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Chemistry Β· 2026 Β· 3 marks
CBSE 2026 Β· Region 4 Β· Set 1 Β· Q27
Consider the decomposition of hydrogen peroxide ( H_2 O_2) as per the equation given below: 2 H_2 O_2 β[ Alkaline medium ] I^- 2 H_2 O + O_2 Theβ¦
Consider the decomposition of hydrogen peroxide $\displaystyle \left(\mathrm{H}_{2} \mathrm{O}_{2}\right)$ as per the equation given below : \[2 \mathrm{H}_{2} \mathrm{O}_{2} \xrightarrow[\text { Alkaline medium }]{\mathrm{I}^{-}} 2 \mathrm{H}_{2} \mathrm{O}+\mathrm{O}_{2} \] The mechanism for this reaction was found to be : Step I : $\displaystyle \mathrm{H}_{2} \mathrm{O}_{2}+\mathrm{I}^{-} \longrightarrow \mathrm{H}_{2} \mathrm{O}+\mathrm{IO}^{-}$(slow) Step II : $\displaystyle \mathrm{H}_{2} \mathrm{O}_{2}+\mathrm{IO}^{-} \longrightarrow \mathrm{H}_{2} \mathrm{O}+\mathrm{I}^{-}+\mathrm{O}_{2}$ (fast)
(a)
Write the rate law expression.
(b)
Determine the order of reaction with respect to $\displaystyle \mathrm{H}_{2} \mathrm{O}_{2}, \mathrm{I}^{-}$and overall order of reaction.
(c)
What is the molecularity of the reaction in Step II ?
Marking-scheme solution
(a)
Rate =π[π»$\displaystyle 2$π$\displaystyle 2$][πΌβ]
(b)
Order w.r.t. [π»$\displaystyle 2$π$\displaystyle 2$] ππ $\displaystyle 1$
Order w.r.t [πΌβ] is $\displaystyle 1$
Overall order of reaction is $\displaystyle 2$
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CBSE Class 12 Chemistry past-paper question from the 2026 board exam, with the answer as CBSEβs own marking scheme gives it. Where our answers come from.