Molecularity

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Molecularity

Kinetic studies involve not only measurement of a rate of reaction but also proposal of a reasonable reaction mechanism. Each and every single step in a reaction mechanism is called an elementary reaction.

An elementary step is characterized by its molecularity. The total number of reactant species that are involved in an elementary step is called molecularity of that particular step. Let us recall the hydrolysis of t butyl bromide studied in XI standard. Since the rate determining elementary step involves only t-butyl bromide, the reaction is called a Unimolecular Nucleophilic substitution (S_N¹) reaction.

Let us understand the elementary reactions by considering another reaction, the decomposition of hydrogen peroxide catalysed by I^–.

2H₂O₂(aq) → 2H₂O(l) + O₂(g)

It is experimentally found that the reaction is first order with respect to both H₂O₂ and I^–, which indicates
that I^– is also involved in the reaction. The mechanism involves the following steps.

Step: 1

H₂O₂(aq) + I^–(aq) → H₂O(l) + Ol^–(aq)

Step: 2

H₂O₂(aq) + OI^–(aq) → H₂O(l) + I^–(aq) + O₂(g)

Overall Reaction is

2H₂O₂(aq) → 2H₂O(l) + O₂(g)

These two reactions are elementary reactions. Adding equ (1) and (2) gives the overall reaction. Step 1 is the rate determining step, since it involves both H₂O₂ and I^–, the overall reaction is bimolecular.

Differences Between Order and Molecularity:

Order of a Reaction	Molecularity of a Reaction
1. It is the sum of the powers of concentration terms involved in the experimentally determined rate law.	1. It is the total number of reactant species that are involved in an elementary step.
2. It can be zero (or) fractional (or) integer	2. It is always a whole number, cannot be zero or a fractional number.
3. It is assigned for a overall reaction	3. It is assigned for each elementary step of mechanism.