Order and Molecularity of Reaction

Order of reaction: It is defined as the concentration of reactions expressed in rate law.

Let us consider a general reaction,
$${\text{mA + nB} \rightarrow \text{Product}}$$
$$\text{Then, Rate} = \text{K[A]}^p.[B]^q$$
$$\text{Order of reaction = p + q}$$

The overall order of reaction may be fractional or whole number.

For a general reaction:
$${\text{A} \rightarrow \text{Product}}$$
$$\text{Rate} = \text{K[A]}^m $$

i. When m = o, rate = k
Here, the rate of reaction is independent of the concentration of the reactants.
So, order of reaction is zero and it follows zero order kinetics.

ii. When m = 1,
Rate = k[A]1
It depends on only one concentration term and has order of reaction equals to 1. So, it follows first order kinetics.

iii. When m =2,
Rate = k[A]2
It depends upon two concentration terms and has order of reaction equals to 2. So, it follows 2nd order kinetics.

Molecularity of the reaction:

It is defined as the number of molecules which simultaneously collide to give the product.

We know that the reaction which occur in single step is known as elementary reactions. In elementary reaction, reactions are classified as unimolar, bimolar, trimolar according to the number of molecules present in the reaction. For elementary reactions, the molecularity and the order of reaction are same.

For example:
i. O2F2 → O2 + F2 (Unimolar)
ii. 2HI → HI + I2 (Bimolar)
iii. 2NO + O2 → 2NO2

Difference between order of reaction and molecularity of reaction:

Order of reaction Molecularity of reaction
It is defined as the sum of concentration of reactants expressed in rate law. It is defined as the number of molecules which simultaneously collide to give new product.
It is determined experimentally but cannot be calculated. It is a theoretical concept and and can be calculated.
It may have fractional or zero value. It is always whole number.
It changes with temperature, pressure, concentration etc. It is invariable with conditions.

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