Oxidation-reduction (redox) reactions are among the most common and important chemical reactions. They are involved in a wide variety of important processes including the rusting of iron, the manufacture and action of bleaches, and the respiration of animals. As we discussed earlier, oxidation refers to the loss of electrons. Conversely, reduction refers to the gain of electrons. (Section 4.4) Thus, oxidation-reduction reactions occur when electrons are transferred from the atom that is oxidized to the atom that is reduced. When zinc metal is added to a strong acid (Figure 20.1), for example, electrons are transferred from zinc atoms (zinc is oxidized) to hydrogen ions (hydrogen is reduced):
Zn(s) + 2H+(aq) → Zn2+(aq) + H2(g) [20.1]
The transfer of electrons that occurs in the reaction in Figure 20.1 produces energy in the form of heat; the reaction is thermodynamically "downhill" and proceeds spontaneously. The transfer of electrons that occurs during oxidation-reduction reactions can also be used to produce energy in the form of electricity. In other instances we use electrical energy to make certain nonspontaneous chemical processes occur. Electrochemistry is the study of the relationships between electricity and chemical reactions. Our discussion of electrochemistry will provide insight into such diverse topics as the construction of batteries, the spontaneity of reactions, the corrosion of metals, and electroplating. We will begin by reviewing how oxidation numbers are used to identify redox reactions.