Consider a typical chemical reaction in which two reactants A and B combine to form a product C:
This can also be written
The prefactors −1, −2 and 3 (with negative signs for reactants because they are consumed) are known as stoichiometric coefficients. One molecule of A combines with two of B to form 3 of C, so if we use the symbol [X] for the molar concentration of chemical X,
If the reaction takes place in a closed system at constant temperature and volume, without a build-up of reaction intermediates, the reaction rate  is defined as
where  is the stoichiometric coefficient for chemical X, with a negative sign for a reactant.
The initial reaction rate  has some functional dependence on the concentrations of the reactants,
and this dependence is known as the rate equation or rate law. This law generally cannot be deduced from the chemical equation and must be determined by experiment.