Q.iii. A reaction takes place in two steps,

1. $\mathrm{NO(g) + Cl_2(g) \longrightarrow NOCl_2(g)}$
2. $\mathrm{NOCl_2(g) + NO(g) \longrightarrow 2\ NOCl(g)}$

a. Write the overall reaction. b. Identify reaction intermediate. c. What is the molecularity of each step?

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 completely from the overall balanced equation's coefficients. A species produced in one step and fully consumed in a later step, so that it never appears in the overall equation, is a reaction intermediate; its concentration never appears in the rate law, distinguishing it from a genuine reactant.