CATALYST

A substance that alters the rate of a chemical reaction without itself being changed chemically in the reaction. The catalyst can, however, undergo physical change; for example, large lumps of catalyst can, without loss in mass, be converted into a powder. Small amounts of catalyst are often sufficient to increase the rate of reaction considerably. A positive catalyst increases the rate of a reaction and a negative catalyst reduces it. Homogeneous catalysts are those that act in the same phase as the reactants (i.e. in gaseous and liquid systems). For example, nitrogen( II) oxide gas will catalyze the reaction between sulfur(IV) oxide and oxygen in the gaseous phase. Heterogeneous catalysts act in a different phase from the reactants. For example, finely divided nickel (a solid) will catalyze the hydrogenation of oil (liquid).
The function of a catalyst is to provide a new pathway for which the rate-determining step has a lower activation energy than in the uncatalyzed reaction. A catalyst does not change the products in an equilibrium reaction and their concentration is identical to that in the uncatalyzed reaction; i.e. the position of the equilibrium remains unchanged. The catalyst simply increases the rate at which equilibrium is attained.
In autocatalysis, one of the products of the reaction itself acts as a catalyst. In this type of reaction the reaction rate increases with time to a maximum and finally slows down. For example, in the hydrolysis of ethyl ethanoate, the ethanoic acid produced catalyzes the reaction.

A catalyst is a substance which, in small quantities, facilitates a reaction or favorably alters the rate of a chemical reaction, without itself undergoing any permanent chemical change. This phenomenon is called catalysis. Acid-based catalysis is an important type of catalysis.
Catalysts that have the same phase as the reactants are known as homogeneous catalysts while those having a different phase are called heterogeneous catalysts. Generally, a catalyst that increases the reaction rate is known as a positive catalyst while one that slows down the rate is known as a negative catalyst, retardant or inhibitor.
Some examples of catalysts and their roles are as follows: (i) Protein catalysts, called enzymes, speed up many chemical reactions in living organisms. These play a key role in all metabolic or biological processes and in some manufacturing processes, such as the fermentation industry. (ii) Catalytic converters, in automobiles, employ a mixed oxide bed over which the exhaust gases pass after they are mixed with additional air. These catalysts promote oxidation of carbon monoxide and hydrocarbons to carbon dioxide and water.
Catalysts may be inorganic, organic or complexes of organic groups and metal halides. (Zeigler-Natta cataIysts). They may be gases, liquids or solids. In some cases, their action is undesirable and destructive, as in the oxidation of iron to iron oxide, which is catalyzed by water vapor (rusting). The life of an industrial catalyst varies from 1000 to 10,000 hours, after which it must be replaced.
Catalysts are highly specific in their action and application and very essential for many industrial chemical reactions, as in petroleum refining and organic syntheses. The activity of a solid catalyst is centerd in a small fraction of its surface; adding promoters increases the surface area by increasing the porosity or the number of active points. Substances that act as poisons clog and weaken the catalyst surface, thus decreasing the catalytic activity.