MCAT General Chemistry Review - Alexander Stone Macnow, MD 2019-2020
Electrochemistry
Answers to Concept Checks
· 12.1
1. In a galvanic cell, the anode is the site of oxidation, has current flowing toward it, and has a (—) designation. The cathode has electrons flowing toward it and attracts cations.
2. In an electrolytic cell, the anode is the site of oxidation and has current flowing toward it. The cathode has electrons flowing toward it, has a (—) designation, and attracts cations.
3. Pb (s) | H_{2}SO_{4} (4 M) || H_{2}SO_{4} (4 M) | PbO_{2} (s)
4. Electrolytic cells are nonspontaneous and have a positive ΔG. Galvanic cells are spontaneous and have a negative ΔG; therefore, they have a positive E_{cell}.
5.
6.
Battery |
State of Use |
Galvanic or Electrolytic |
Anode Material |
Anode Charge |
Cathode Material |
Cathode Charge |
Ni—Cd |
Discharging |
Galvanic |
Cd |
Negative |
NiO(OH) |
Positive |
Ni—Cd |
Charging |
Electrolytic |
Cd(OH)_{2} |
Positive |
Ni(OH)_{2} |
Negative |
Molten NaCl |
Discharging |
Electrolytic |
Any |
Positive |
Any |
Negative |
Daniell cell |
Discharging |
Galvanic |
Zn |
Negative |
Cu |
Positive |
Lead—acid |
Charging |
Electrolytic |
PbSO_{4} |
Positive |
PbSO_{4} |
Negative |
Lead—acid |
Discharging |
Galvanic |
Pb |
Negative |
PbO_{2} |
Positive |
· 12.2
1. A sample is measured by setting up a cell relative to a standard hydrogen electrode, which is given a reduction potential of 0 V by convention.
2. A positive emf means the cell is spontaneous (galvanic); a negative emf means the cell is nonspontaneous (electrolytic).
3. The first cell is electrolytic because it has a negative emf. The second cell is galvanic because it has a positive emf.
4. The reduction potential of triiodide is higher than iron(III), so triiodide will be reduced and iron will be oxidized: 2 Fe + 3 I_{3}^{−} → 2 Fe^{3+} + 9 I^{−} (E^{°}_{cell} = +0.57 V)
· 12.3
1.
K_{eq} |
ΔG°: (+) or (−)? |
Reaction: Spontaneous or Nonspontaneous? |
E^{°}_{cell}: (+) or (−)? |
1.2 × 10^{—2} |
+ |
Nonspontaneous |
— |
2 × 10^{2} |
— |
Spontaneous |
+ |
1 |
0 |
Not applicable—applies to any cell at equilibrium |
0 |
2. Remember that ΔG° = —RT ln K_{eq}; if K_{eq} < 1, ln K_{eq} < 0, and ΔG° > 0. If K_{eq} > 1, ln K_{eq} > 0, and ΔG° < 0. If K_{eq} = 1, ln K_{eq} = 0, and ΔG° = 0.
3.
Q |
K_{eq} |
Reaction Direction (Forward, Backward, or Equilibrium) |
Sign of E_{cell} |
10^{−3} |
10^{−2} |
Forward |
+ |
10^{2} |
1.1 |
Backward |
— |
1 |
1 |
Equilibrium |
0 |
4. Note that these calculations do not assume standard conditions, unlike question 1.