Question

Differentiate between the Order of Reaction and Molecularity.

Hint:

For elementary reactions, order and molecularity may be the same, but for complex reactions they usually differ.

Answer 1

Step 1: Definition of Order of Reaction.
The order of reaction refers to the sum of the exponents of the concentration terms in the experimentally determined rate equation. It shows how the rate of reaction depends on the concentration of reactants.

For example, if the rate law is written as:
Rate = k [A]¹[B]²
Then the order of reaction = 1 + 2 = 3 (third order reaction).

Step 2: Definition of Molecularity.
The molecularity of a reaction is the total number of reacting species that participate simultaneously in a single elementary step of the reaction mechanism. It indicates how many molecules collide at the same time to bring about a reaction.

For example:
If three molecules collide in one step, the reaction is termolecular and its molecularity is 3.

Step 3: Differences Between Order of Reaction and Molecularity.
1. Basis of Concept:
Order of reaction is based on the rate law and is obtained experimentally.
Molecularity is based on the reaction mechanism and is a theoretical concept.

2. Nature of Values:
Order of reaction can be zero, fractional, or even negative in some cases.
Molecularity is always a positive integer (1, 2, or 3) and never fractional or zero.

3. Scope:
Order applies to the overall reaction.
Molecularity is defined only for individual elementary steps.

4. Relation with Stoichiometry:
Order does not necessarily match the stoichiometric coefficients of the balanced equation.
Molecularity directly corresponds to the number of reacting species in an elementary step.

5. Experimental Verification:
Order must be determined through experimental rate studies.
Molecularity can be predicted from the proposed mechanism of the reaction.

Conclusion.
In summary, the order of reaction describes how concentration affects reaction rate and is experimentally determined, whereas molecularity describes the number of particles colliding in a single step and is always a fixed whole number.