CHAPTER 12:

OXIDATION-REDUCTION REACTIONS

Questions on oxidation-reduction reactions can be divided into four categories: recognizing redox reactions, balancing redox equations, using balanced redox equations, and determining the relative strength of oxidizing and reducing agents. A representative sample of each question is included in this section. Many of the following questions assume that a truncated table of standard redox potentials is attached to the exam. The following information is often presented under the heading: “potentially useful information.”

E = E° - (0.0257/n) ln Q 1 F = 96,500 C

N = 6.02 x 10²³ particles per mole 1 amp = 1 C/s

Oxidation-Reduction Reactions

12-1. Which of the following isn’t an example of an oxidation-reduction reaction?

(a) Ca₃P₂(s) + 6 H₂O(l) ➝ 3 Ca²⁺(aq) + 6 OH^-(aq) + 2 PH₃(g)

(b) 2 PH₃(g) + 4 O₂(g) ➝ H₃PO₄(s)

(d) 6 Ca(s) + P₄(s) ➝ 2 Ca₃P₂(s)

(e) All of the above are oxidation-reduction reactions.

Answer: (a)

12-2. Which of the following isn’t an example of an oxidation-reduction reaction?

(a) H₂ + Cl₂ ➝ 2 HCl

(b) Ag⁺ + 2 NH₃ ⇌ Ag(NH₃)₂⁺

(d) Cl₂ + 2 Br^- ➝ 2 Cl^- + Br₂

Answer: (b)

12-3. Identify the oxidizing agents in the following reaction.

P₄(s) + HNO₃(aq) ➝ H₃PO₄(aq) + NO(g)

(a) P₄ and H₃PO₄ (b) P₄ and NO (c) P₄ and HNO₃

(d) HNO₃ and H₃PO₄ (e) HNO₃ and NO

Answer: (d)

12-4. For the reaction:

3 Sn²⁺(aq) + Cr₂O₇^2-(aq) + 14 H⁺(aq) ➝ 3 Sn⁴⁺(aq) + 2 Cr³⁺(aq) + 7 H₂O(l)

which of the following statements is true?

(a) both Sn²⁺ and H⁺ are oxidizing agents.

(b) Cr₂O₇^2- is the oxidizing agent.

(d) the acid isn’t important to the reaction.

(e) none of the above are true.

Answer: (b)

12-5. For the reaction: 2 S₂O₃^2-(aq) + I₃^-(aq) + H⁺(aq) ➝ S₄O₆^2-(aq) + 3 I^-(aq), which of the following statements is true?

(a) S₂O₃^2- is the oxidizing agent. (b) I₃^- is the oxidizing agent.

(e) none of the above.

Answer: (b)

Balancing Oxidation-Reduction Reactions

12-6. Write a balanced chemical equation for the following reaction.

MnO₂(s) + PbO₂(s) + H⁺(aq) ➝ MnO₄^-(aq) + Pb²⁺(aq)

Answer: 2 MnO₂(s) + 3 PbO₂(s) + 4 H⁺(aq) ➝ 2 MnO₄^-(aq) + 3 Pb²⁺(aq) + 2 H₂O(l)

12-7. Write a balanced chemical equation for the following reaction, which can be used to standardize aqueous permanganate ion solutions.

H₂C₂O₄(aq) + MnO₄^-(aq) + H⁺(aq) ➝ CO₂(g) + Mn²⁺(aq)

Answer: 5 H₂C₂O₄(aq) + 2 MnO₄^-(aq) + 6 H⁺(aq) ➝ 10 CO₂(g) + 2 Mn²⁺(aq) + 8 H₂O(l)

12-8. Balance the following redox reaction. Clearly show the two balanced half-reactions and indicate which represents oxidation and which reduction.

H₂O₂(aq) + NO(g) + H⁺(aq) ➝ NO₃^-(aq)

Answer: 3 H₂O₂(aq) + 2 NO(g) ➝ 2 NO₃^-(aq) + 2 H₂O(l) + 2 H⁺(aq)

12-9. Balance the following oxidation-reduction equation.

HI(aq) + HNO₃(aq) ➝ NO(g) + I₂(aq)

Answer: 6 HI(aq) + 2 HNO₃(aq) ➝ 2 NO(g) + 3 I₂(aq) + 4 H₂O(l)

12-10. Balance the following oxidation-reduction equation.

MnO₄^-(aq) + Fe²⁺(aq) + H⁺(aq) ➝ Mn²⁺(aq) + Fe³⁺(aq)

Answer: MnO₄^-(aq) + 5 Fe²⁺(aq) + 8 H⁺(aq) ➝ Mn²⁺(aq) + 5 Fe³⁺(aq) + 4 H₂O(l)

12-11. Balance the following oxidation-reduction equation.

OH^-

CrO₄^2-(aq) + HSnO₂^-(aq) ➝ HSnO₃^-(aq) + CrO₂^-(aq)

Answer:

2 CrO₄^2--(aq) + 3 HSnO₂^-(aq) + 2 H⁺(aq) ➝ 3 HSnO₃^-(aq) + 2 CrO₂^-(aq) + H₂O(l)

12-12. What is the correct coefficient for Na₂MnO₄ in the balanced equation for the air oxidation of manganese dioxide to sodium manganate in the presence of sodium carbonate?

MnO₂(s) + Na₂CO₃(aq) + O₂(g) ➝ CO₂(g) + Na₂MnO₄(aq)

(a) 1 (b) 3/2 (c) 2 (d) 5/2 (e) 3

Answer: (c)

12-13. What is the coefficient for sulfur in the following redox reaction?

S(s) + KClO₃(s) ➝ SO₂(g) + KCl(s) + bang!

(a) 1 (b) 2 (c) 3 (d) 4 (e) 5

Answer: (c)

12-14. Complete and balance the following equations:

Cr₂O₇^2-(aq) + I^-(aq) + H⁺(aq) ➝ Cr³⁺(aq) + I₂(aq)

PbO₂(s) + Cl^-(aq) + H⁺(aq) ➝ Pb²⁺(aq) + Cl₂(aq)

Answer:

Cr₂O₇^2-(aq) + 6 I^-(aq) + 14 H⁺(aq) ➝ 2 Cr³⁺(aq) + 3 I₂(aq) + 7 H₂O(l)

PbO₂(s) + 2 Cl^-(aq) + 4 H⁺(aq) ➝ Pb²⁺(aq) + Cl₂(aq) + 2 H₂O(l)

12-15. The following equation, when balanced, involves a total of ___ electrons.

ClO₃^-(aq) + I₂(aq) + H₂O(l) ➝ IO₃^-(aq) + Cl^-(aq) + H⁺(aq)

(a) 5 (b) 6 (c) 12 (d) 24 (e) 30

Answer: (e)

Questions 16 and 17 refer to the following incomplete, unbalanced equation

H⁺

Cr₂O₇^2-(aq) + NO(g) ➝ Cr³⁺(aq) + NO₃^-(aq)

12-16. How many electrons are in the half-reaction involving Cr₂O₇^2-?

(a) 2 (b) 3 (c) 4 (d) 5 (e) 6

Answer: (e)

12-17. What is the coefficient of the NO in the balanced equation?

(a) 1 (b) 2 (c) 3 (d) 4 (e) 6

Answer: (b)

Questions 18-20 refer to the following reaction:

OH^-

OCl^-(aq) ➝ ClO₂^-(aq) + Cl₂(aq)

12-18. How many OCl^- ions are consumed in the balanced equation for this reaction?

(a) 2 (b) 3 (c) 4 (d) 5 (e) 6

Answer: (b)

12-19. How many OH^- ions are involved in the simplest balanced equation for this reaction? (a) 0 (b) 1 (c) 2 (d) 3 (e) 4

Answer: (c)

12-20. What is the net effect on the OH^- ion concentration in this reaction?

(a) the OH^- ion concentration increases.

(b) the OH^- ion concentration decreases.

(d) there is no way to predict what happens to the OH^- ion concentration in this reaction.

Answer: (a)

Questions 21 through 25 refer to the following reaction.

OH^-

CrO₄^2- + PH₃ ➝ Cr(OH)₄^- + P₄

12-21. Which of the following are oxidizing agents in this reaction?

(a) CrO₄^2- and PH₃ (b) CrO₄^2- and P₄ (c) Cr(OH)₄^- and PH₃

(d) Cr(OH)₄^- and P₄ (e) CrO₄^2- and OH^-

Answer: (b)

12-22. How many electrons are transferred in the half-reaction which involves CrO₄^2-?

(a) 1 (b) 2 (c) 3 (d) 4 (e) 5

Answer: (c)

12-23. How many OH^- ions are produced in the half-reaction which involves CrO₄^2-?

(a) 1 (b) 2 (c) 3 (d) 4 (e) 5

Answer: (d)

12-24. In the simplest balanced chemical equation, is OH^- produced or consumed?

(a) produced (b) consumed

Answer: (a)

12-25. How many CrO₄^2- ions are consumed in the balanced chemical equation?

(a) 1 (b) 2 (c) 3 (d) 4 (e) 6

Answer: (d)

Problems 26 through 28 refer to the following reaction in acid solution.

H⁺

CuS(s) + NO₃^-(aq) ➝ Cu²⁺(aq) + SO₄^2-(aq) + NO(g)

12-26. In the balanced half-reaction for the NO₃^- ion, how many electrons are involved?

(a) 1 (b) 2 (c) 3 (d) 4 (e) none of these

Answer: (c)

12-27. In the complete balanced overall reaction, what is the coefficient of the NO₃^- ion? (a) 3 (b) 4 (c) 5 (d) 6 (e) none of these

Answer: (e)

12-28. Which of the following isn’t true?

(a) the reducing agent is CuS.

(b) the CuS is oxidized.

(d) the NO₃^- ion is reduced.

(e) the oxidation number of the copper changes from 0 to +2.

Answer: (e)

Using Balanced Redox Equations

12-29. How many moles of Fe²⁺ can be oxidized to Fe³⁺ by 0.5 moles of sodium permanganate, NaMnO₄? (Note: The final oxidation state of manganese is Mn²⁺)

(a) 1 (b) 2 (c) 2.5 (d) 3 (e) none of the above

Answer: (c)

12-30. How many moles of iodide will be oxidized by one mole of permanganate ion in the following reaction?

MnO₄^-(aq) + I^-(aq) + H⁺(aq) ➝ Mn²⁺(aq) + I₂(aq) + H₂O(l)

(a) 1 (b) 2 (c) 3 (d) 4 (e) 5

Answer: (e)

12-31. If permanganate reacts with oxalic acid to form carbon dioxide and manganese(II) according to the following unbalanced equation,

H⁺

MnO₄^-(aq) + H₂C₂O₄(aq) ➝ CO₂(g) + Mn²⁺(aq)

and 50.1 mL of MnO₄^- is needed to titrate 115 mL of 0.0250 M H₂C₂O₄, what is the concentration of the MnO₄^- ion solution?

(a) 0.0115 M (b) 0.0230 M (c) 0.0574 M

(d) 0.115 M (e) 0.143 M

Answer: (b)

12-32. A sample of impure zinc metal of mass 2.45 g is analyzed by titration with a KBrO₃ solution. The sample required 40.4 mL of 0.247 M KBrO₃ solution. Find the percentage of zinc metal in the sample, assuming that the sample does not contain any other reducing agents. (AW: Zn = 65.38 g/mol) The unbalanced equation for this reaction is:

Zn(s) + BrO₃^-(aq) ➝ Zn²⁺(aq) + Br₂(aq)

Answer: 66.5%

12-33. If a 10.00 mL sample of a K₂C₂O₄ solution is diluted to a total volume of 50.00 mL and then requires 15.00 mL of a 0.25 M KMnO₄ solution to titrate in acidic medium, what was the molarity of the original K₂C₂O₄ solution?

Answer: 0.94 M

12-34. What weight of ferrous chloride (FeCl₂) can be oxidized by 3.2 mL of 3 M KMnO₄? (AW: Fe = 55.85 g/mol, Cl = 35.45 g/mol)

Answer: 6.08 g

Relative Strength of Oxidizing/Reducing Agents

12-35. Cobalt(III) oxide reacts with hydrogen gas to form cobalt metal and water.

Co₂O₃(s) + 3 H₂(g) ➝ 2 Co(s) + 3 H₂O(g)

What does this tell you about the relative strength of the oxidizing and reducing agents in this reaction?

(a) the Co³⁺ ion is a stronger reducing agent than cobalt metal.

(b) cobalt metal is a better reducing agent than water.

(d) the Co³⁺ ion is a weaker oxidizing agent than water.

(e) statements (a) through (d) above are all false.

Answer: (c)

12-36. Which is the strongest oxidizing agent among the following?

(a) Cl₂ gas at 1 atm pressure.

(b) O₂ gas at 1 atm pressure in contact with 1 M acid.

(d) Ag metal.

(e) H₂O₂ in acid solution.

Answer: (c)

12-37. Which of the following is the strongest reducing agent?

(a) H⁺ (b) H₂ (c) H^- (d) H₂O (e) O₂

Answer: (c)

Voltaic Cells: General

12-38. Which of the following statements correctly describes the voltaic cell represented by the following line notation?

Zn(s)|Zn²⁺ (1.0 M)||H⁺(1.0 M)|H₂(g)|Pt

(a) H⁺ ions flow toward the cathode.

(b) the Zn²⁺ ion concentration increases with time.

(d) electrons flow from the Zn electrode to the Pt electrode.

(e) statements (a) through (d) are all true.

Answer: (e)

12-39. Use cell notation to describe the following galvanic cell. The anode is the standard hydrogen electrode and the cathode is a fluorine gas electrode. The latter, however, isn’t a standard fluorine electrode. The gas is in the standard state, but the solution with which the gas is in contact has [F^-] = 0.10 M. Use Pt as the wire for the both electrodes.

Answer: Pt∣H₂(g)∣H⁺(aq), 1 M∣∣F₂(g)∣F^-(aq), 0.10 M∣Pt

12-40. Consider the following generalized half-reactions.

A_ox + e^- ⇌ A_red

B_ox + e^- ⇌ B_red

If E° for the half-reaction involving A is positive and if E° for the half-reaction involving B is negative, then at standard conditions:

(a) A_ox will reduce B_ox

(b) A_red will reduce B_ox

(d) Br_red will reduce A_red

(e) no reaction will occur

Answer: (d)

12-41. The following electrochemical cell has a voltage of 0.48 V.

Pt|PtCl₄^2-, Cl^-||MnO₂|H⁺,Mn²⁺|Pt

For the spontaneous reaction:

(a) Mn²⁺ will be oxidized to MnO₂

(b) PtCl₄^2- will be reduced to Pt.

(d) MnO₂ will be oxidized to Mn²⁺.

(e) it isn’t possible to identify the species reduced or oxidized from the information given.

Answer: (c)

12-42. Write balanced anode and cathode half-reactions and overall cell reactions for the following cell:

Pt|Fe²⁺, Fe³⁺||Cl^-|AgCl, Ag

Answer: Fe²⁺ + AgCl ➝ Ag + Fe³⁺ + Cl^-

12-43. Determine E° for the cell in question 42.

Answer: -0.548 V

12-44. Indicate whether the cell reaction in question 42 is spontaneous or nonspontaneous as written.

Answer: nonspontaneous

12-45. Determine the cell potential of the cell in question 42 if [Fe²⁺] = 0.05 M, [Fe³⁺] = 0.01 M, and [Cl^-] = 0.25 M.

Answer: -0.581

Voltaic Cells: Calculations

12-46. Which of the following cells has a potential larger than the standard-state potential for this cell?

(a) Zn|Zn²⁺(1 M)||Cu²⁺(1 M)|Cu (b) Zn|Zn²⁺(2 M)||Cu²⁺(1 M)|Cu

(e) Zn|Zn²⁺(0.1 M)||Cu²⁺(0.1 M)|Cu

Answer: (c)

12-47. Calculate the standard-state potential for the following reaction.

4 Cr²⁺(aq) + O₂(g) + 4 H⁺(aq) ⇌ 4 Cr³⁺(aq) + 2 H₂O(l)

Calculate the cell potential when the reaction is run at pH = 7.

Answer: 1.639 V, 1.225 V

12-48. From the following shorthand notation, write the balanced cell reaction. What is the coefficient for zinc?

Zn(s)|Zn²⁺||H⁺|O₂(g)|H₂O|Pt(s)

(a) 1/2 (b) 1 (c) 2 (d) 3 (e) 4

Answer: (c)

12-49. What is the voltage generated by the cell in the previous question?

(a) 1.99 V (b) 3.22 V (c) 1.229 V (d) -1.99 V (e) -1.229 V

Answer: (a)

12-50. Use a table of standard electrode potentials to decide which of the following reactions doesn’t occur spontaneously.

(a) Zn(s) + 2 H⁺(aq) ⇌ Zn²⁺(aq) + H₂(g)

(b) I₂(s) + 2 Fe²⁺(aq) ⇌ 2 I^-(aq) + 2 Fe³⁺(aq)

(d) Mg(s) + Cl₂(g) ➝ MgCl₂(s)

(e) All of these reactions are spontaneous.

Answer: (b)

12-51. What is the potential for a voltaic cell produced from the following half-reactions, if all ions are present at 1.0 M concentrations?

Oxidation: Al ⇌ Al³⁺ + 3 e^-

Reduction: Zn²⁺ + 2 e^- ⇌ Zn

(a) -3.59 V (b) -0.943 V (c) 0.943 V (d) 1.12 V (e) 3.59 V

Answer: (c)

12-52. What is the cell potential in a voltaic cell containing the Cu²⁺|Cu and Zn²⁺|Zn half-reactions if [Zn²⁺] = 0.500 M and [Cu²⁺] = 0.0100 M?

(a) 1.00 V (b) 1.05 V (c) 1.10 V (d) 1.15 V (e) 1.20 V

Answer: (b)

12-53 What happens to the concentration of the Zn²⁺ ion in the voltaic cell in question 52 with time?

(a) The concentration of Zn²⁺ increases.

(b) The concentration of Zn²⁺ decreases.

Answer: (a)

12-54. Which of the following statements about the voltaic cell in question 52 is correct? (a) Cu metal is produced at the cathode.

(b) Cu metal is produced at the anode.

(d) Zn metal is produced at the anode.

(e) no metal is produced at either the cathode or the anode.

Answer: (a)

12-55. In the voltaic cell described in question 52, electrons flow:

(a) from the Zn electrode to the Cu electrode.

(b) from the Cu electrode to the Zn electrode.

Answer: (a)

12-56. What is the potential of the following cell?

Zn|Zn²⁺ (1 M)||Sn²⁺ (1 M)|Sn

(a) -0.90 V (b) -0.63 V (c) 0.63 V (d) 0.90 V

Answer: (c)

12-57. What is the potential of the following cell?

Pt|Sn²⁺ (1 M), Sn⁴⁺ (1 M)||I₂ (1 M), I^- (1 M)|Pt

(a) -0.69 V (b) -0.39 V (c) 0.39 V (d) 0.69 V

Answer: (c)

12-58. What is the potential of the following cell?

Mn|Mn²⁺ (0.00010 M)||Zn²⁺ (1.5 M)|Zn

(a) 0.15 (b) 0.28 V (c) 0.40 V (d) 1.80 V (e) none of the above

Answer: (c)

12-59. What is the potential for the following cell?

Ag|Ag+ (0.0010 M)||Ag⁺(0.10 M)|Ag

(a) -0.12 V (b) 0.12 V (c) 0.68 V (d) 0.92 V

Answer: (b)

12-60. What is the standard cell potential for the following redox reaction?

2 Fe³⁺(aq) + 2 I^-(aq) ➝ 2 Fe²⁺(aq) + I₂(aq)

(a) -1.30 V (b) -0.24 V (c) 0.24 V (d) 1.30 V (e) none of the above

Answer: (c)

12-61. What is the magnitude of the standard-state cell potential for the following redox reaction?

2 Al(s) + 3 Pb²⁺(aq) ➝ 2 Al³⁺(aq) + 3 Pb(s)

(a) 1.58 V (b) 1.83 V (c) 3.03 V (d) 3.790 V (e) 4.866 V

Answer: (a)

Equilibrium Constants and Cell Potentials

12-62. The equilibrium constant for the following reaction is 1.2 x 10⁵.

H₂(g) + Sn⁴⁺(aq) ➝ 2 H⁺(aq) + Sn²⁺(aq)

What is the value of E° for the reaction?

(a) -0.15 V (b) 0.15 V (c) 0.30 V (d) 0.35 V

Answer: (b)

12-63. What is the equilibrium constant at 25°C for the reaction:

Zn(s) + Cu²⁺(aq) ➝ Zn²⁺(aq) + Cu(s)

if the value of E° is 1.10 V?

(a) 1.9 (b) 37 (c) 1.6 x 10³⁷ (d) 1.6 x 10^-37 (e) -37

Answer: (c)

12-64. Calculate the value of the K_sp for CdS from the following data.

CdS + 2 e^- ⇌ Cd + S^2- E° = -1.21 V

Cd²⁺ + 2 e^- ⇌ Cd E° = -0.40 V

(a) 3 x 10^-55 (b) 4 x 10^-28 (c) 2 x 10^-14 (d) 3 x 10²⁷

Answer: (b)

12-65. There are two half-reactions for the reduction of O₂ to H₂O.

O₂ + 4 H⁺ + 4 e^- ⇌ 2 H₂O E° = 1.229 V

O₂ + 2 H₂O + 4 e^- ⇌ 4 OH^- E° = 0.401V

Use these half-reactions to calculate the ionization constant for water, K_w.

H₂O(l) ⇌ H⁺(aq) + OH^-(aq) K_w = ?

Answer: 1 x 10^-14

12-66. The half-reaction reduction potentials for the mercury(I) and silver(I) ions are given below.

Hg₂²⁺ + 2 e^- ⇌ 2 Hg E° = 0.7961 V

Ag⁺ + e^- ⇌ Ag E° = 0.7996V

Calculate the equilibrium constant for the following reaction under standard-state conditions:

Hg₂²⁺(aq) + 2 Ag(s) ⇌ 2 Hg(l) + 2 Ag⁺(aq)

Answer: 0.761

12-67. Calculate the complex dissociation equilibrium constant (K_d) for the Cd(NH₃)₄²⁺ complex from the following data.

Cd(NH₃)₄²⁺ + 2 e^- ⇌ Cd + 4 NH₃E° = -0.597 V

Cd²⁺ + 2 e^- ⇌ Cd E° = -0.403 V

(a) 2.7 x 10^-7 (b) 0.19 (c) 6.6 (d) 3.6 x 10⁶

(e) none of the above

Answer: (a)

Electrolytic Cells

12-68. Which of the following describes what happens when an aqueous solution of MgCl₂ is electrolyzed?

(a) Mg metal forms at the cathode.

(b) Mg²⁺ ions are oxidized at the cathode.

(d) Cl^- ions flow toward the cathode.

(e) H₂ gas is formed at the cathode.

Answer: (e)

12-69. Which of the following is the correct half-cell reaction for the anode process in the electrolysis of an aqueous solution of potassium sulfate?

(a) SO₄^2- ➝ SO₂(g) + O₂(g) + 2 e^-

(b) H₂O ➝ 1/2 O₂(g) + 2 H⁺ + 2 e^-

(d) SO₄^2- + 4 H⁺ + 2 e^- ➝ SO₂ + 2 H₂O

(e) 3 H₂O + e^- ➝ 1/2 H₂(g) + 3 OH^- + 2 H⁺

Answer: (b)

12-70. Which of the following statements about the electrolysis of an aqueous solution of NaCl is true?

(a) O₂ is liberated at the cathode.

(b) H₂ is liberated at the cathode.

(d) Cl₂ is liberated at the cathode.

(e) the decomposition potential must not be exceeded.

Answer: (b)

12-71. In the electrolysis of NaBr(aq), which of the following products is least likely to be found?

(a) H₂(g) (b) O₂(g) (c) Br₂(l) (d) Na(s) (e) NaOH(aq)

Answer: (d)

Electrolysis Reactions

12-72. Which of the following compounds would give the largest weight of metal if a 10.0-amp current is passed through molten samples of these salts for 2.0-hours?

(a) KCl (AW: K = 39.08 g/mol, Cl = 35.45 g/mol)

(b) CaCl₂(AW: Ca = 40.08 g/mol, Cl = 35.45 g/mol)

(d) TiCl₄ (AW: Ti = 47.90 g/mol, Cl = 35.45 g/mol)

(e) the same weight of metal would be obtained from the electrolysis of each of these salts.

Answer: (a)

12-73. Calculate the weight of sodium metal that would be produced by the electrolysis of molten sodium chloride for 1.00 hour with a 10.0-amp current.

Answer: 8.58 g

12-74. What is the ratio of the weight of Cl₂ produced at the anode to the weight of Al produced at the cathode when 5 Faraday's of electric charge are passed through a molten sample of AlCl₃?

(AW: Al = 27.0, Cl = 35.5 amu).

(a) about 2 g Cl₂ per 1 g Al. (b) about 4 g Cl₂ per 1 g Al.

(e) none of the above.

Answer: (b)

12-75. Calculate the amount of aluminum produced in 1.00 hour by the electrolysis of molten AlCl₃ if the current is 10.0 amperes.

Answer: 3.35 g

12-76. Suppose a beer can weighs 40.0 g. For how long would a current of 100.0 amp need to be passed through a molten AlF₃ electrolysis cell to produce enough Al to replace a discarded beer can? Sketch the electrolysis cell, labeling the electrodes and showing the direction of electron flow in the external circuit as part of your answer.

Answer: 1.2 hours

12-77. Which of the following electrolysis processes will produce the largest volume of Cl₂ gas at STP?

(a) passing 35 amps for 20 hours through an aqueous Na₂SO₄ solution.

(b) passing 10 amps for 2.69 hours through a 1 M NaCl solution.

(d) passing 1.5 Faradays through a 5 M CaCl₂ solution.

(e) passing 1.4 Faradays through a 5 M AlCl₃ solution.

Answer: (c)

12-78. A molten sample of TiCl₄ was electrolyzed for 10 hours at 12 amps. What is the ratio of the weight of Cl₂ produced compared to that of Ti? (A.W.: Ti = 47.90 g/mol; Cl = 35.45 g/mol)

(a) 0.34 g Cl₂/1.00 g Ti (b) 0.68 g Cl₂/1.00 g Ti

(e) 2.96 g Cl₂/1.00 g Ti

Answer: (e)

12-79. A current of 1.5 amps is applied to a 1.00 L solution of 0.100 M hydrochloric acid for 1 hour. What is the pH of the solution after electrolysis is complete?

Answer: 1.4

12-80. An electric current is passed through a solution of CuSO₄(aq) producing Cu(s) at the cathode and O₂(g) at the anode. If 3.48 L of O₂(g) measured at STP is produced at the anode, how many grams of Cu(s) must have been deposited on the cathode? (AW: Cu = 63.55; S = 32.06, 0 = 16.00 amu; 1 F = 96,500 C; R = 0.08206 L-atm/mol-K)

(a) 3.48 g (b) 4.93 g (c) 9.87 g (d) 19.7 g (e) none of these

Answer: (d)

12-81. What is the ratio by weight of Br₂ to Cr if a molten sample of CrBr₂ is electrolyzed for 4 hours at 10 amps? (AW: Cr = 52.0, Br = 79.9 amu)

(a) 1.05 g Br₂/1 g Cr (b) 1.54 g Br₂/1 g Cr

(e) 4.61 g Br₂/1 g Cr

Answer: (d)

12-82. How much Cl₂ gas would be collected when a 2.0 M NaCl(aq) solution is electrolyzed for 2.0 hours with a current of 15 amps? (MW: Cl₂ = 70.9 g/mol)

(a) 0.0220 g (b) 39.7 g (c) 79.4 g (d) 159 g (e) none of the above

Answer: (b)

12-83. A current of 10.0 amp over a period of 3 hours is passed through a solution of molten KCl. What weight of K metal is produced? (A.W.: K = 39.1 am)

(a) 0.365 g (b) 0.729 g (c) 21.9 g (d) 43.8 g (e) 87.5 g

Answer: (d)

12-84. A current of 2.0 amp passed through a molten solution of the salt MCl₂ for a period of 30 minutes produces 1.32 g of Cl₂ gas and 0.453 g of M. Which of the following metals is present in this compound?

(a) Mg (AW: 24.3 amu) (b) Ca (AW: 40.1 am)

(e) Hg (AW: 200.6 amu)

Answer: (a)

12-85. What is the oxidation state of the osmium atom in an unknown salt if 26.7 grams of osmium plate out when a current of 15 amps is passed through a solution of this salt for 1 hour? (Os: AW = 190.2 amu)

(a) Os⁺ (b) Os²⁺ (c) Os³⁺ (d) Os⁴⁺ (e) Os⁵⁺

Answer: (d)

12-86. What is the oxidation state of the cerium atom in Ce_xZ_y if a current of 1.2 amps for 3.0 hours deposits 4.70 g of Ce at the cathode? (AW: Ce = 140.1 amu)

(a) 0 (b) 1 (c) 2 (d) 3 (e) 4

Answer: (e)

12-87. What is the oxidation state of the tin atom in an unknown salt if 14.8 grams of tin are plated out when a current of 40 amps is passed through a molten solution of this salt for 10 minutes? (AW: Sn =118.7 amu)

(a) +1 (b) +2 (c) +3 (d) +4 (e) +6

Answer: (b)

Relative Strength of Oxidizing/Reducing Agents

12-88. Which of the following is the strongest reducing agent?

(a) H⁺ (b) H₂ (c) H^- (d) H₂O (e) O₂

Answer: (c)

12-89. Which of the following is the strongest oxidizing agent?

(a) H₂O₂ in base (b) H₂O₂ in acid (c) O₂ in acid

(d) CrO₄^2- in acid (e) Br₂

Answer: (b)

12-90. Based on demonstrations you have seen in class, which of the following is the strongest reducing agent?

(a) KMnO₄ (b) Zn (c) Cu (d) O₂ (e) Na

Answer: (e)

12-91. Which is the strongest oxidizing agent among the following?

(a) Cl₂ gas at 1 atm pressure.

(b) O₂ gas at 1 atm pressure in contact with 1 M acid

(d) Ag metal.

(e) H₂O₂ in acid solution.

Answer: (c)

For questions 92-93, use the following information.

Mg²⁺+ 2 e^- ⇌ Mg E^°= -2.37 V

Cr²⁺+ 2 e^- ⇌ Cr E^°= -0.91 V

2 H⁺+ 2 e^- ⇌ H₂ E° = 0.00 V

Pd²⁺+ 2 e^- ⇌ Pd E^◦= 0.987 V

Ce⁴⁺+ e^- ⇌ Ce³⁺ E° = 1.61 V

12-92. Which is the strongest oxidizing agent?

(a) Ce⁴⁺ (b) Ce³⁺ (c) H⁺ (d) Cr²⁺ (e) Mg

Answer: (a)

12-93. Which of the following reagents should react with H⁺to produce H₂?

(a) Both Mg²⁺ and Cr²⁺ (b) Both Pd and Cr²⁺ (c) Only Pd²⁺

(d) Both Mg and Cr (e) Mg, Cr, and Pd

Answer: (d)

12-94. Use the table of electrode potentials to determine which of the following reactions isn’t spontaneous.

(a) Zn(s) + 2 H⁺(aq) ➝ Zn²⁺(aq) + H₂(g)

(b) 2 Ag(s) + Zn²⁺(aq) ➝ 2 Ag⁺(aq) + Zn(s)

(d) 2 Al(s) + 3/2 O₂(g) + 6 H⁺(aq) ➝ 2 Al³⁺(aq) + 3 H₂O(l)

(e) Mg(s) + Cl₂(g) ➝ MgCl₂(s)

Answer: (b)

12-95. Which of the following pairs of ions can't coexist in aqueous solution under standard-state conditions because a spontaneous redox reaction should take place?

(a) Sn⁴⁺ and Fe³⁺ (b) Sn⁴⁺ and Fe²⁺ (c) Sn²⁺ and Fe³⁺

(d) Sn²⁺and Fe²⁺ (e) None of these pairs of ions can coexist

Answer: (c)

12-96. Which of the following pairs of ions cannot coexist in solution because a spontaneous redox reaction should take place?

(a) Cu²⁺ and Zn²⁺ (b) Fe³⁺ and Hg₂²⁺ (c) Cu²⁺and I^- (d) Al³⁺and S^2-

(e) All of these pairs of ions can coexist

Answer: (b)