5 Steps to a 5 AP Chemistry

Redox Reactions

Electrochemical reactions involve redox reactions. In the chapter on Reactions and

Periodicity we discussed redox reactions, but here is a brief review: 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, suppose a piece of zinc metal is placed in a solution

containing Cu^2 +. Very quickly, 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 Zn^2 +and Cu^2 +is gaining electrons to form copper metal. These two processes

can be shown as:

Zn(s) → Zn^2 +(aq) + 2e− (oxidation)

Cu^2 +(aq) + 2e−→ Cu(s) (reduction)

The electrons that are being lost by the zinc metal are the same electrons that are being

gained by the cupric ion. The zinc metal is being oxidized, and the cupric cation is being

reduced.

Something must cause the oxidation (taking of the electrons), and that substance is

called the oxidizing agent (the reactant being reduced). In the example above, the oxidizing

agent is Cu^2 +. The reactant undergoing oxidation is called the reducing agent, because it is

furnishing the electrons 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. The electrons must cancel—that is, there must be the same

number of electrons lost as electrons gained:

Zn(s) + Cu^2 +(aq) + 2e– → Zn^2 +(aq) + 2e−+ Cu(s) or

Zn(s)+Cu^2 + (aq) → Zn^2 +(aq) + Cu(s)

In these redox reactions, like the electrochemical reactions we will show you, 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 elec-

trons 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).

In this chapter, we will show you both processes and the calculations associated with each.

The balancing of redox reactions is beginning to appear on the AP exam, so we have

included the half-reaction method of balancing redox reactions in the Appendix, just in case

you are having trouble with the technique in your chemistry class.

The definitions for oxidation and reduction given above are the most common and the

most useful ones. A couple of others might also be useful: Oxidation is the gain of oxygen

or loss of hydrogen and involves an increase in oxidation number. Reduction is the gain of

hydrogen or loss of oxygen and involves a decrease in oxidation number.

Electrochemical Cells

In the example above, the electron transfer was direct, that is, the electrons were exchanged

directly from the zinc metal to the cupric ions. But such a direct electron transfer doesn’t

allow for any useful work to be done by the electrons. Therefore, in order to use these elec-

trons, indirect electron transfer must be done. The two half-reactions are physically sepa-

rated and connected by a wire. The electrons that are lost in the oxidation half-reaction are

allowed to flow through the wire to get to the reduction half-reaction. While those electrons

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