Appeared in: 2023 (3 sets)
Given below is the decomposition of hydrogen peroxide in alkaline medium, which is catalysed by iodide ions :
$2\text{H}_2\text{O}_2 \xrightarrow{\text{I}^-, \text{alkaline medium}} 2\text{H}_2\text{O} + \text{O}_2$
The above reaction takes place in two steps :
Step I : $\text{H}_2\text{O}_2 + \text{I}^- \longrightarrow \text{H}_2\text{O} + \text{IO}^-$ (slow)
Step II : $\text{H}_2\text{O}_2 + \text{IO}^- \longrightarrow \text{H}_2\text{O} + \text{I}^- + \text{O}_2$
Molecularity of Step I and Step II is :
(a) Step I – 2, Step II – 2
(b) Step I – 1, Step II – 2
(c) Step I – 2, Step II – 1
(d) Step I – 3, Step II – 1
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Official Evaluation Steps & EL10 Insight
Step 1 (Governing Definition): [0.25 Mark]
The molecularity of an elementary reaction is defined as the number of reacting species (atoms, ions, or molecules) taking part in an elementary step, which must collide simultaneously in order to bring about a chemical reaction.
Step 2 (Step-by-Step Analysis): [0.5 Mark]
- Step I: The elementary reaction is given as: $$\text{H}_2\text{O}_2 + \text{I}^- \longrightarrow \text{H}_2\text{O} + \text{IO}^-$$ Here, two reactant species ($\text{H}_2\text{O}_2$ and $\text{I}^-$) are involved. Therefore, the molecularity of Step I is 2.
- Step II: The elementary reaction is given as: $$\text{H}_2\text{O}_2 + \text{IO}^- \longrightarrow \text{H}_2\text{O} + \text{I}^- + \text{O}_2$$ Here, two reactant species ($\text{H}_2\text{O}_2$ and $\text{IO}^-$) are involved. Therefore, the molecularity of Step II is 2.
Step 3 (Final Concluded Answer): [0.25 Mark]
Comparing the derived values with the given options, we conclude that:
Correct Option: (a) Step I – 2, Step II – 2
💡 EL10 Concept Insight:
Think of molecularity like a dance floor: it simply counts how many "partners" (reactant molecules) need to bump into each other at the exact same time to make the reaction happen in that specific step.