of 3.45. 13. A 0.10 M aqueous solution of an acid has a pH of 5.44. What is the value of Ka for this acid? 14. What is the percent dissociation of a weak acid if its initial concentration is 1.5 M and its Ka = 4.5 x 104? 15. Upon losing a proton, a Brønsted acid becomes… A. highly reactive B. its conjugate base C. its conjugate acid D. a hydronium ion E. a hydroxide ion 16. What is the pH of a solution that contains 0.033 M boric acid, H3BO3? The Ka for boric acid is 5.4  1010. 17. Which of the following solutions will have the lowest pH? A. 0.10 M HCN (Ka = 6.2 1010) B. 0.10 M HIO3 (Ka = 1.6 101) C. 0.10 M HF (Ka = 6.8 104) D. 0.10 M CH3COOH (Ka = 1.8 105) E. 0.10 M HNO2 (Ka = 7.1 104) 18. Which of the acids in Question 11 has the strongest conjugate base? A. HCN D. CH3COOH B. HIO3 E. HNO2 C. HF 19. In this reaction the equilibrium favors reactants (lies to the left as written). Which species is the strongest base? 205740102235 A. NH3 C. NH4+ B. H2O D. OH– 20. The figure below shows three acids HX, HY and HZ, (water molecules are omitted for clarity). Which acid has the largest Ka? center8890 A. HX B. HY C. HZ D. it depends on the amount of acid present. 21. What is the pH of a solution for which [Ba(OH)2] = 0.015 M? 22. The ionization constant, Kw, of water at 60 oC is 1.00 1013. What is the pH of pure water at this temperature? 23. An aqueous solution of which of the following salts will give the most basic solution? A. NaOCl C. NaClO2 B. NaCl D. NaClO3 24. You have the following bases and want to prepare a buffer solution with pH = 8.90. Which one do you choose? A. cinnoline Kb = 2.3 1012 B. pyridine Kb = 1.7 109 C. ketamine Kb = 3.0 107 D. codeine Kb = 1.6 106 E. dibutylamine Kb = 1.8 103 25. In an aqueous solution which is 0.1 M sodium acetate, CH3CO2Na, the concentration of hydronium ions [H3O+] is…. A. less than the [H3O+] in 0.1 M acetic acid. B. equal to the [H3O+] in 0.1 M acetic acid. C. greater than the [H3O+] in 0.1 M acetic acid. D. impossible to compare with the [H3O+] in 0.1 M acetic acid. 26. Which of the following salt solutions is neutral? A. Na2CO3 C. CaBr2 B. NH4Cl D. KNO2 27. What is the pH of a solution which is 0.0100 M in HA (a weak acid) and also 0.0020 M in NaA (Ka = 9.0 106)? A. 2.40 C. 4.35 B. 5.65 D. 6.65 28. Blood is buffered by the carbonic acid/bicarbonate buffer system (H2CO3/HCO3–). If the pH of the blood starts to rise (for example if your kidneys fail to metabolize urea properly) which component of the buffer system will react to maintain the correct pH? A. H2O C. HCO3 B. CO32 D. H2CO3 29. What is the pH of a solution prepared by mixing 50 mL of a 0.10 M solution of HF with 25 mL of a 0.20 M solution of NaF? (The pKa of HF is 3.14.) 30. Shown below is a titration curve for ascorbic acid (Vitamin C). What is the approximate pKa for this material? center0 A. 2.80 D. 4.10 B. 6.00 E. 8.70 C. 10.60 31. What is the [H3O+] concentration of a solution formed by mixing 50.0 mL of 0.20 M HCl and 50.0 mL of 0.10 M NaOH? 32. Given 100.0 mL of a buffer that is 0.50 M in HOCl and 0.40 M in NaOCl, what is the pH after 10.0 mL of 1.0 M NaOH has been added? (Ka for HOCl = 3.5 108). 33. A reaction releases 250 kJ of heat but has a decrease in disorder of 800 J/K. Calculate ?G for this reaction at 27 ºC. Is the reaction spontaneous or non-spontaneous? A. -10 kJ, spontaneous B. -10 kJ, non-spontaneous C. +10 kJ, spontaneous D. +10 kJ, non-spontaneous 34. What is ?G for the reaction below at 25 C if ?G = 141.8 kJ and the partial pressures of the gases involved in the reaction are 10 atm SO2, 100 atm O2, and 1.0 atm SO3? 2SO2(g) + O2(g) ? 2SO3(g) 35. The normal freezing point of sulfur dioxide is 72 C. For the process, SO2(l) ? SO2(s) at 80 C, the signs of ?H, ?S, and ?G would be: ?H ?S ?G 36. Consider the following gas-phase reaction of A2 (grey spheres) and B2 (black spheres) molecules. A2 + B2 2AB Which of the following reaction mixtures (not at equilibrium) has the largest (most positive) G of reaction? 45720040005I II III 00I II III A. Mixture I D. Mixtures I and II B. Mixture II E. Mixtures I and III C. Mixture III 37. Using the values below, calculate Grxn for the reaction below at 25 C. Hg2+(aq) + 2Cl(aq) HgCl2(s) Hf (kJ/mol) S (J/Kmol) Hg2+(aq) +171.1 32.2 Cl(aq) 167.2 56.5 HgCl2(s) 224.0 146 38. What is G for the vaporization of hydrazine, N2H4, at 70 C if 2.0 mol of hydrazine liquid are present and the pressure of the hydrazine gas is 0.15 atm? N2H4(l) N2H4(g) Gf = 159 kJ/mol for N2H4(g) Gf = 149 kJ/mol for N2H4(l) 39. What can be said about the spontaneity of the dimerization of sulfur in the gas phase? 2S(g) S2(g) Hf kJ/mol S J/(mol•K) S(g) 277 238 S2(g) 129 80. A. The formation of S2(g) (forward reaction) is spontaneous at all temperatures. B. The formation of S(g) (reverse reaction) is spontaneous at all temperatures. C. The formation of S2(g) (forward reaction) is spontaneous at low temperatures and the formation of S(g) (reverse reaction) is spontaneous at high temperatures. D. The formation of S(g) (reverse reaction) is spontaneous at low temperatures and the formation of S2(g) (forward reaction) is spontaneous at high temperatures. 40. For the reaction below, S = 269 J/(mol·K) and H = 103.8 kJ/mol. Calculate the equilibrium constant at 25 C. 3C(s) + 4 H2(g) C3H8(g) 41. Copper will spontaneously reduce which of the following? A. Fe2+ and Ag+ B. Fe2+ C. Ag+ D. Al3+ E. Fe2+ and Al3+ 42. For the galvanic cell pictured below, which statement is true? 14859059690 A. Potassium ions from the salt bridge flow toward the Fe electrode as the electrode is oxidized. B. Potassium ions from the salt bridge flow toward the half-cell where Fe3+ is reduced. C. Potassium ions from the salt bridge flow toward the Cr electrode as the electrode is oxidized. D. Potassium ions from the salt bridge flow toward the half-cell where Cr3+ is reduced. 43. How many electrons are transferred in the balanced redox reaction for the electrochemical cell summarized below? Al(s) | Al3+(aq) || Cd2+(aq) | Cd(s) 44. Based on the cell information below, determine the standard reduction potential for the following reaction. Mn2+(aq) + 2e Mn(s) Mn(s) | Mn2+ (1 M) ? 2H+ (1 M) | H2 (1 atm) | Pt(s) Ecell = +1.18 V 45. Which of the following statements is true for the galvanic cell depicted below? 571500101600 A. The mass of the Sn electrode decreases during the reaction. B. Sn is the cathode. C. The concentration of Cd2+ increases as the reaction proceeds. D. Cd2+ is the anode. 46. What is ?G for the reaction below? 2 AlI3(aq) 2 Al(s) + 3I2(s) 47. Which of the following is true for the cell represented here? Zn(s) | Zn+2(aq) || Cr+3(aq) | Cr(s) A. The electrons flow from the zinc to the chromium. B. The electrons flow from the chromium to the zinc. C. The chromium is oxidized. D. The zinc is reduced. 48. What is the potential at 25.0 °C of the cell? Zn(s) | Zn2+ (aq, 0.010 M) || Ag+ (aq, 0.20 M) | Ag(s) A. 1.53 V C. 1.57 V B. 1.55 V D. 1.58 V 49. Consider the following reaction at 25 C. Cu2+(aq) + Fe(s) Cu(s) + Fe2+(aq) What is [Fe2+] if [Cu2+] = 0.040 M and Ecell = 0.76 V? USEFUL INFORMATION 08128000 Ho = Hof (products) - Hof (reactants) Go = Gof (products) - Gof (reactants) So = So (products) - So (reactants) G = H TS G = Go + RT ln Q Go = RT ln K Go = nFE At 298 K: At 298 K: Ka · Kb = Kw = 1.00 x 1014 at 25 °C 1 V = 1 J/C 1 F = 96,500 C/mol e R = 8.314 J/K·mol = 0.08206 L·atm/ K·mol 011620500 Table 1. Standard Reduction Potentials at 25 C Reduction Half Reaction Eo Cl2(g) + 2e- 2Cl(aq) 1.36 V Ag+(aq) + e- ? Ag(s) 0.80 V Fe3+(aq) + e- ? Fe2+(aq) 0.77 V I2(s) + 2e- 2I(aq) 0.54 V Cu+(aq) + e- Cu(s) 0.52 V Cd2+(aq) + 2e- Cd(s) 0.40 V Cu2+(aq) + 2e- ? Cu(s) 0.34 V 2H+(aq) + 2e- H2(g) 0.00 Fe3+(aq) + 3e- Fe(s) 0.04 V Sn2+(aq) + 2e- Sn(s) 0.14 V Tl+(aq) + e- Tl(s) 0.34 V Fe2+(aq) + 2e- ? Fe(s) 0.45 V Cr3+(aq) + 3e- Cr(s) 0.74 V Zn2+(aq) + 2e- Zn(s) 0.76 V Al3+(aq) + 3e- ? Al(s) 1.66 V Mg2++ 2 e- ? Mg -2.37 V