Section 10.7: Electrolysis
Mini Investigation: Pencil Electrolysis, page 666
A. The anode of this cell is one pencil. The cathode of this cell is the other pencil. The graphite
in the pencils is the inert material acting as the electrodes of the electrolytic cell.
B. While the pencils were connected, water in the potassium idodide solution was reduced at the
cathode. When phenolphthalein indicator was added to the petri dish, the solution turned pink.
Bubbles appeared in the potassium idodide solution. The end of the anode pencil turned slightly
brown.
C. Answers may vary. Sample answer: I predict that at the cathode end of the electrolytic cell,
hydroxide ions, OH–(aq), were produced as indicated by the phenolphthalein. The bubbles
indicated the production of hydrogen gas, H2(g). At the anode end, the brown colouring of the
pencil wood indicated the production of iodine, I2(s). This occurs because the iodine ions, I–(aq),
are a stronger reducing agent than water. The reaction equations are given below:
2 I – (aq) ! I 2 (s) + 2e –
2 H 2O(l) + 2e – ! 2 OH – (aq) + H 2 (g)
2 I – (aq) + 2 H 2O(l) ! I 2 (s) + 2OH – (aq) + H 2 (g)
2 H2O(l)
Section 10.7 Questions, page 670
1. Answers may vary. Sample answer:
Galvanic cell
Type of reaction
–spontaneous redox reaction
Electrolytic cell
–non-spontaneous redox
reaction
Energy flow
–energy flows from one
–energy flows into the cell
electrode to the other, driven
from an outside source, then
from one electrode to the other
by the reduction potential of
each metal
–electrons flow from the
–electrons flow from the
electrode that is the weaker
electrode that is the stronger
reducing agent to the electrode
reducing agent to the electrode that is the stronger reducing
agent
that is the weaker reducing
agent
2. Answers may vary. Sample answer: A car battery is an example of a galvanic cell that
generates and distributes electrical energy for engine ignition and electronics. The battery is then
charged by the car’s alternator as part of an electrolytic system or cell.
3. (a) Cathode half-reaction equation: Ni2+(aq) + 2 e– ! Ni(s)
Anode half-reaction equation: 2 Br–(aq) ! Br2(s) + 2 e–
(b) Cathode half-reaction equation: Al3+(aq) + 3 e– ! Al(s)
Anode half-reaction equation: 6 F–(aq) ! 3 F2(s) + 6 e–
(c) Cathode half-reaction equation: Mn2+(aq) + 2 e– ! Mn(s)
Anode half-reaction equation: 2 I–(aq) ! I2(s) + 2 e–
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Chapter 10: Electrochemical Cells
10.7-1
4. Aluminum recycling can be good for both business and the environment because the
production of aluminum from raw materials is an energy-intensive and costly process.
Aluminum production generates greenhouse gas emissions such as sulfur dioxide, SO2(g), and
carbon dioxide, CO2(g), and toxic chemicals such as perfluorocarbons (PFCs), polycyclic
aromatic hydrocarbon (PAH), and fluoride, F–(aq). Recycling is less expensive than raw
production of aluminum, which saves businesses money. Recycling is less energy intensive and
produces fewer greenhouse gases and toxic compounds, which is good for the environment.
5. Diagrams may vary. Sample diagram:
6. Diagrams may vary. Sample diagram:
7. Anode half-reaction equation:
2 Cl–(aq) ! Cl2 (g)+ 2 e–
Cathode half-reaction equation:
Sn2+(aq) + 2 e– ! Sn(s)
Net ionic reaction equation: Sn2+(aq) + 2 Cl–(aq) ! Cl2(g) + Sn(s)
8. Anode half-reaction equation:
2 H2O(l) ! O2(g) + 4H+(aq) + 4 e–
Cathode half-reaction equation: 2 Cu2+(aq) + 4 e– ! 2 Cu(s)
At the anode, water is electrolyzed to produce hydrogen ions. At the cathode, copper ions are
reduced and plated to the cathode to increase its mass.
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Chapter 10: Electrochemical Cells
10.7-2
9. (a) Cathode half-reaction equation:
AlF63–(l) + 3 e– ! Al(s) + 6 F–(l)
2–
Anode half-reaction equation: 2 Al2OF6 (l) + 12 F–(l) + C(s) ! + 4 AlF63–(l) + CO2(g) + 4 e–
Overall cell reaction equation: 6 Al2OF62–(l) + 12 F–(l) + 3 C(s) !
8 AlF63– (l) + 4 Al(s) + 3 CO2(g)
(b) Aluminum oxide-cryolite anion reaction equation:
Al2O3(s) + 4 AlF63–(l) ! 3 Al2OF62–(l) + 6 F–(l)
Cathode half-reaction equation:
AlF63–(aq) + 3 e– ! Al(s) + 6 F–(aq)
2–
Anode half-reaction equation: 2 Al2OF6 (l) + 12 F–(l) + C(s) ! + 4 AlF63–(l) + CO2(g) + 4 e–
Overall reaction equation for aluminum: 2 Al2O3(s) + 3 C(s) ! 4 Al(s) + 3 CO2(g)
10. Answers may vary. Sample answer: Silver tarnish is the result of a chemical reaction
between silver and sulfur-containing substances in the air that produces silver sulfide, Ag2S(s).
To remove tarnish from silver objects, follow these steps: obtain a pan large enough to contain
the object so that the object can be immersed completely in water. Cover the bottom of the pan
with aluminum foil. Boil enough water to cover the object. Add baking soda, NaHCO3(s), to the
boiling water (250 mL of baking soda to 1 L of water). Pour the baking soda solution over the
object, immersing it completely. The tarnish will be removed immediately if the object is lightly
tarnished. Several attempts may be needed for heavily tarnished objects.
The baking soda solution acts as an electrolyte and allows electrons to flow through the solution.
Since aluminum is a stronger reducing agent, electrons flow from the aluminum and reduce the
silver sulfide. The chemical reaction for the removal of tarnish is
3 Ag2S(s) + 2 Al(s) ! 6 Ag(s) + Al2S3(s)
11. Answers may vary. Sample answer: The electroplating industry uses toxic heavy metals and
ions like lead, cadmium, chromium, and cyanide. When these metals are released into the
environment in waste water or in the air, they affect a wide variety of metabolic functions in
organisms. Heavy metals like cadmium and lead can displace other metal ions like magnesium
and zinc, which are beneficial metals that play structural and functional roles in enzymes. Heavy
metals alter or destroy the functions of the good metals leading to metabolic failure. Heavy
metals can also cross-link DNA structures, which may lead to increased mutation rates or altered
gene expression.
Electroplating companies attempt to remove the heavy metals and toxic ions by making them
insoluble, and then coagulate, flocculate, and clarify them by sedimentation.
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Chapter 10: Electrochemical Cells
10.7-3