Difference Between Molecularity and Order of Reaction with Examples

Order of reaction and molecularity both play important roles in understanding chemical reactions and their rates, but they have different meanings and implications. Here is a breakdown of their differences along with some examples.

Order of Reaction

{tocify} $title={Table of Contents}

The sum of powers in the rate equation of the reactants participating in the reaction is called the order of a reaction.

-r_A = K . C_A . C^2_B

Order of Reaction = 2H = 3

1. It is defined as the sum of exponents of the concentration terms in the law equation.

2. On this basis the reaction is done as follows.

• a). Zero order reaction.
• b). First-order reaction.
• c). Second-order reaction.
• d). Third-order reaction.
• e). Fraction order reaction.

3. This is an experimental quality. 

4. It can be an integer zero or a fraction.

5. Examples

a.) Zero-order reaction

H2 + Cl2 ⟶ 2HCl

b.) First-order reaction 

CH3COOC2H5 + H2O  ⟶ CH3COOH + C2H5OH

c.) Second-order reaction 

CH3COOC2H5 + NaOH  ⟶ CH3COONa + C2H5OH

Molecularity 

The total number of molecules that participate in the reaction is called Molecularity. The molecularity of a reaction is one or sometimes even three.

1. It is defined as the total molecules used in the reaction.

2. On this basis reactions are of the following types.

• a). Mono molecular
• b). Bi Molecular
• c). Tri molecular

3. This is a theoretical quality. 

4. Molecularity is always an integer.

5. Examples

a). Mono molecular

C4H4 + C2H4  ⟶ 2C2H4

b). Bi Molecular

2HI  ⟶ H2 + I

c). Tri molecular

2NO + O2  ⟶ 2NO