JEE Main · 2005 · 08 May 2005 · Offline · Chemical Kinetics

A reaction involving two different reactants can never be

(A)Unimolecular reaction✓ Correct
(B)First order reaction
(C)second order reaction
(D)Bimolecular reaction

Answer: (A)

Solution

Molecularity counts the molecules that come together in a single elementary step, and a unimolecular step by definition involves just one molecule rearranging or breaking apart. If a reaction genuinely requires two different reactants, then at least two molecules must meet in that step, so its molecularity is two or more and it can never be unimolecular. It may still turn out first-order overall when one reactant is in large excess, but order and molecularity are separate ideas.

JEE Main · 2005 · 08 May 2005 · Offline · Single correct option

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