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266 CHEMISTRY FOR THE UTTERLY CONFUSED
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Our goal in this chapter is to help you understand how to balance redox equations,
know the different types of electrochemical cells, and how to solve
electrolysis problems. Have your textbook handy—you may need to find some
information in electrochemical tables. We will be using the mole concept, so if
you need some review refer to Chapter 3, especially the mass/mole relationships.
You might also need to review the section concerning net-ionic equations in
Chapter 4. And don’t forget to Practice, Practice, Practice.
18-1 Redox Reactions
Electrochemical reactions involve redox reactions. Redox is a term that stands
for reduction and oxidation. Reduction is the gain of electrons and oxidation is
the loss of electrons. For example, if you place a piece of zinc metal in a solution
containing the Cu 2 ion. A reddish solid forms on the surface of the zinc
metal. That substance is copper metal. At the molecular level, the zinc metal is
losing electrons to form the Zn 2 cation and the Cu 2 ion is gaining electrons to
form copper metal. These two processes (called half-reactions) are:
Zn(s) l Zn 2 (aq) 2e (oxidation)
Cu 2 (aq) 2e l Cu(s) (reduction)
The electrons lost by the zinc metal are the same electrons gained by the
copper(II) cation. The zinc metal is oxidized (loses electrons and increases
its oxidation number) and the copper(II) ion is reduced (gains electrons and
decreases its oxidation number).
The reactant causing the oxidation to take place is the oxidizing agent (the reactant
undergoing reduction). In the example above, the oxidizing agent is the
Cu 2 ion. The reactant undergoing oxidation is the reducing agent because it
furnishes the electrons that are necessary for the reduction half-reaction. Zinc
metal is the reducing agent. The two half-reactions, oxidation and reduction,
can be added together to give you the overall redox reaction. When doing this,
the electrons must cancel—that is, there must be the same number of electrons
lost as electrons gained:
Zn Cu2 (aq) 2e l Zn2 (aq) 2e Cu or
Zn Cu2 (aq) l Zn2 (aq) Cu