chemistry 12th class cbse notes

Molecularity of a Reaction

The number of reacting species (atoms, ions or molecules) taking part in an elementary reaction, which must collide simultaneously in order to bring about a chemical reaction is called molecularity of a reaction.

Classification of reaction on the basis of Molecularity:

Unimolecular reactions:

When one reacting species is involved, For example, decomposition of ammonium nitrate.

Unimolecular reactions

Bimolecular reactions:

involve the simultaneous collision between two species, for example, dissociation of hydrogen iodide.

Bimolecular reactions

Trimolecular or termolecular reactions:

involve the simultaneous collision between three reacting species, for example,

Trimolecular or termolecular reactions

Reactions with molecularity greater than three are very rare.


The reactions taking place in one step are called elementary reactions.
When a sequence of elementary reactions (called mechanism) gives us the products, the reactions are called complex reactions.
The different steps in which the complex reaction takes place is called the mechanism of the reaction.

Rate determining step:

The overall rate of the reaction is controlled by the slowest step in a reaction called the rate-determining step.
A complex reaction can be represented as a series of elementary steps.

Reaction intermediates:

There are some species which are formed during the course of the reaction but do not appear in the overall reaction. They are called reaction intermediates.

The distinction between Order and Molecularity of a reaction

Order is the sum of the powers of the concentration of the reactions in the rate law expression.Molecularity is the number of reacting species taking part in an elementary reaction, which must collide simultaneously in order to bring about a chemical reaction.
It can be zero or even a fraction.It cannot be zero or a non-integer.
It is applicable to elementary as well as complex reactions.It is applicable only for elementary reactions. For complex reaction molecularity has no meaning.
It can be determined experimentally only and cannot be calculated.It can be calculated by simply adding the molecules of the slowest step.
For complex reaction, order is given by the slowest step.Generally, molecularity of the slowest step is same as the order of the overall reaction.


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