AP chem Review - 13 Name: ____________________ Text: chapters 17 (Electrochemistry)

• Eo = Eo

ox + Eored

• ΔG = -nFEo

• E = Eo – (RT/nF)lnQ = Eo – (0.0592/n)logQ • log K = nEo / 0.0592

PROBLEMS The following takes place in a voltaic cell: Zn(s) + Cu2+(1 M) → Cu(s) + Zn2+(1 M). The cell has a voltage of +1.10 V. Choose the answer that best relates to questions 1-4. (A) there is no change in the voltage (B) the voltage becomes zero (C) the voltage increases (D) the voltage decreases, but stays

positive (E) the voltage becomes negative 1. What happens to the voltage when a

saturated ZnSO4 solution is added to the zinc compartment of the cell?

2. What happens to the cell voltage when the copper electrode is made smaller?

3. What happens to the cell voltage when the salt bridge is filled with deionized water instead of 1 M KNO3?

4. What happens to the cell voltage

after the cell has operated for 10 minutes?

5. MnO4-(aq) + H+(aq) + C2O4

2-(aq) → Mn2+(aq) + H2O(l) + CO2(g). What is the coefficient of H+ when the above reaction is balanced? (A) 16 (B) 2 (C) 8 (D) 5 (E) 32

6. S2O3

2- + OH- → SO42- + H2O + e-.

After the above half-reaction is balanced, which of the following are the respective coefficients of OH- and SO4

2-? (A) 8 and 3 (B) 6 and 2 (C) 10 and 2 (D) 5 and 2 (E) 5 and 1

7. How many moles of Pt may be

deposited on the cathode when 0.80 F of electricity is passed through a 1.0 M solution of Pt4+? (A) 1.0 mol (B) 0.60 mol (C) 0.20 mol (D) 0.80 mol (E) 0.40 mol

8. All of the following may serve as

reducing agents, EXCEPT: (A) Mg (B) Cs (C) Fe2+ (D) MnO4

- (E) Br-

9. Cr2O72- + 14H+ + 3S2- → 2Cr3+ + 3S

+ 7H2O. For the above reaction, pick the true statement from the following. (A) The S2- is reduced by Cr2O7

2-. (B) The oxidation number of

chromium changes from +7 to +3.

(C) The oxidation number of sulfur remains -2.

(D) The S2- is oxidized by Cr2O72-.

(E) The H+ oxidizes the S.

10. H+ + NO3- + e- → NO + H2O. What

is the coefficient for water arising when the above half-reaction is balanced? (A) 3 (B) 4 (C) 2 (D) 1 (E) 6

11. Co2+ + 2e- → Co, Eo = -0.28 V

Cd2+ + 2e- → Cd, Eo = -0.40 V Given the above standard reduction potentials, estimate the approximate value of the equilibrium constant for the following reaction: Cd + Co2+ ⇌ Co + Cd2+ (A) 10-4 (B) 10-2 (C) 104 (D) 1016 (E) 102

12. When a basic solution of KMnO4 is added to an SnCl2 solution, a brown precipitate of MnO2 forms and Sn4+ remains in solution. When the same basic solution of KMnO4 is added to an NaF solution, no reaction occurs. Which of the substances involved in these reactions served as the reducing agent? (A) SnCl2 (B) KMnO4 (C) NaF (D) MnO2 (E) Sn4+

13. A sample of silver is to be purified by

electrorefining. This will separate the silver from an impurity of gold. The impure silver is made into an electrode. Which of the following is the best way to set up the electrolytic cell? (A) an impure silver cathode and an

inert anode (B) an impure silver cathode and a

a pure gold anode (C) A pure silver cathode with an

impure silver anode (D) A pure gold cathode with an

impure silver anode (E) An impure silver cathode with a

pure silver anode

14. 2MnO4- + 16H+ + 5S2- → 2Mn2+ + 5S

+ 8H2O. The reducing agent in the above reaction is which of the following? (A) MnO4

-

(B) H+ (C) S (D) S2- (E) Mn2+

15. 2Fe3+ + Zn → Zn2+ + 2Fe2+. The

reaction shown above was used in an electrolytic cell. The voltage measured for the cell was not equal to the calculated Eo for the cell. This discrepancy could be caused by which of the following? (A) The anion in the anode

compartment was chloride, instead of nitrate as in the cathode compartment.

(B) One or more of the ion concentrations was not 1 M.

(C) One or more of the ion concentrations were at 25oC instead of 0oC.

(D) The solution in the salt bridge was Na2SO4 instead of KNO3.

(E) The anode and cathode were different sizes.

16. How many grams of mercury could

be produced by electrolyzing 1.0 M Hg(NO3)2 solution with a current of 2.00 A for 3.00 h? (A) 22.4 g (B) 201 g (C) 11.2 g (D) 44.8 g (E) 6.00 g

17. An electrolysis cell was constructed with two platinum electrodes in a 1.00 M aqueous solution of KCl. An odorless gas evolves from one electrode, and a gas with a distinctive odor evolves from the other electrode. Choose the correct statement from the following list. (A) The gas with the distinctive odor

was evolved at the anode (B) The odorless gas was oxygen. (C) The gas with the distinctive odor

was evolved at the negative electrode.

(D) The odorless gas was evolved at the positive electrode.

(E) The odorless gas was evolved at the anode.

18. H2O2(aq) + KIO4(aq) → KIO3(aq) + O2(g) + H2O(l). Choose the true statement from the following list: (A) The iodine oxidation state is

reduced from +8 to +6. (B) This is not an oxidation-

reduction reaction. (C) H2O2 behaves as a reducing

agent. (D) Hydrogen is reduced from +2 to

+1. (E) H2O2 behaves as an oxidizing

agent. 19. Based on the reaction 2BrO3

- + 12H+ + 10e- → Br2 + 6H2O, choose the correct statement: (A) The BrO3

- undergoes oxidation at the anode.

(B) Br goes from a -1 oxidation state to a 0 oxidation state.

(C) Br2 is oxidized at the anode. (D) H+ is a catalyst. (E) The BrO3

- undergoes reduction at the cathode.

20. Based on the reaction 2BrO3- + 12H+

+ 10e- → Br2 + 6H2O, if a current of 5.0 A is passed through the electrolytic cell for 0.50 h, how should you calculate the number of grams of Br2 that will form? (A) !.! !.!" !"## (!"#.!)

(!")

(B) !.! !.!" !"## (!"#.!)!",!"" (!")

(C) !.! !.!" !" (!"#.!)!",!"" (!")

(D) !.! !.!" !"## (!".!)!",!"" (!")

(E) !.! !.!" (!"#.!)!",!"" (!")

21. 2M + 3Zn2+→2M3+ +3Zn Eo=0.90 V

Zn2+ + 2e- → Zn, Eo = -0.76 V M3+ + 3e- → M, Eo = ??? V Using the above information to determine the missing standard reduction potential. (A) 0.90 V (B) +1.66 V (C) 0.00 V (D) -0.62 V (E) -1.66 V

22. In the electroplating of nickel, 0.200 faraday of electrical charge is passed through a solution of NiSO4. What mass of nickel is deposited? (A) 2.94 g (B) 5.87 g (C) 11.7 g (D) 58.7 g (E) 294 g

23. If a copper sample containing some

zinc impurity is to be purified by electrolysis, the anode and the cathode must be which of the following?

Anode Cathode (A) pure copper pure zinc (B) pure zinc pure copper

(C) pure copper impure Cu (D) impure copper pure copper (E) impure copper pure zinc

24. Which of the following is true

regarding the reaction represented below?

H2Se + 4O2F2 → SeF6 + 2HF + 4O2

(A) The oxidation number of O does

not change. (B) The oxidation number of H

changes from -1 to +1. (C) The oxidation number of F

changes from +1 to -1. (D) The oxidation number of Se

changes from -2 to +6. (E) This is a disproportionation

reaction for F.

25. When the equation below is balanced using lowest whole-number coefficients, the coefficient for H+ is

Cr2O72- + H2S + H+ → Cr3+ + S + H2O

(A) 2 (B) 4 (C) 6 (D) 8 (E) 14

26. What is Keq for the reaction:

Fe(s) + Ni2+ → Fe2+ + Ni(s) given: Ni2+ + 2e- → Ni Eo = -0.25 V Fe2+ + 2e- → Fe Eo = -0.44 V

(A) 1.9 x 10-23 (B) 7.6 x 10-8 (C) 3.6 x 103 (D) 2.6 x 106 (E) 5.2 x 1022

FREE-RESPONSE QUESTIONS 27. A galvanic cell is construted with a cobalt electrode in a 1.0 M Co(NO3)2 solution in the left compartment, and a silver

electrode in a 1.0 M AgNO3 solution in the right compartment. The salt bridge contains KNO3(aq). The voltage is positive. (A) What is the balanced net ionic equation for the reaction, and what is the cell potential?

Co2+ + 2e- → Co Eo= -0.28 V Ag+ + e- → Ag Eo = +0.80 V

(B) Which electrode is the anode? Justify your answer. (C) Could KCl be substituted for the KNO3 in the salt bridge? Justify your answer. (D) If solid Co(NO3)2 is added to the cobalt compartment, what happens to the total voltage? (E) If the cell is allowed to operate until equilibrium is established, what will the potential be?

28. The above galvanic cell is constructed with a cadmium electrode in a 1.0 M Cd(NO3)2 solution in the left compartment, and silver electrode in a 1.0 M AgNO3 solution in the right compartment. The salt bridge contains KNO3(aq). The cell voltage is positive. (NOTE: Cd2+ + 2e- → Cd, Eo = -0.41 V) (A) What is the balanced net ionic equation for the reaction, and what is the cell potential? (B) Show how to calculate the equilibrium constant for the cell. (C) Write the expression for Q that would be needed in the Nernst equation. (D) Show how to calculate the free energy for the reaction. (E) Identify the anode, the cathode, the oxidizing agent, and the reducing agent.

27. a.Co(s) + 2Ag+(aq) → Co2+(aq) + 2Ag(s), E = +1.08 V b. cobalt c. KCl cannot be substituted because Cl- will precipitate to form AgCl d. the voltage will decrease. Reaction shifts left. e. At equilibrium, voltage will be 0 V.

1. D 2. A 3. B 4. D 5. A 6. C 7. C 8. D 9. D 10. C 11. C 12. A 13. C 14. D 15. B 16. A 17. A 18. C 19. E 20. B 21. E 22. B 23. D 24. D 25. D 26. D

28. a. 2Ag+ + Cd → Cd2+ + 2Ag, E = +1.21 V b. log K = nEo/0.0592 c. Q = [Cd2+]/[Ag+]2 d. ΔGo = -nFEo = (-2)(96,500)(1.21) e. anode = Cd, cathode = Ag, oxidizing agent = Ag+, reducing agent = Cd