Order of Reaction

a concept in chemical kinetics. The order of a reaction is determined as the sum of the exponents n₁ and n₂ in the equation

r = k [A₁]^n₁[A₂]^n₃

which expresses the dependence of the reaction rate r on the concentrations [A₁] and [A₂] of the initial materials, where k is the rate constant. Reactions in which n₁ + n₂ = 1, 2, … are called first-order reactions, second-order reactions, and so on. The individual exponent in equation (1) is called the order of the reaction for the corresponding substance.

In simple reactions the rate in one direction, according to the mass action law, is subject to equation (1), and n₁ and n₂ coincide with the number of molecules of substances A₁ and A₂ that take part in the elementary stage of the reaction. The rates of complex reactions are sometimes also expressed by equations of type (1); at the same time, however, the order of a reaction may be different from the stoichiometric coefficient of the substance in the reaction equation (written with the smallest integral stoichiometric coefficients), and it may be a fraction. Fractional and zero orders, as well as integral orders, are common for heterogenous catalytic reactions; negative orders are also known.

M. I. TEMKIN