Why is the concept of molecularity limited to elementary reactions?

The concept of molecularity is limited to elementary reactions because only these reactions occur in a single step.

Molecularity refers to the number of molecules or ions that participate in an elementary reaction, the simplest form of a reaction that occurs in a single step. Elementary reactions are the basic building blocks of complex reactions, which can consist of multiple elementary reactions happening in sequence or simultaneously.

In an elementary reaction, the reactants directly form the products in a single collision or interaction. Therefore, the molecularity of an elementary reaction is simply the number of reactant molecules or ions involved in that collision or interaction. This can be unimolecular (involving one molecule), bimolecular (involving two molecules), or termolecular (involving three molecules), with termolecular reactions being quite rare due to the low probability of three particles colliding simultaneously.

On the other hand, complex reactions consist of multiple elementary reactions, each with its own molecularity. The overall reaction may involve more molecules or ions than any individual elementary reaction. However, the molecularity of the overall reaction is not simply the sum of the molecularities of the elementary reactions. Instead, it is determined by the rate-determining step, the slowest elementary reaction in the sequence.

Therefore, the concept of molecularity does not apply to complex reactions as a whole, because they do not occur in a single step. Instead, each elementary reaction within the complex reaction has its own molecularity. This is why the concept of molecularity is limited to elementary reactions.

In summary, molecularity is a characteristic of elementary reactions, which occur in a single step. It refers to the number of molecules or ions that participate in the reaction. Complex reactions, which consist of multiple elementary reactions, do not have a single molecularity, because they do not occur in a single step. Instead, each elementary reaction within the complex reaction has its own molecularity.