Electrochemistry Review – Answer Key
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1. Balance the following in basic solution. MnO4- + C2O42- → MnO2 + CO2 (basic)
( MnO4- + 4H+ + 3e- → MnO2 + 2H2O ) x 2
( C2O42- → 2CO2 + 2e-) x 3
Acid: 2MnO4- + 3C2O42- + 8H+ → 2MnO2 + 6CO2 + 4H2O
Water: 8 H2 O → 8 H+ + 8 OH-
Basic: 2MnO4- 2-
+ 3C2O4 + 4H2O → 2MnO2 + 6CO2 + 8OH-
2. Consider the electrolysis of 1.0 M H2SO4 using platinum electrodes.
a) Write the oxidation half-reaction H2O → 1/2O2 + 2H+ + 2e- Eo = -0.82 V
b) Write the reduction half-reaction SO42- + 4H+ + 2e- → H2SO3 + H2O Eo = +0.17 V
c) Write the overall reaction and determine the minimum theoretical voltage required.
SO42- + 4H+ → 1/2O2 + 2H+ + H2SO3 Eo = -0.65 V MTV = +0.65 V
3. Consider the following diagram for the electrorefinning of lead.
Pure Pb Impure Pb
a) On the diagram, label the anode and cathode.
b) Write the formula for a suitable electrolyte Pb(NO3)2
c) Write the equation for the reduction half-reaction. Pb2+ + 2e- → Pb(s)
4. Describe two chemically different methods that can be used to prevent corrosion of iron and
explain why each method works.
Method 1: Painting
Explanation: Prevents reactants H2O and O2 from contacting the iron
Method 2: Attach some Zn
Explanation: Fe is forced to be the cathode and cannot oxidize
5. The data below were obtained in a redox titration of a 25.00 mL sample containing Sn2+ ions using
0.125 M KMnO4 according to the following reaction:
2MnO4- + 16H+ + 5Sn2+ → 2Mn2+ + 8H2O + 5Sn4+
Volume of KMnO4 used (mL)
Trial 1 Trial 2 Trial 3
Initial burette reading 2.00 13.80 24.55
Final burette reading 13.80 24.55 35.32
11.80 10.75 10.77 Average last two to get 10.76 mL
Calculate the [Sn ]
[Sn2+] = 0.01076 L x 0.125 mole x 5 moles Sn2+
1L 2 moles MnO4-
= 0.135 M
6. A student wanted to electroplate a coin with copper.
a) Identify a suitable anode Cu
b) Identify an appropriate electrolyte CuSO4
c) To which battery terminal (positive or negative) should the coin be connected? Negative
7. Consider the electrolysis of molten magnesium chloride.
a) Identify the product at the anode. Cu2+
b) Write the equation for the reduction half-reaction. Mg2+ + 2e- Mg(s)
c) Write the equation for the overall reaction. Mg2+ + 2Cl- Mg(s) + Cl2
8. Consider the following electrochemical cell.
1.0 M KNO3
1M Sn(NO3)2 1M Mn(NO3)2
a) Write the anode reaction Sn2+ + 2e- Sn
b) Write the cathode reaction Mn Mn 2+
c) Write the overall reaction and determine the initial cell potential (Eo).
Sn2+ + Mn Mn2+ + Sn
d) Explain in words how the electrons move through the cell. From anode Mn to cathode Sn
e) List all of the ions that migrate toward the Sn electrode. Sn2+ Mn2+ K+
f) Which electrode loses mass? Anode Sn
g) What is the purpose of the salt bridge?
To allow cations and anions to flow from one half cell to the other
h) What is the cell potential once equilibrium is achieved? 0.00 v