Molecularity of reaction:- It is defined as, the total number of molecules of all the reactants taking part in a chemical reaction as represented by a simple equation.

Example:- (i) Inversion of cane sugar [molecularity = 2]

C₁₂H₁₂O₁₁ + H₂O + H⁺ ions → C₆H₁₂ + C₆H₁₂O₆

(ii) Dissociation of ammonia [molecularity = 2]

2NH₃ ⇌ N₂ + 3H₂

(iii) Decomposition of dibromo succinic acid into bromomaleic acid and hydrobromic acid [molecularity = 1]

C₄H₄O₄Br₂ → C₄H₃O₄Br + HBr

Reactions having molecularity 1, 2, 3 etc. are known as unimolecular, bimolecular and trimolecular reactions respectively.

Order of reaction:- Reactions are generally classified on the basis of their order or reaction. It is defined as the total number of reacting molecules whose concentration changes during the chemical change. In other words it is also defined as the total number of reacting molecules whose concentration determines the rate of reaction.

If A → products and dx/dt = kC_A, then

Order of reaction = 1

If A + B → Products and dx/dt = kC_A.C_B, then

order or reaction = 2

If n₁A + n₂B + n₃C.... → Products and

dx/dt = kCⁿ¹_A.Cⁿ²_B.Cⁿ³_C...... then

Order of reaction = n₁ + n₂ + n₃ .....

In general order of reaction is defined as, the sum of the powers to which the concentration terms of the reactants must be raised in order to determine the rate of a reaction.

Distinction between order and molecularity:

Order:-

1. Kinetic order is a purely experiment property connected with the rate equation. It is not necessarily connected with the chemical equation of the reaction.

2. In a reaction A + B .....

If the observed rate ∝ [A]^α [B]^β the order of the reaction = α + β.

3. Order may change with physical condition such as pressure, solvent etc. For example, order change from 1 to 2 at low pressure for many gaseous reactions.

Molecularity:-

1. Molecularity is a theoretical property. In its classical definition it is connected with the overall chemical equation and in the modern definition with the mechanism.

2. Molecularity is defined as the number of molecules taking part in a rate determining steps of its mechanism.

3. Molecularity (classical) is an invariant number for a given chemical reaction.