In the expression for the rate of a chemical reaction, the sum of the powers of the concentrations is the overall order of the reaction. For instance, in a reaction A+B → C the rate equation may have the form R=k[A][B] 2 This reaction would be described as first order in A and second order in B. The overall order is three. The order of a reaction depends on the mechanism and it is possible for the rate to be independent of concentration (zero order) or for the order to be a fraction. See also molecularity; pseudo order.
A+B → C