Why molecularity is applicable only for elementary reactions and order is applicable for elementary as well as complex reactions


 

A complex reaction occurs through a number of steps ie., elementary reactions. The number of molecules involved in each elementary reaction may be different, i.e., the molecularity of each step may be different. Therefore, it is meaningless to talk of the molecularity of the overall Complex reaction.

On the other hand, the order of complex reactions depends upon the molecularity of the slowest step. Hence, it is not meaningless to talk about the order of a complex reaction.