placed in a solution containing the blue Cu^2 +cation. Very quickly a reddish solid forms on
the surface of the zinc metal. That substance is copper metal. As the copper metal is
deposited, the blue color of the solution begins to fade. At the molecular level, the more
active zinc metal is losing electrons to form the Zn^2 +cation, and the Cu^2 +ion is gaining
electrons to form the less active copper metal. These two processes can be shown as:The electrons that are being lost by the zinc metal are the same electrons that are being
gained by the copper(II) ion. The zinc metal is being oxidized and the copper(II) ion is
being reduced. Further discussions on why reactions such as these occur can be found in
the section on single-displacement reactions later in this chapter.
Something must cause the oxidation (taking the electrons) and that substance is called
the oxidizing agent(the reactant being reduced). In the example above, the oxidizing agent
is the Cu^2 +ion. The reactant undergoing oxidation is called the reducing agentbecause it
is furnishing the electrons that are being used in the reduction half-reaction. Zinc metal is
the reducing agent above. 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:On the AP exam, you might be asked to identify what is being oxidized and reduced
or to identify the oxidizing and reducing agents. (Be careful.)
In these redox reactions there is a simultaneous loss and gain of electrons. In the oxida-
tion reaction (commonly called a half-reaction) electrons are being lost, but in the reduc-
tion half-reaction those very same electrons are being gained. So, in redox reactions
electrons are being exchanged as reactants are being converted into products. This electron
exchange may be direct, as when copper metal plates out on a piece of zinc, or it may be
indirect, as in an electrochemical cell (battery).
Another way to determine what is being oxidized and what is being reduced is by look-
ing at the change in oxidation numbers of the reactant species. (See the Basics chapter for
a discussion of oxidation numbers and how to calculate them.) On the AP exam you may
be asked to assign oxidation numbers and/or identify changes in terms of oxidation num-
bers. Oxidation is indicated by an increase in oxidation number. In the example above, the
Zn metal went from an oxidation state of zero to +2. Reduction is indicated by a decrease
in oxidation number. Cu^2 +went from an oxidation state of +2 to zero. In order to figure
out whether a particular reaction is a redox reaction, write the net ionic equation. Then
determine the oxidation numbers of each element in the reaction. If one or more elements
have changed oxidation number, it is a redox reaction.
There are several types of redox reaction that are given specific names. In the next few
pages we will examine some of these types of redox reaction.Combination Reactions
Combination reactionsare reactions in which two or more reactants (elements or com-
pounds) combine to form one product. Although these reactions may be of a number of
different types, some types are definitely redox reactions. These include reactions of metalsor Zn s()+→+Cu^22 ++()aq Zn ()aq Cu s()Zn s()++→++Cu^22 +−+−()aq 22 e Zn ()aq e Cu s()Zn s Zn aq e oxidation
C()→+^2 +−( ) 2 ( )
uuaq e Cus^2 +−()+→ 2 () (reduction)72 STEP 4. Review the Knowledge You Need to Score High