Question

Why is molecularity applicable only for elementary reactions whereas order is applicable for elementary as well as complex reactions? 

Hint:

Molecularity applies only to elementary reactions, while order applies to the overall reaction, regardless of whether it is elementary or complex.

Answer 1

Molecularity vs Order in Reactions

In chemical kinetics, molecularity and order are distinct concepts describing reaction behavior. Molecularity pertains to elementary reactions, while order applies to both elementary and complex reactions. Below are the distinctions:

1. Molecularity:

Molecularity defines the number of reactant species (molecules, atoms, or ions) that participate in a single, simultaneous collision or interaction within an elementary reaction. An elementary reaction is a single step in a reaction mechanism. For example, in a bimolecular elementary reaction: \[ \text{A} + \text{B} \rightarrow \text{C} \] the molecularity is 2, as two reactant molecules (A and B) are involved in the collision.

Molecularity is exclusively defined for elementary reactions because: - In a single step, the count of reacting species directly corresponds to molecularity. - It is not applicable to complex reactions as they comprise multiple elementary steps, and a single-step molecularity is not meaningful for the overall process.

2. Order of Reaction:

The order of a reaction quantifies how the reaction rate changes with reactant concentrations, as represented by the exponents in the rate law. This order is determined experimentally and may not align with the reaction's molecularity. For instance, a reaction with the rate law: \[ \text{Rate} = k[\text{A}]^2[\text{B}] \] has an order of 3 (second-order with respect to A and first-order with respect to B), derived from the sum of the concentration exponents.

The order of reaction can be determined for both elementary and complex reactions because: - For elementary reactions, the order typically mirrors the molecularity. - For complex reactions, experimental determination is necessary to establish the order, as their overall rate law reflects the interplay of multiple elementary steps.

3. Key Differences:

• Molecularity applies solely to elementary reactions, indicating the number of species involved in a single mechanistic step.
• Order is applicable to both elementary and complex reactions, reflecting the experimentally determined relationship between rate and reactant concentrations.

Conclusion:

- Molecularity is defined only for elementary reactions, representing the number of reactant molecules in a single step. - Order can be determined for all reaction types (elementary and complex) and indicates how reactant concentrations affect the reaction rate, irrespective of whether the reaction proceeds in one step or multiple steps.