An order of chemical reaction in which the rate of the reaction depends on the concentration of only one reactant, and is proportional to the amount of the reactant.
It may be represented by the equation, rate = k[A], where k is the reaction rate constant, and A is the concentration of the reactant.
An example of a reaction in a first-order is the decomposition of N2o5:
N2o5 → 2NO2 + ½ O2
because the rate of reaction depends on the concentration of N2o5 raised to the first power.
Compare: second-order kinetics
See also: chemical kinetics, reactant, reaction rate constant, chemical reaction.