substance that can cause a change in the rate of a chemical reaction without itself being consumed in the reaction; the changing of the reaction rate by use of a catalyst is called catalysis. Substances that increase the rate of reaction are called positive catalysts or, simply, catalysts, while substances that decrease the rate of reaction are called negative catalysts or inhibitors.
Catalysts work by changing the activation energy for a reaction, i.e., the minimum energy needed for the reaction to occur. This is accomplished by providing a new mechanism or reaction path through which the reaction can proceed. When the new reaction path has a lower activation energy, the reaction rate is increased and the reaction is said to be catalyzed.
If the activation energy for the new path is higher, the reaction rate is decreased and the reaction is said to be inhibited. Inhibitors can provide an interesting challenge to the chemist. For example, because oxygen is an inhibitor of free-radical reactions, many of which are important in the synthesis of polymers, such reactions must be performed in an oxygen-free environment, e.g., under a blanket of nitrogen gas.
In some reactions one of the reaction products is a catalyst for the reaction; this phenomenon is called self-catalysis or autocatalysis. An example is the reaction of permanganate ion with oxalic acid to form carbon dioxide and manganous ion, in which the manganous ion acts as an autocatalyst. Such reactions are potentially dangerous, since the reaction rate may increase to the point of explosion.
Some substances that are not themselves catalysts increase the activity of a catalyst when added with it to some reaction; such substances are called promoters. Alumina is a promoter for iron when it is used to catalyze the reaction of hydrogen and nitrogen to form ammonia. Substances that react with catalysts to reduce or eliminate their effect are called poisons.
Enzymes are the commonest and most efficient of the catalysts found in nature. Most of the chemical reactions that occur in the human body and in other living things are high-energy reactions that would occur slowly, if at all, without the catalysis provided by enzymes. For example, in the absence of catalysis, it takes several weeks for starch to hydrolyze to glucose; a trace of the enzyme ptyalin, found in human saliva, accelerates the reaction so that starches can be digested. Some enzymes increase reaction rates by a factor of one billion or more.
Enzymes are generally specific catalysts; that is, they catalyze only one reaction of one particular reactant (called its substrate). Usually the enzyme and its substrate have complementary structures and can bond together to form a complex that is more reactive due to the presence of functional groups in the enzyme, which stabilize the transition state of the reaction or lower the activation energy. The toxicity of certain substances (e.g., carbon monoxide and the nerve gases) is due to their inhibition of life-sustaining catalytic reactions in the body.
Catalysis is also important in chemical laboratories and in industry. Some reactions occur faster in the presence of a small amount of an acid or base and are said to be acid catalyzed or base catalyzed. For example, the hydrolysis of esters is catalyzed by the presence of a small amount of base. In this reaction, it is the hydroxide ion, OH-, that reacts with the ester, and the concentration of the hydroxide ion is greatly increased over that of pure water by the presence of the base. Although some of the hydroxide ions provided by the base are used up in the first part of the reaction, they are regenerated in a later step from water molecules; the net amount of hydroxide ion present is the same at the beginning and end of the reaction, so the base is thought of as a catalyst and not as a reactant.
Finely divided metals are often used as catalysts; they adsorb the reactants onto their surfaces (see adsorption), where the reaction can occur more readily. For example, hydrogen and oxygen gases can be mixed without reacting to form water, but if a small amount of powdered platinum is added to the gas mixture, the gases react rapidly. Hydrogenation reactions, e.g., the formation of hard cooking fats from vegetable oils, are catalyzed by finely divided metals or metal oxides. The commercial preparation of sulfuric acid and nitric acid also depends on such surface catalysis. Other commonly used surface catalysts, in addition to platinum, are copper, iron, nickel, palladium, rhodium, ruthenium, silica gel (silicon dioxide), and vanadium oxide.
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Energy profiles for catalytic and thermal (noncatalytic) reactions in the gaseous phase. Credit:Encyclopædia Britannica, Inc. Modification (usu