Law of Mass Action

For an elementary chemical reaction the instantaneous reaction rate is proportional to the product of reactant concentrations each raised to a power equal to its stoichiometric coefficient: for aA + bB → products, rate = k [A]^a [B]^b.

For an elementary homogeneous chemical reaction, the instantaneous reaction rate is proportional to the product of the reactant activities (or concentrations for ideal dilute systems), each raised to the power of its stoichiometric coefficient; mathematically, for a step with reactants i, rate = k · Π a_i^{ν_i}.

A chemical and pharmacological principle that, for an elementary reversible reaction in a well‑mixed dilute system, the instantaneous rate of an elementary step is proportional to the product of the reactant concentrations (each raised to the stoichiometric power), and at equilibrium the ratio of rate constants equals the ratio of concentration products (equilibrium constant).