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In chemistry or biology a substance that accelerates the rate of a chemical reaction but is otherwise unconsumed by the reaction (see also catalysis). A catalyst participates in the reaction, but is neither a chemical reactant or a chemical product of the net reaction.

Generally, catalysts work by providing a lower energy pathway for the overall reaction to occur. This usually occurs by the reaction of the catalyst with one or more reactants to form a stable intermediate that subsequently reacts to form the final reaction product and regenerate the catalyst.

The following is a typical catalytic reaction scheme, where C represents the catalyst:

A + C ->  AC      (1)
B + AC -> AB + C  (2)

Note that while the catalyst (C) is consumed by reaction 1, it is subsequently produced by reaction 2, so for the overall reaction:

A + B + C -> AB + C

the catalyst is neither consumed nor produced.

Enzymes are bio-catalysts.

Use of the term in a broader cultural sense is in rough analogy to the sense described here and to the process described as catalysis.