CATALYSIS

in chemistry, the modification of the rate of a chemical reaction, usually an acceleration, by addition of a substance not consumed during the reaction. The term catalysis was first used in 1835 by the Swedish chemist Jons Jacob Berzelius to characterize phenomena observed by chemists in the late 18th and early 19th centuries. Examples of catalysis, however, were known much earlier. Such ancient techniques as fermenting wine to acetic acid and manufacturing soap from fats and bases are both achieved through catalytic reactions. In a catalyzed reaction the catalyst generally enters into chemical combination with the reactants but is ultimately regenerated so that the amount of catalyst remains unchanged. Since the catalyst is not consumed, each catalyst molecule may induce the transformation of many molecules of reactant. For an active catalyst, the number of molecules transformed per minute by one molecule of catalyst may be as high as several million. Where a given substance or a combination of substances undergoes two or more simultaneous reactions that yield different products, the distribution of products may be influenced by the use of a catalyst that selectively accelerates one reaction relative to the other(s). By choosing the appropriate catalyst, a particular reaction can be made to occur to the extent of practically excluding another. Many important applications of catalysis are based on selectivity of this kind. Since a reverse chemical reaction may proceed by reversal of the steps comprising the mechanism of the forward reaction, the catalyst for a given reaction accelerates the reaction in both directions equally. Therefore, a catalyst does not affect the position of equilibrium of a chemical reaction; it affects only the rate at which equilibrium is attained. Apparent exceptions to this generalization are those reactions in which one of the products is also a catalyst for the reaction. Such reactions are termed autocatalytic. Cases are also known in which the addition of a foreign substance, called an inhibitor, decreases the rate of a chemical reaction. This phenomenon, properly termed inhibition or retardation, is sometimes called negative catalysis. Concentrations of the inhibitor may, in some cases, be much lower than those of the reactants. Inhibition may result from (1) decrease in the concentration of one of the reactants because of complex formation between the reactant and the inhibitor; (2) decrease in the concentration of an active catalyst (poisoning of the catalyst) because of complex formation between the catalyst and the inhibitor; or (3) termination of a chain reaction because of destruction of the chain carriers by the inhibitor. Additional reading