Question

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

Answer 1

1️⃣The number of reacting species (atoms, molecules or ions) which collide simultaneously to bring about a chemical reaction is called molecularity of a reaction.

2️⃣Complex reaction takes place in a number of  elementary steps.

3️⃣Complex reaction does not proceed by simultaneous collision of all  the particles  involved  in the reaction. Therefore for complex reaction molecularity has no meaning.

Answer 2

Hey !!

A complex reaction proceeds through several elementary reactions. Number of molecules involved in each elementary reaction may be different, the molecularity of each step may be different. Therefore, discussion of molecularity of overall complex reaction is meaningless. On the other hand, order of  a complex reaction is determined by the slowest step in its mechanism and is not meaningless even in the case of complex reactions.

Hope it helps you !!