Rate Law and Order of a Reaction

The kinetic equation, which shows the relationships between the rate (v) of a reaction and the concentrations of reactants, and in some cases products also, is called the rate law. For example, the rate law for the reaction 

The order of a reaction is defined as the sum of the exponents appearing in the rate law. For elementary reactions, the order of reaction is equal to the sum of the coefficients of the left-hand side of a written balanced chemical equation. For a reaction that obeys a complex rate law, such as Equation 7.12, the reaction order cannot be defined, and such reactions are said to have no simple reaction order. 

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