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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 (SN1) 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
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.