ANSWERS TO END-OF-CHAPTER QUESTIONS

Exploring Extensions
52. The aluminum–air battery is being explored for use in automobiles. In this battery, aluminum
metal undergoes oxidation to Al 3+ and forms Al(OH)3. Oxygen from the air undergoes
reduction to OH – ions.
a. Write equations for the oxidation and reduction half-reactions. Use H2O as needed to
balance the number of hydrogen atoms present, and add electrons as needed to balance the
charge.
b. Add the half-reactions to obtain the equation for the overall reaction in this cell.
c. Specify which half-reaction occurs at the anode and which occurs at the cathode in the
battery.
d. What are the benefits of the widespread use of the aluminum–air battery? What are
some of the limitations? Write a brief summary of your findings.
e. What is the current state of development of this battery? Is it in use in any vehicles at
the present time? What is its projected future use?
Answer:
a. Al + 3 H2O ⎯→ Al(OH)3 + 3 H + + 3 e – half-reaction of oxidation
O2 + 2 H2O + 4 e – ⎯→ 4 OH – half-reaction of reduction
b. To make the number of electrons the same, the first half-reaction must be multiplied by 4,
and the second half-reaction must be multiplied by 3. In this way, 12 moles of electrons are
exchanged in the overall reaction.
4 (Al + 3 H2O ⎯→ Al(OH)3 + 3 H + + 3 e – )
3 (O2 + 2 H2O + 4 e – ⎯→ 4 OH – )
The net equation is 4 Al + 3 O2 + 6 H2O ⎯→ 4 Al(OH)3.
c. The half-reaction of oxidation, Al + 3 H2O ⎯→ Al(OH)3 + 3 H + + 3 e – , occurs at the
anode.
The half-reaction of reduction, O2 + 2 H2O + 4 e – ⎯→ 4 OH – , occurs at the cathode.
d. Potential benefits are that aluminum is recyclable, non-combustible, and it is capable of
storing a lot of energy. An aluminum–air battery is quite light and uses air as a reactant. It is a
non-polluting source of power. Some of the limitations are that obtaining aluminum from its
ore is a very energy-intensive process. Even recycling aluminum in a large volume requires
considerable energy, potentially limiting the cost and availability of aluminum. In early
versions of the battery, the electrolyte degraded the aluminum even when the battery was not
in use. Recently, companies have overcome this problem.
e. Despite twenty-five years of research to develop the aluminum–air battery, its use has been
mostly in military applications and no commercial products use the technology. A French
company, Métalectrique (http://www.metalectrique.com), is developing aluminum–air
batteries for use in mobile refrigerators, electric cars, and electric wheelchairs.
53. An iron-based “superbattery” is a promising alternative for delivering more power with
fewer environmental effects than alkaline batteries. Find out how the superbattery is designed
and its state of commercial acceptance.
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