CHM152 Exam 4 100 Pts Spring 2017 Name: Due Friday May 5 th at the beginning of class. Show all work. You are to work alone on this exam, but may see me for questions. 1. Arrange these compounds in order of increasing standard molar entropy at 25 C: C3H8(g), C2H4(g), ZnS(s), and H2O(l). A) ZnS(s) < H2O(l) < C3H8(g) < C2H4(g) Explain your choice: B) C2H4(g) < H2O(l) < C3H8(g) < NaCl(s) C) ZnS(s) < C3H8(g) < C2H4(g) < H2O(l) D) C3H8(g) < C2H4(g) < H2O(l) < ZnS(s) E) ZnS(s) < H2O(l) < C2H4(g) < C3H8(g) 2. Which one of the following reactions would you expect to have the lowest S? You must explain your answer. A) CH4(g) + 2O2(g) CO2(g) + 2H2O(g) Explain your choice: B) C2H2(g) + 5 /2O2(g) 2CO2(g) + H2O(g) C) C2H4(g) + O2(g) 2CO2(g) + 2H2O(g) D) C2H6(g) + 7 /2O2(g) 2CO2(g) + 3H2O(g) 3. Using the thermodynamic data provided below, calculate the standard change in entropy when one mole of sodium nitrate is dissolved in water? S (J/K mol) NaNO3(s) 116.3 Na + (aq) 60.25 NO3 (aq) 146.4 Will the solubility of sodium nitrate increase or decrease if the temperature of the system is increased? Justify your answer. 4. Using the thermodynamic data provided below, calculate the standard change in entropy when one mole of sodium sulfate is dissolved in water? S (J/K mol) Na2SO4(s) 149.49 Na + (aq) 60.25 SO4 2 (aq) 17.15 Will the solubility of sodium nitrate increase or decrease if the temperature of the system is increased? Justify your answer. Page 1
5. Rubidium has a heat of vaporization of 69.0 kj/mol at its boiling point (686 C). Calculate S for this process, Rb(l) Rb(g), at 1 atm and 686 C. 6. Assuming S and H do not vary with temperature, at what temperature will the reaction shown below become spontaneous? Show work to support your answer. C(s) + H2O(g) H2(g) + CO(s) ( S = 133.6 J/K mol; H = 131.3 kj/mol) 7. Consider the following standard reduction potentials in acid solution: E (V) Al 3+ + 3e Al(s) 1.66 Sn 4+ + 2e Sn 2+ +0.14 I2(s) + 2e 2I (aq) +0.53 Which is the weakest oxidizing agent in this list? Justify your answer. A) Al 3+ (aq) B) Al(s) C) I (aq) D) I2(s) E) Sn 4+ (aq) 8. Using a table of standard electrode potentials, decide which of the following statements is completely true. A) Cu 2+ can oxidize H2, and Fe can reduce Mn 2+. B) Ni 2+ can oxidize Cu 2+, and Fe 2+ can reduce H +. C) Fe 2+ can oxidize H2, and Fe 2+ can reduce Au 3+. D) Br2 can oxidize Ni, and H2 can reduce Mn 2+. E) H + can oxidize Fe, and Ni can reduce Br2. 9. Complete and balance the following redox reaction under acidic conditions: Cu(s) + NO3 - (aq) Cu 2+ (aq) + NO2(g) Page 2
10. Write the balanced overall redox equation for the following cell diagram: Pt(s) Cu + (aq) Cu 2+ (aq) Br2(l) Br - (aq) Pt(s) 11. Gold can be electrochemically "plated" onto a less expensive metal by submerging the object in a solution of gold(iii) and applying an electric current to drive the following half-reaction: Au 3+ + 3e Au(s) How many grams of gold will be formed if a current of 2.37 A is applied for 71 minutes? Show work. 12. Calculate G (kj/mol) for the following electrochemical cell: Be(s) Be 2+ (aq) Mn 2+ (aq) Mn(s) 13. Determine the equilibrium constant for the following reaction at 25 C using reduction potentials from a table in your book. Show work. 2I (aq) + Br2(l) I2(s) + 2Br (aq). 14. Calculate the cell emf for the following reaction at 25 C: 2Ag + (0.010 M) + H2(1 atm) 2Ag(s) + 2H + (ph = 6.0) Page 3
15. How many grams of copper are deposited on the cathode of an electrolytic cell if an electric current of 2.00 A is passed through a solution of CuSO4 for a period of 19.0 min? 16. Many different ways have been proposed to make batteries. One cell is set up with copper and lead electrodes in contact with CuSO4(aq) and Pb(NO3)2 (aq), respectively. If the Pb 2+ and Cu 2+ concentrations are each 1.0 M, what is the overall cell potential? Pb 2+ + 2e Pb E = 0.22 V Cu 2+ + 2e Cu E = +0.34 V 17. On a plot of the number of neutrons versus the number of protons, a nucleus of aluminum-28 lies above the belt of stability. Write an equation for the radioactive decay of aluminum-28. 18. Complete and balance the nuclear equation 223 Fr 0 β +. 87 1 19. Complete and balance the nuclear equation n + U Br + 3 n +. 1 235 87 1 0 92 35 0 20. Tritium is a radioisotope of hydrogen having a half-life of 12.3 years. If you initially had 1.0 10 7 mole of tritium, how many moles of tritium would be left after 78 years? 21. Balance the equation 222 Rn 4 He +? 86 2 22. Balance the equation U + C? + 4 n. 236 12 1 92 6 0 Page 4
23. Radioactive nitrogen-13 has a half-life of 10 minutes. After 40 minutes, how much of this isotope would remain in a sample that originally contained 96 mg? 24. The blood volume of a patient can be measured by using chromium-51, a positron emitter, administered as a solution of sodium chromate. Write the nuclear equation for the decay of chromium-51. 25. The isotope of iodine with mass number 128 undergoes beta decay with a half-life of 25.00 minutes. How long will it take for 93.75% of a sample of pure 128 I to decay? Page 5