OXIDATION-REDUCTION REACTION

also called Redox Reaction, any chemical process in which the oxidation number (q.v.) of a participating chemical species changes. The term oxidation originally was applied to reactions in which an element combines with oxygen, as in the combustion of carbon, C + O2 CO2; the term reduction designated a process by which an element dissociates from oxygen, as in the reaction of copper(II) oxide with hydrogen, CuO + H2 Cu + H2O. Oxidation-reduction reactions are now considered to be those involving the transfer of electrons from one chemical species to another. Such reactions are usually coupled. Whenever an oxidation reaction occurs, a reduction reaction occurs simultaneously and in an equivalent amount; i.e., the number of electrons lost during oxidation is equal to the number gained during reduction. In the process that describes the rusting of iron, for example, the overall reaction (4Fe + 3O2 2Fe2O3) may be written as two separate equations: 4Fe 4Fe3+ + 12e- and 3O2 + 12e- 6O2-. The iron loses electrons (e-) and its oxidation number increases from 0 to +3, so it is said to be oxidized; it acts as a reducing agent. The oxygen gains an equal number of electrons and its oxidation number decreases from 0 to -2, so it is said to be reduced; it acts as an oxidizing agent. Among the most powerful oxidizing agents are fluorine and ozone, which readily liberate oxygen from water. The most powerful reducing agents include such highly electropositive metals as sodium and lithium, which readily reduce the compounds formed by the noble metals and also liberate hydrogen from water. The two equations that describe electron transfer during redox reactions are called half reactions, or oxidation-reduction couples. In theory, every oxidation-reduction reaction may be broken up into two couples that indicate the mechanism by which electrons are transferred. Half reactions can be carried out in separate compartments of an electrochemical cell, with electrons flowing through a connecting wire between both electrodes. Oxidation takes place at the anode, while reduction occurs at the cathode. The relative electric potentials, or voltages, of the ensuing half reactions determine the direction in which the overall reaction will proceed. See also electrolytic cell; electromotive series.