Question

Higher order (>3) reactions are rare due to

• A. Shifting of equilibrium towards reactants due to elastic collisions
• B. Loss of active species on collision
• C. Low probability of simultaneous collision of all reacting species
• D. Increase in entropy as more molecules are involved

Answer 1

The Correct Option is C

1. Understand Reaction Order and Collision Theory

- Reaction Order: The order of a reaction is the sum of the exponents of the concentration terms in the rate law. It reflects how the rate of the reaction changes with the concentrations of the reactants.

- Collision Theory: For a reaction to occur, reactant molecules must collide with sufficient energy (activation energy) and with the correct orientation.

2. Analyze the probability of simultaneous collisions
Higher-order reactions (order > 3) imply that more than three reactant molecules must simultaneously collide in the correct orientation for the reaction to proceed. The probability of such a simultaneous collision of multiple molecules with sufficient energy and proper orientation is extremely low.
For example, for a 4th-order reaction, you need four molecules to collide at the same time with enough energy and correct orientation for the reaction to happen.

3. Evaluate the given options

- (A) Shifting of equilibrium towards reactants due to elastic collisions: Elastic collisions do not directly affect equilibrium in a way that makes higher-order reactions rare.

- (B) Loss of active species on collision: While collisions can sometimes lead to the loss of active species, this is not the primary reason for the rarity of higher-order reactions.

- (C) Low probability of simultaneous collision of all reacting species: This is the most accurate explanation. The probability of multiple molecules colliding simultaneously with sufficient energy and proper orientation decreases drastically as the number of molecules increases.

- (D) Increase in entropy as more molecules are involved: While entropy changes are relevant to reactions, they don't directly explain why higher-order reactions are rare.

Final Answer:
(C) Low probability of simultaneous collision of all reacting species

Answer 2

Higher-order reactions involving more than three reactant molecules are rare due to the extremely low probability of all reactant molecules colliding simultaneously with the correct orientation and sufficient energy to react. The probability of such collisions decreases as the number of reactants involved increases, making higher-order reactions less likely to occur.

So, the correct answer is (C) : Low probability of simultaneous collision of all reacting species.