Chemistry 1312, Exam 4
Spring, 1997

I. Multiple Choice (84 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. Oxidation is often associated with each of the following except
a. anode
b. increase in oxidation state
c. corrosion
d. loss of electrons
e. free energy

2. In the dry cell
a. MnO₂ is the reducing agent
b. zinc is the cathode
c. electrons flow from cathode to anode
d. NH₄⁺(aq) + OH^-(aq) = NH₃(aq) + H₂O(l)
e. electrons flow from graphite to zinc

3. For the galvanic cell, Fe|Fe²⁺(aq)||Ag⁺(aq)|Ag
a. ferrous ion is reduced to iron
b. silver metal is the cathode
c. electrons flow from silver metal to iron metal
d. silver ion is oxidized to silver
e. ferrous ion is the anode

4. In the electrolysis of an aqueous solution of sodium chloride (in an electrolytic cell), the most probable reaction occurring at the cathode is
a. Na⁺(aq) + e^- = Na(l)
b. Cl₂(g) + 2 e^-= 2 Cl^-(aq)
c. 2 H₂O(l) + 2 e^- = H₂(g) + 2 OH^-(aq)
d. 2 Cl^-(aq) = Cl₂(g) + 2 e^-
e. 2 H₂O(l) = O₂(g) + 4 H⁺(aq) + 4 e^-

5. At the 17th Fred M. Garland Memorial Lecture, Dr. Mario Molina of MIT spoke on the current status of the ozone layer in the stratosphere. I hope you found the talk worthwhile. "My" basic intent for your attendance was to experience the presentation of a Nobel Laureate, so the following question is designed to determine if you attended.
At the beginning of the lecture a number of similar black and white photos were shown . . . what was the major subject content of the photos?
a. weather balloons
b. refrigerators
c. former colleagues of Dr. Molina
d. sewing machines
e. automobiles

6. In the comparison of a galvanic cell with an electrolytic cell, which statement is false?
a. For both a galvanic cell and an electrolytic cell, electrons flow from the anode to the cathode.
b. In a galvanic cell oxidation occurs at the anode, but in an electrolytic cell oxidation occurs at the cathode.
c. In both a galvanic cell and an electrolytic cell, cations migrate toward the cathode and anions toward the anode.
d. In both a galvanic cell and an electrolytic cell, electrons flow externally to the cell and ions flow internally in the cell.
e. In a galvanic cell the anode is assigned the negative electrode, but in an electrolytic cell the anode is assigned the positive electrode.

7. In the discharge of the lead-acid (automobile) battery, all of the following are true except
a. PbO₂ is the reducing agent
b. the anode and cathode are alloys of lead
c. at the anode: Pb(s) + H₂SO₄(aq) = PbSO₄(s) + 2 H⁺(aq) + 2 e^-
d. the electrolyte is sulfuric acid
e. the density of the electrolyte decreases

8. The loss of metal through corrosion is a major industrial and domestic problem. Which statement regarding the study of corrosion is incorrect?
a. To protect underground steel pipelines and storage tanks by a redox reaction, the pipeline/storage tank is set up as the anode.
b. The oxidizing agent for the corrosion of iron to form rust is oxygen.
c. Galvanized steel is more resistant to corrosion than unprotected steel because the zinc coating has a lower reduction potential than does the iron of the steel.
d. Tin-coated steel is more resistant to corrosion than unprotected steel because the tin forms a metal layer through which the oxygen cannot penetrate, much like characteristics of paint or tar.
e. Passivated metals generally do not show extensive corrosion because of the formation of an impermeable metal oxide coating.

9. Which of the following is the strongest reducing agent?
a. Na⁺
b. Au⁺
c. Ag
d. Al
e. H₂O

10. A galvanic cell was constructed from the two half-cells, Mg²⁺/Mg and Fe³⁺/Fe²⁺. What is true about the cell?
a. magnesium metal is the anode
b. Mg²⁺ flow toward the cathode
c. E°_cell = 3.13 V
d. a nonreactive conductor is the cathode
e. all of the statements are true

11. Which ligand/name is not a match?
a. H₂O, aqua
b. Cl^-, chloro
c. CO, carbon monoxide
d. NH₃, ammine
e. SO₄^2-, sulfato

12. The complex and coordination number agree for each of the following except
a. [Fe(CN)₆]^4-, six
b. [Cu(H₂O)₄]²⁺, four
c. [Co(C₂O₄)₂(H₂O)₂]^-, six
d. [Ni(en)₂I₂]⁺, four
e. [Cr(NH₃)₂Cl₄]^-, six

13. All of the following relate to the commercial production of aluminum from its ore (bauxite)except
a. Al³⁺(aq) + 4 OH^-(aq) = Al(OH)₄^-(aq)
b. Fe³⁺(aq) + 3 OH^-(aq) = Fe(OH)₃(s)
c. 2 AlCl₃(l) = 2 Al(l) + 3 Cl₂(g)
d. Na₃AlF₆
e. 2 Al₂O₃(l) = 4 Al(l) + 3 O₂(g)

14. In the Dow Process for the production of magnesium,
a. magnesium is produced at the cathode
b. oxygen is produced at the anode
c. magnesium is oxidized at the cathode
d. chlorine is produced at the cathode
e. water is oxidized at the anode

Standard Reduction Potentials at 25°C

Au⁺ + e^- = Au, E° = 1.69 V
Cl₂ + 2 e^- = 2 Cl^-, E° = 1.36 V
O₂ + 4 H⁺ + 4 e^- = 2 H₂O, E° = 1.23 V
Ag⁺ + e^- = Ag, E° = 0.80 V
Fe³⁺ + e^- = Fe²⁺, E° = 0.77 V
2 H⁺ + 2 e^- = H₂, E° = 0.00 V
Pb²⁺ + 2 e^- = Pb, E° = –0.13 V
Sn²⁺ + 2 e^- = Sn, E° = –0.14 V
Ni²⁺ + 2 e^- = Ni, E° = –0.23 V
Zn²⁺ + 2 e^- = Zn, E° = –0.76 V
2 H₂O + 2 e^- = H₂ + 2 OH^-, E° = –0.83 V
Al³⁺ + 3 e^- = Al, E° = –1.66 V
Mg²⁺ + 2 e^-= Mg, E° = –2.36 V
Na⁺ + e^- = Na, E° = –2.71 V

II. Nomenclature. (14 points) Complete the following table.

III. Exercises. (67 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) How much time (in seconds) is required to electroplate 100 µg gold from a gold(III) chloride solution using a current of 13.0 milliamps of current?

11.3 seconds

2. (10 points) For the following cell, the measured cell potential, E_cell, is 0.13 V. What is the molar concentration of the nickel(II) ion in the cell?
Ni|Ni²⁺(? mol/L)||Pb²⁺(0.10 mol/L)|Pb

9.68 x 10^-3 mol/L

3. (42 points) Consider the galvanic cell,
Sn|Sn²⁺(0.010 M)||O₂(1 atm)|H⁺(pH = 3.0)|Pt
a. (3 pts) Write the cathodic half reaction.

O₂ + 4 H⁺ + 4 e^- = 2 H₂O

b. (3 pts) Write the anodic half reaction.

Sn = Sn²⁺ + 2 e^-  

c. (3 pts) Write the cell reaction.

O₂ + 4 H⁺ + 2 Sn = Sn²⁺ + 2 H₂O

d. (3 pts) Determine the standard cell potential for the cell.

1.37 V

e. (15 pts) Determine the actual cell potential for the cell as designed.

1.25 V

f. (5 pts) Determine the actual free energy change for the cell reaction.

–483 kJ

g. (10 pts) Calculate the equilibrium constant for the cell reaction.

K = 4.02 x 10⁹²

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