Question

Define : Molecularity of reaction

Hint:

Never confuse Molecularity with Order. Molecularity is theoretical and only for elementary steps, while Order is experimental and applies to the overall reaction.

Answer 1

Step 1: Understanding the Concept:
Chemical reactions can occur in a single step (elementary) or multiple steps (complex). Molecularity is a theoretical concept strictly applicable to elementary reactions.
Step 2: Detailed Explanation:
Molecularity represents the stoichiometry of the rate-determining elementary step.
1. Unimolecular: Only one reacting species is involved (e.g., decomposition of \( O_2F_2 \)).
2. Bimolecular: Two species collide simultaneously (e.g., \( H_2 + I_2 \rightarrow 2HI \)).
3. Termolecular: Three species collide at once. Reactions with molecularity greater than three are extremely rare because the probability of more than three molecules colliding at the exact same time and with proper orientation is very low.
Unlike "order of reaction", molecularity is always a whole number and cannot be zero or fractional.
Step 3: Final Answer:
Molecularity is the total number of reactant particles involved in an elementary step of a reaction.