Assertion (A): In the decomposition of hydrogen peroxide catalysed by iodide ion in an alkaline medium, IO− is an intermediate.
Reason (R): In the decomposition of hydrogen peroxide catalysed by iodide ion in an alkaline medium, both the steps, \( \ce{H2O2 + I^- -> H2O + IO^-} \) and \( \ce{H2O2 + IO^- -> H2O + I^- + O2} \), are bimolecular elementary reactions.
In the light of the above statements, choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Both (A) and (R) are true but (R) is not the correct explanation of (A).
The iodide-catalysed decomposition of hydrogen peroxide, \( \ce{2H2O2 -> 2H2O + O2} \), takes place in two steps:
\( \displaystyle \text{(1)}\quad \ce{H2O2 + I^- -> H2O + IO^-} \)
\( \displaystyle \text{(2)}\quad \ce{H2O2 + IO^- -> H2O + I^- + O2} \)
Assertion (A): In the decomposition of hydrogen peroxide catalysed by iodide ion in an alkaline medium, IO− is an intermediate. — True. IO− is formed in step (1) and used up in step (2), so it does not appear in the net equation.
Reason (R): In the decomposition of hydrogen peroxide catalysed by iodide ion in an alkaline medium, both the steps, \( \ce{H2O2 + I^- -> H2O + IO^-} \) and \( \ce{H2O2 + IO^- -> H2O + I^- + O2} \), are bimolecular elementary reactions. — True. Each step is a single collision between two species.
Why R does not explain A?
R gives the number of species that collide in each step; a species formed in a bimolecular step need not be an intermediate.
The correct reason for A is that IO− is formed during the course of the reaction but does not appear in the balanced equation for the whole reaction.
NCERT (2026-27 edition):
3.2.4 Molecularity of a Reaction – “Both the steps are bimolecular elementary reactions. Species IO- is called as an intermediate since it is formed during the course of the reaction but not in the overall balanced equation.”