where k = the rate constant (in reciprocal time and concentration units), [A] and [B] represent the molar concentrations of reactants A and B, and x

and y are exponents denoting the order of the reaction for the reactant species. The exponents x and y must be determined experimentally. The sum of the exponents (x + y) is called the overall order of the reaction. In the present case, it is said that the reaction is x order in A, y order in B, and (x + y) order overall.

Consider, for example, the following reaction:

Experimentally, it is determined that the rate law for this reaction is

This means that the rates are second order in NO2, zeroth order in CO2, and second order overall.

Consider, as another example, the decomposition of dinitrogen pent- oxide, N2O5:

Here, it is experimentally determined that the rate law for this reaction is

Thus, the rate of decomposition is first order in N2O5. The rate constant k is also experimentally measured: