CBSE 2025, Set 1

Question

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

Edvaya · Accepted Answer

The correct answer is Subjective. **Molecularity** is defined as the number of molecules/atoms/ions that must collide simultaneously in a **single elementary step** to bring about a chemical reaction. It is a theoretical concept defined step-by-step. In a complex (multi-step) reaction, each elementary step has its own molecularity — they may all be different. Assigning a single molecularity to the overall complex reaction would therefore be **meaningless and incorrect**. **Order**, on the other hand, is **experimentally determined** from the rate law expression. For a complex reaction, the overall order is governed by the rate-determining (slowest) step. It does not require any assumption about mechanism. Hence, order is applicable to both elementary and complex reactions.

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Chemistry > Physical Chemistry > Chemical Kinetics

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