Q.Distinguish between order and molecularity of a reaction.

An elementary reaction occurs in a single step and cannot be broken down further; its molecularity is the number of reactant molecules that collide and take part in that one step -- one reactant molecule makes it unimolecular, two makes it bimolecular. For a genuinely elementary reaction, the experimentally observed order and the theoretical molecularity are numerically equal (e.g. $2NO_2\rightarrow2NO+O_2$ is both second order and bimolecular). Complex, multi-step reactions proceed through a sequence of elementary steps, one of which -- the slowest -- is the rate determining step; the overall reaction's rate law is predicted from the stoichiometry of just that slow step, and can differ …