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# Explain the molecularity of a reaction.

Last updated date: 25th Jun 2024
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Hint: At first think about the chemical reaction. The Molecularity of a reaction is defined as the number of molecules that come together in an elementary reaction and is equal to the sum of stoichiometric coefficients of reactions in elementary reactions.

Molecularity of a reaction depends on how many molecules come together, a reaction can be unimolecular, bimolecular or trimolecular.
The kinetic order of any elementary reaction or reaction step is equal to its molecularity, and the rate equation of an elementary reaction can be determined from the molecularity. The kinetic order of a complex reaction cannot be equated to molecularity since it only describes only elementary reactions or steps.
Unimolecular reaction:- In a unimolecular reaction, a single molecule rearranges atoms forming different molecules.
$A \to P$
The reaction or reaction step is an isomerization if there is only one product molecule, or dissociation if there is more than one product.
The rate of the reaction for the above reaction is,
$\dfrac{{d[A]}}{{dt}} = - {k_r}[A]$
Bimolecular reaction:-In a bimolecular reaction, two molecules collide and exchange energy , the reaction is given by
$A + B \to P$
The rate of reaction for the above reaction is,
$\dfrac{{d[A]}}{{dt}} = - {k_r}[A][B]$
Trimolecular reaction:-A trimolecular reaction in solutions or gas mixtures involves three reactant molecules simultaneously colliding. Trimolecular reactions are very rare because three molecules hitting at the same time is very unlikely.

Note:
The molecularity of a reaction is only applicable to elementary reactions. In complex reactions, the molecularity of a reaction is not applicable to the whole but the slowest step in a complex reaction is used as the molecularity of that reaction.