Also contains definition of: negative feedback
https://doi.org/10.1351/goldbook.C01210
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:
    are called
    parallel reactions
    or
    simultaneous reactions
    . When there are
    simultaneous reactions
    there is sometimes
    competition
    , as in the scheme:
    where B and C compete with one another for A.
  • Reactions occurring in forward and reverse directions are called
    opposing reactions
    :
  • Reactions occurring in
    sequence
    , such as
    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:
    The intermediate Y may catalyse the reaction (
    positive feedback
    ) or it may inhibit it (negative feedback).
  • Chain reactions
  • Source:
    PAC, 1996, 68, 149. (A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)) on page 161 [Terms] [Paper]