chemical reactions in which two particles participate in the elementary events—for example,
NOI + NOI → 2NO + I2
The bimolecular reaction is the most common class of chemical reactions. Both the simple (one-stage) reactions between two molecules (in the example above) and the majority of elementary reactions that are part of complex reactions belong to this class. In the case of complex reactions, atoms, radicals, and ions can enter into the bimolecular reaction in addition to molecules—for example,
ĊH3 + ĊH3 = C2H6
(recombination of free radicals). The rate of a bimolecular reaction, expressible in terms of the number of elementary reaction events per unit time per unit volume, is proportional to the frequency of collisions between the particles of the original substances (in the first example, NOI molecules). The frequency of collisions between particles is proportional to the product of their concentrations. Therefore, the rate of a bimolecular reaction is proportional to the product of the concentrations of the particles that enter into the reaction. For example, for the reaction NOI + NOI → 2NO + I2, the rate r is related to the concentration of original substances CNOI by the equation
where k is the rate constant of the reaction, which depends on the nature of the reacting particles. For the majority of reactions it increases rapidly as the temperature increases.