(^74) › STEP 4. Review the Knowledge You Need to Score High
ENRICHMENT
involved in redox reactions is the exchange of electrons from a more active species to a less
active one. You can predict the relative activities from a table of activities or a half-reaction table.
Chapter 16, Electrochemistry, goes into depth about electrochemistry and redox reactions.
The AP free-response booklet includes a table of half-reactions, which you may use for
help during this part of the exam. A similar table can be found in the back of this book.
Alternatively, you may wish to memorize the common oxidizing and reducing agents.
Redox is a term that stands for reduction and oxidation. Reduction is the gain of elec-
trons, and oxidation is the loss of electrons. For example, suppose a piece of zinc metal is
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:
Zn(s)Zn(aq)2e(oxidation)
Cu (aq) 2e Cu(s)(reduction)
2
2
→+
+→
+−
+−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 agent because
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:Zn(s)C++u(^22 +−aq)2eZ→+n(+−aq)2eC+ u(s)or Zn(s)C+→u(^22 ++aq)Zn(aq)C+ u(s)In these redox reactions, there is a simultaneous loss and gain of electrons. In the
oxidation reaction (commonly called a half-reaction) electrons are being lost, but in the
reduction 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 Chapter 5, Basics, 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 numbers.
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 deter-
mine the oxidation numbers of each element in the reaction. If one or more elements have
changed oxidation number, it is a redox reaction.TIP