Chemistry 1312, Exam 4
Spring, 1998

I. Multiple Choice (90 points) Select the best response for each of the following and record on the Scantron. Do not erase; erasures may result in a misgrading of your response.

1. Consider the following half-reactions and standard reduction potentials:
Br₂ + 2 e^- = 2 Br^- E° = +1.07 V
Sn⁴⁺ + 2 e^- = Sn²⁺ E° = +0.15 V
2 H⁺ + 2 e^- = H₂ E° = 0.00 V
Zn²⁺ + 2 e^- = Zn E° = – 0.76 V
Which substance is the strongest oxidizing agent?
a. Zn
b. H₂
c. H⁺
d. Sn⁴⁺
e. Br^-

2. For the voltaic cell consisting of the two redox couples:
Ce⁴⁺/Ce³⁺ E° = +1.61 V
Co²⁺/Co E° = – 0.28 V
Which statement is true?
a. oxidation of Ce4+ occurs at the anode
b. the standard cell potential is 1.33 V
c. C(gr) or Pt(s) can be used as the cathode
d. the cell reaction is: 2 Ce⁴⁺ + Co²⁺ = 2 Ce³⁺ + Co
e. Cobalt metal is reduced at the cathode

3. The anodic reaction in the dry cell is
a. NH₄⁺ + OH^- = NH₃ + H₂O
b. Zn²⁺ = Zn + 2 e^-
c. 2 MnO₂ + H₂O + 2 e^- = Mn₂O₃ + 2 OH^-
d. Zn2+ + 4 NH₃ = Zn(NH₃)₄²⁺
e. Zn = Zn²⁺ + 2 e^-

4. For the reaction occurring in an acidic solution, Mo + ReO₄^- = MoO₃ + ReO₂, which statement is true?
a. Molybdenum (VI) oxide is the oxidizing agent
b. ReO₄^- is the oxidizing agent
c. The decrease in the oxidation state of rhenium is 2
d. The reaction is not a redox reaction
e. Molybdenum metal is the oxidizing agent

5. In an electrochemical cell, cations flow from ______ and electrons flow from ______.
a. anode to cathode, anode to cathode
b. cathode to anode, anode to cathode
c. anode to cathode, cathode to anode
d. cathode to anode, cathode to anode
e. cathode to cathode, anode to anode

6. How many moles of electrons are required to produce one mole of titanium from molten TiCl₄?
a. one
b. 6.02 x 10²³
c. four
d. 96500
e. need to know the balanced half-reaction

7. Which of the following is the reducing agent for the production of magnesium by the Dow Process?
a. CaCO₃
b. Cl₂
c. HCl
d. OH^-
e. none of the above

8. For the electroplating silver onto a spoon (silverware) from an aqueous nitric acid solution of [Ag(CN)₂]^-
a. the silver ion is electroplated at the anode
b. the anodic reaction is Ag = Ag⁺ + e^-
c. the spoon should be the cathode
d. the cathode should be a nonreactive electrode
e. the nitrate ion is oxidized at the anode

9. Consider the voltaic cell at right.
Fe³⁺ + e^- = Fe²⁺ E° = +0.77 V
Ni²⁺ + 2 e^- = Ni E° = –0.25 V
What is true of the cell?
a. Electrons flow from the nickel electrode to the platinum electrode.
b. Chloride ions flow toward the nickel electrode
c. The platinum electrode is the cathode
d. Ni²⁺ ions flow toward the platinum electrode
e. All of the above are true

10. Again, consider the voltaic cell at (upper) right. Which statement is true?
a. The function of the "barrier" in the middle of the cell is to prevent electron flow
b. A piece of iron can be substituted for the nickel electrode with no consequent change in cell potential
c. The "barrier" in the middle of the cell allows cation and anion flow.
d. The "barrier" in the middle of the cell allows anion flow only.
e. A piece of platinum can be substituted for the nickel electrode with no consequent change in cell potential

11. Iron metal undergoes corrosion in a humid, salty environment. What is the cathodic reaction?
a. Fe²⁺ + 2 e^- = Fe
b. Fe = Fe²⁺ + 2 e^-
c. 2 Fe³⁺ + 3 O^2- = Fe₂O₃
d. O₂ + 4 H⁺ + 4 e^- = 2 H₂O
e. Fe³⁺ + e^- = Fe²⁺

12. For the electrolysis of molten calcium chloride in the electrolytic cell at right, which of the following statements is true?
a. Ca²⁺ is reduced at the platinum electrode
b. Cl^- migrates to the graphite electrode
c. Electrons flow from the platinum electrode to the graphite electrode
d. Chloride ions are reduced to chlorine gas
e. All of the above statements are true

13. In the discharge of the lead acid battery, the oxidizing agent is
a. H₂SO₄
b. "spongy" lead
c. PbO₂
d. PbSO₄
e. CaSO₄

14. For the commercial production of aluminum, which of the following statements is false?
a. Aluminum metal is reduced at the cathode from molten alumina.
b. Cryolite, Na₃AlF₆, is added to reduce the temperature of the molten alumina.
c. Water is oxidized at the carbon anodes to produce oxygen and hydronium ion.
d. Molten aluminum metal is more dense than molten alumina
e. The graphite anodes are "wasted" during the electrolysis of molten alumina.

15. To balance the half-reaction, MnO₄^- = MnO₂, in a basic solution,
a. 4 OH^-’s must appear on the left side of the balanced half-reaction
b. 4 H₂O’s must appear on the left side of the balanced half-reaction
c. 2 H₂O’s must appear on the right side of the balanced half-reaction
d. 4 OH^-’s must appear on the right side of the balanced half-reaction
e. 3 e^-’s must appear on the right side of the balanced half-reaction

II. Exercises. (68 points) All work must be shown clearly and legibly in order to receive credit. Answers, without a logical presentation, will receive only 10% credit.

1. (15 points) For the commercial production of aluminum metal from a molten Al₂O₃ bath by the Hall Process, aluminum metal is electroplated on the steel wall of the electrolytic cell.
a. (5 pts) Does the steel cell wall serve as the anode or cathode? Write the corresponding (balanced) half-cell reaction.

Cathode: Al³⁺ + 3 e^- = Al

b. (10 pts) How many minutes are required to electroplate 1700 g (3.74 lb) of aluminum using 1.0 x 10⁴ amperes? The molar mass of aluminum is 26.9815 g/mol.

30.4 minutes

2. (15 points) Write the two balanced half-reactions, followed by an overall balanced equation for the reaction occurring in an acidic solution.
TcO₄-(aq) + Ti(s) = Tc2+(aq) + Ti3+(aq)

Ti(s) = Ti³⁺(aq) + 3 e^-
TcO₄^-(aq) + 8 H⁺(s) + 5 e^- = Tc²⁺ + 4 H₂O
3 TcO₄^-(aq) + 5 Ti(s) + 24 H⁺ = 3 Tc²⁺(aq) + 5 Ti³⁺(aq) + 12 H₂O(l)

2. (38 points) Consider the use of the two half-reactions for constructing a voltaic cell, using 0.100 M Pb(NO₃)₂, 0.010 M CrCl₂, and 1.0 x 10^-4 M CrCl₃ as electrolytes.

Pb²⁺ + 2 e^- = Pb E° = – 0.13 V
Cr³⁺ + e^- = Cr²⁺ E° = – 0.41 V

Identify in the cell at right,
a. (3 pts) the anode and indicate the anode material
b. (3 pts) the cathode and indicate the cathode material
c. (2 pts) the direction of electron flow
d. (2 pts) the direction of flow of chloride ions
e. (5 pts) Write the balanced cell reaction.

2 Cr²⁺ + Pb²⁺ = Pb + 2 Cr³⁺

f. (3 pts) What is the standard cell potential?

0.28 V

g. (10 pts) Determine the actual potential under the existing conditions of the cell.

0.37 V

h. (5 pts) What is the actual energy that can be obtained with the existing conditions of the cell?

DG = -71.4 kJ

i. (5 pts) What is the equilibrium constant for the cell reaction?

K = 3.0 x 10⁹

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