Question

Define and explain the term ‘molecularity of a reaction’ with a suitable example.

Answer 1

Hint: The term molecularity of a reaction involves the collision of particles (atoms or
molecules) of the products. The rate of reaction or the speed of formation of the product depends on
the effective collisions of the participating molecules. One can use this data to give a definition of the
term and its well-detailed elaboration.
Complete step by step answer:
1) First of all we will see the definition of molecularity of a reaction where it is the number of participating molecules (or atoms or ions) colliding to form products in an elementary step.
2) The molecularity of a reaction has the following properties:
(A) It is a theoretical quantity and does not change with the experimental conditions.
(B) It is always an integer and can’t be negative, fraction, or zero.
(C) It can’t be greater than three as the possibility of more than three molecules colliding and forming
products at the same time are negligible.
3) When a single molecule gets rearranged forming different molecules then the reaction is called
unimolecular reaction. In bimolecular reaction two atoms or molecules collide and exchange energy to
form a single product and this goes on further as molecularity increases.
4) Now let us see an example, the molecularity of the combination of hydrogen and iodine to form
hydrogen iodide can be determined by writing the chemical equation:
${H_2} + {I_2} \to 2HI$
In the above example the two molecules, one of hydrogen and one of iodine combine to give two
molecules of hydrogen iodide.
Thus, the molecularity of the reaction is two.


Note:
It can be summarized that the total sum of stoichiometric coefficients of reactants in a balanced
a chemical equation is termed as the molecularity of the reaction. The equation has to be balanced in
order to calculate the molecularity of the reaction. Sometimes the order of the reaction is equal to the
molecularity of the reaction. The order of reaction tells us about the number of molecules of reactants
whose concentration changes during the reaction.