Which one of the following statements is correct for a first order reaction?

Infinite time is taken for 100% completion of the reaction.

The correct answer is Option (3) → Infinite time is taken for 100% completion of the reaction.

Option 1: Half life is directly proportional to the initial concentration of reactant

This is incorrect because in a first order reaction, half-life does not depend on initial concentration. It remains constant throughout the reaction.

Option 2: Half life is inversely proportional to the initial concentration of reactant

This is also incorrect because dependence of half-life on concentration is a feature of zero order reactions, not first order reactions.

Option 3: Infinite time is taken for 100% completion of the reaction

This is correct because in first order reactions, the concentration decreases exponentially. It approaches zero but never becomes exactly zero, meaning complete consumption theoretically takes infinite time.

Option 4: Rate of the reaction is proportional to the square of initial concentration of reactant

This is incorrect because this describes a second order reaction, not a first order reaction.