5.0 Molecularity of a reaction

  Chemical Kinetics

There are two different types of chemical reactions.

First one is the reactions which takes place in one step called elementary reactions and the other one is the reactions which takes place in more than $1$ step using particular mechanisms.

Molecularity is defined for elementary reactions only.

It is defined as the number of molecules or ions of the reactants required for the reaction to occur. Generally, it is equal to the sum of stoichiometric coefficients of the reactants.

If the reaction takes place in more than $1$ steps, then the overall rate is limited by the slowest step called the rate determining step. For those types of reactions, molecularity is defined as the number of molecules or ions of the reactants that takes part in the rate determining step.

On the basis of molecularity, reactions are termed as unimolecular, bimolecular, trimolecular and so on.

Note:

• Molecularity can not be $0$, negative, fraction, imaginary or infinite.

• Molecularity can not be greater than $3$. Since the reaction takes place by collision between reactant molecules and more than $3$ molecules may not mutually collide with each other.

For example:

$$\begin{equation} \begin{aligned} PC{l_5} \to PC{l_3} + C{l_2}\quad {\text{Molecularity}} = 1 \\ 2S{O_2} + {O_2} \to 2S{O_3}\quad {\text{Molecularity}} = 2 + 1 = 3 \\\end{aligned} \end{equation} $$