Molecularity

Molecularity

Aug 26, 2022, 16:45 IST

About Molecularity

A chemical reaction that takes place in one and only one step is called elementary reaction while a chemical reaction occurring in the sequence of two or more steps is called complicated reaction. The sequence of steps through which a complicated reaction takes place is called reaction – mechanism. Each step in a mechanism is an elementary step reaction.The molecularity of an elementary reaction is defined as the minimum number of molecules, atoms or ions of the reactants(s) required for the reaction to occur and is equal to the sum of the stoichiometric coefficients of the reactants in the chemical equation of the reaction. Thus, the molecularity of some elementary reactions are as mentioned in pdf given below

What is unimolecular, biomolecular, trimolecular Molecularity

Reaction with molecularity equal to one, two, three etc; are called unimolecular, biomolecular, trimolecular etc. respectively.

A complicated reaction has no molecularity of its own but molecularity of each of the steps (elementary reactions) involved in the mechanism.

For example; see the example in the given below pdf

Properties of Molecularity

The molecularity of each step in the mechanism is two, so that we say that the reaction takes in the sequence of three steps each of which is bimolecular. There is another way also. According to which molecularity of a complicated reaction is taken to be equal to the molecularity of the slowest step i.e. rate determining step (r.d.s) in the mechanism.

For example, the reaction .Reactions of higher molecularity (molecularity > 3) are rare. This is because a reaction takes place by collision between reactant molecules and as number of a reactant molecules i.e. molecularity increases the chance of their coming together and colliding simultaneously decreases.

Difference between Order and Molecularity

i)  Order is an experimental property while molecularity is the theoretical property.

ii) Order may be any number, fractional, integral zero or even negative whereas molecularity is always an integer excepting zero.

iii) Order may change with change in experimental condition while molecularity can’t.