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Rates of Consumption and Formation

The rate of consumption of a reactant of concentration [A] is defined as

........ (1)

in which t represents time. Square brackets may be used without definition to indicate concentrations, e.g. [A] is the concentration of A. Other symbols, such as a for the concentration of A, may be used for typographical convenience, but these should be defined. The rate of formation of a substance of concentration [Z] is defined as

........ (2)

The usual unit of rate is mol dm^-3 s^-1 or mol L^-1 s^-1 (where L = liter dm³) or M s^-1 (where M, molarity, = mol dm^-3). In all contexts the term velocity may be used as a synonym for rate. The alternative will only be mentioned in these Recommendations, however, when it appears to be the more common usage.

In a given reaction the rates of consumption and rates of formation are equal to one another only if there is a one-to-one stoichiometric relationship between the species consumed and formed. For example, in a reaction with stoichiometry

A + 3B 2Z

the rate of formation of Z is twice the rate of consumption of A, and the rate of consumption of B is three times that of A. In most enzyme-catalysed reactions, however, there is a one-to-one stoichiometric ratio between all substrates and products. It is then permissible to omit the subscript from v and use the term rate of reaction.

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