Question

Why is molecularity applicable for only elementary reactions whereas order of a reaction is applicable for elementary and complex reactions? Explain with suitable examples.

Answer 1

1. Reactions occuring in a single step which cannot be broken down further into simpler reactions are called elementary reactions. Molecularity refers to how many reactant molecules are involved in reactions. Hence, the molecularity of each elementary reaction is fixed.
2. A number of chemical reactions are complex. They take place as a series of elementary steps. The molecularity of each step in a complex reaction may be different. Hence, the concept of molecularity is meaningless for complex reactions.
3. The slowest step in a complex reaction determines the rate of the overall reaction. Hence, the order of the reaction is applicable for both; elementary and complex reactions.

Example:

Consider the reaction: \[\ce{2NO2Cl_{(g)} -> 2NO_{2(g)} + Cl_{2(g)}}\]

The reaction takes place in two steps:

1.	\[\ce{NO_2Cl_{(g)} ->[k1] NO_{2(g)} + Cl_{(g)}}\]	(slow)
2.	\[\ce{NO_2Cl_{(g)} + Cl ->[k2] NO_{2(g)} + Cl_{2(g)}}\]	(fast)
	Overall: \[\ce{2NO_2Cl_{(g)} -> 2NO_{2(g)} + Cl_{2(g)}}\]

The molecularity of elementary reaction (1) is 1 and that of elementary reaction (2) is 2. The first step being slower than the second and is the rate determining step. Hence, the reaction is of first order.