Chemistry 1312
Exam 4, Spring, 2001

I. Multiple Choice (102 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. The following redox couples are used to construct a voltaic cell:
2 Hg²⁺ + 2 e^- ® Hg₂²⁺    E° = +0.91 V
Fe²⁺ + 2e^- ® Fe                 E° = –0.44 V
Identify the substances that can be used as the cathode and anode respectively.
a. Hg and Fe
b. C(gr) and Fe
c. Fe and C(gr)
d. Hg²⁺ and Fe²⁺
e. C(gr) and Pt

2. Consider the voltaic cell, Ni|Ni²⁺(1 M)||Hg₂²⁺(1 M)|Hg. Which substance is considered the reducing agent in the cell reaction?
a. Ni
b. Ni²⁺
c. Hg
d. Hg₂²⁺
e. unknown, need standard reduction potential values

3. Consider the following redox couples and the corresponding standard reduction potentials:
Fe³⁺ + e^- ® Fe²⁺    E° = +0.77 V
Pb²⁺ + 2 e^- ® Pb    E° = –0.13 V
Cd²⁺ + 2 e^- ® Cd    E° = –0.31 V
Which of the following statements is true?
a. Pb is a stronger oxidizing agent than Cd
b. Fe³⁺ is a stronger reducing agent than Cd²⁺
c. Pb²⁺ is a stronger oxidizing agent than Fe³⁺
d. Pb²⁺ spontaneously reacts with Fe²⁺
e. Cd is a stronger reducing agent than Pb

4. For the cell reaction, 3 VO₂⁺ + 6 H⁺ + Al ® 3 VO²⁺ + Al³⁺ + 3 H₂O what is the value of n, the number of moles of electrons exchanged in the reaction?
a. 2
b. 3
c. 4
d. 6
e. 7

5. Which one of the following changes requires a reducing agent?
a. Au ® Au³⁺
b. H₂O ® O₂
c. NO₃^- ® NO
d. ClO₂^- ® ClO₂
e. Pb ® PbO₂

6. Which of the following italicized elements does not have the corresponding oxidation number?
a. MnO₂, 4+
b. K₂MnO₄, 6+
c. BaCO₃, 4+
d. IO₃^-, 1-
e. ClO₄^-, 7+

7. A reducing agent
i. causes oxidation of some other chemical species
ii. causes reduction of some other chemical species
iii. is itself oxidized
iv. is itself reduced
a. i only
b. ii only
c. i and iv
d. ii and iv
e. ii and iii

8. What is the reducing agent in the discharge of a lead-acid battery?
a. Pb
b. PbSO₄
c. PbO
d. H₂SO₄
e. PbO₂

9. What is the substance oxidized in the alkaline battery?
a. Pb
b. NH₄Cl
c. Zn
d. MnO₂
e. H₂O

10. Consider the voltaic cell, Cd|Cd²⁺||Sn⁴⁺, Sn²⁺|C(gr). Which of the following statements is false?
a. Electrons flow from the Cd electrode to the C(gr) electrode.
b. Cadmium metal is oxidized.
c. Sn²⁺ is reduced at the graphite electrode.
d. Sn²⁺ is produced at the graphite electrode.
e. The concentration of Cd²⁺ increases with time.

11. The standard cell voltage for the following cell is +0.61 V.
Tl|Tl⁺||Cl^-|Hg₂Cl₂|Hg
If the standard reduction potential for the Tl⁺/Tl redox couple is –0.34 V, what is the standard reduction potential of the Hg₂Cl₂/Cl^-/Hg redox couple?
a. +0.27 V
b. –0.27 V
c. –0.95 V
d. +0.95 V
e. +0.61 V

12. In the Dow Process for the production of magnesium metal, what chemical process occurs at the anode?
a. Mg ® Mg²⁺ + 2 e^-
b. 2 O^2- ® O₂ + 4 e^-
c. Mg²⁺ ® Mg + 2 e^-
d. 2 Cl^- ® Cl₂ + 2 e^-
e. Mg²⁺ + 2 e^- ® Mg

13. In comparing voltaic cells with electrolytic cells,
a. oxidation occurs at the anode in a voltaic cell, but at the cathode in an electrolytic cell.
b. electrons flow from anode to cathode in a voltaic cell, but cathode to anode in an electrolytic cell.
c. cations migrate to the anode in a voltaic cell, but toward the cathode in an electrolytic cell.
d. cations migrate to the positive electrode in a voltaic cell, but toward the negative electrode in an electrolytic cell.
e. reduction always occurs at the anode in both the voltaic cell and the electrolytic cell.

14. When copper is refined using the electrolysis technique, which statement below is true?
a. Impure copper is oxidized at the anode and pure copper is reduced at the cathode.
b. The impure copper is oxidized at the anode and the pure copper is produced in the "anode mud."
c. The impurities of impure copper are oxidized at the anode and subsequently reduced at the cathode.
d. The "anode mud" contains highly oxidized forms of gold and silver.
e. The electron flow (metallic conduction) is from the pure copper to the impure copper

15. For the cell reaction, 2 Ja + 3 Fe²⁺ ® 2 Ja³⁺ + 3 Fe, E°_cell = 0.99 V. (Ja is the symbol for the element Javelinium!) What is the DG° for the reaction?
a. –479 kJ
b. –573 kJ
c. –287 kJ
d. –95.8 kJ
e. –192 kJ

16. A comparison of the dry cell, the alkaline battery, and the mercury cell reveals that
a. all have the same electrolyte
b. all have the same oxidizing agent
c. all have the same color
d. all have the same potential
e. all have the same reducing agent

17. Consider the following redox couples and the corresponding standard reduction potentials:
S₂O₈^2- + 2 e^- ® 2 SO₄^2-          E° = +2.01 V
O₂ + 4 H⁺ + 4 e^- ® 2 H₂O       E° = +1.23 V
Cu²⁺ + 2 e^- ® Cu                     E° = +0.34 V
2 H₂O + 2 e^- ® H₂ + 2 OH^-    E° = –0.83 V
In the electrolysis of a cupric sulfate, CuSO₄, solution, which one of the following reaction occurs at the cathode?
a. S₂O₈^2- + 2 e^- ® 2 SO₄^2-
b. O₂ + 4 H⁺ + 4 e^- ® 2 H₂O
c. 2 H₂O + 2 e^- ® H₂ + 2 OH^-
d. Cu²⁺ + 2 e^- ® Cu
e. Cu + 2 e^- ® Cu^2-

II. Exercises (63 points). The organization of all exercises must be shown clearly and legibly in order to receive any partial credit. Answers without a logical problem setup will receive only 10% credit.

1. (39 points) Consider the following redox couples and the corresponding standard reduction potentials:
Au³⁺(0.50 M) + 3 e^- ® Au E° = +1.50 V
Cl₂(1 atm) + 2 e^- ® 2 Cl^-(0.060 M) E° = +1.36 V
a. (10 pts) Sketch the cell diagram.

C(gr)|Cl₂|Cl^-||Au³⁺|Au

b. (9 pts) Write the balanced cathodic, anodic, and cell reactions consecutively.

cathodic reaction Au³⁺ + 3 e^- ® Au

anodic reaction 2 Cl^- ® Cl₂ + 2 e^-

cell reaction 2 Au³⁺ + 6 Cl^- ® 2 Au + 3 Cl₂

c. (5 pts) What is the standard cell potential?

E° = 0.14 V

d. (10 pts) What is the actual cell potential?

E_cell = 0.06 V

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

K = 1.6 x 10¹⁴

(10 points) An electrical current of 700 A is passed through a CuSO₄ solution for 6.00 hours during the electrolytic refining of copper metal. What mass of copper could theoretically be produced during this time period?

4980 kg copper

(10 points) Electrolysis of a molten metal chloride (MCl₃) using a current of 6.50 A for 1397 seconds deposits 1.41 g of the metal at the cathode. What is the name of the metal?

scandium

(4 points) Balance the following half-reaction occurring in an acidic (aqueous) solution:

ClO₄^- ® Cl₂

14 e^- + 16 H⁺ + 2 ClO₄^- ® Cl₂ + 8 H₂O

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