Question

Define molecularity of the reaction. State any one condition in which a bimolecular reaction may be kinetically first order.

Hint:

In cases of bimolecular reactions with excess reactant, the rate depends predominantly on the concentration of the limiting reactant, making the reaction appear to be first order with respect to that reactant.

Answer 1

Molecularity of a reaction refers to the number of molecules or ions that must collide simultaneously in an elementary step of the reaction. It is a term used to describe the actual number of reacting particles involved in a single reaction event. For example, a reaction between two molecules is said to be bimolecular. A bimolecular reaction can be kinetically first order if the concentration of one of the reactants is much larger than the other. In such cases, the rate of reaction depends primarily on the concentration of the limiting reactant, effectively making the reaction behave as if it were first order. This can happen in cases where one reactant is present in excess. For instance, in the reaction: \[ A + 2B \rightarrow P \] If the concentration of B is much higher than A, the rate law will be approximately: \[ \text{Rate} = k[A] \] Thus, although the reaction is bimolecular, it behaves like a first-order reaction due to the excess of one reactant.