Rate determining step

A number of chemical reactions are complex: they take place as a series of elementary steps. One of these steps is slower than the others, and this slowest step is the rate determining step — it determines the rate of the overall reaction.

Consider $\mathrm{2NO_2Cl(g) \longrightarrow 2NO_2(g) + Cl_2(g)}$. The reaction takes place in two steps:

i. $\mathrm{NO_2Cl(g) \xrightarrow{\;k_1\;} NO_2(g) + Cl(g)}$ (slow)

ii. $\mathrm{NO_2Cl(g) + Cl(g) \xrightarrow{\;k_2\;} NO_2(g) + Cl_2(g)}$ (fast)

The first step, being slower than the second, is the rate determining step. The rate law is

This also represents the rate law of the overall reaction. The reaction thus is of the first order.

Reaction intermediate:

In the above reaction Cl is formed in the first step and consumed in the second. Such a species represents the reaction intermediate. The concentration of a reaction intermediate does not appear in the rate law.

Distinction between order and molecularity of a reaction :