A reaction that involves more than one elementary reaction is said to occur by a composite mechanism. The terms complex mechanism, indirect mechanism, and step-wise mechanism are also commonly used. There are two main kinds of evidence for a composite mechanism:

1. The kinetic equation for the reaction does not correspond to its stoichiometry.
2. There is experimental evidence, direct or indirect, for intermediates of such a nature that it is necessary to conclude that more than one elementary reaction is involved.

There are many types of composite mechanisms, for example:

Reactions occurring in parallel, such as:

C01210-1.png

C01210-2.png

are called parallel reactions or simultaneous reactions. When there are simultaneous reactions there is sometimes competition, as in the scheme:

C01210-3.png

C01210-4.png

where B and C compete with one another for A.

Reactions occurring in forward and reverse directions are called opposing reactions:

C01210-5.png

Reactions occurring in sequence, such as

C01210-6.png

are known as consecutive reactions.

Reactions are said to exhibit feedback if a substance formed in one step affects the rate of a previous step. For example, in the scheme:

C01210-6.png

The intermediate Y may catalyse the reaction C01210-7.png (positive feedback) or it may inhibit it (negative feedback).

Chain reactions