CAIE A-Level Chemistry 26.1.5 Mechanisms and Rate-Determining Steps

Practise checking mechanisms against a rate equation and overall reaction and identifying intermediates or catalysts.

Exam points

sum elementary steps and cancel intermediates or catalysts to recover the overall equation
match the molecularity of the slow step, or its derived precursor relation, to the observed rate law
identify an intermediate as produced then consumed and a catalyst as consumed then regenerated

[Maximum number: 2]

(a)

A four-step mechanism is suggested for the reaction between hydrogen peroxide and iodide ions in an acidic solution.

$step $1 \quad \mathrm{H}_{2} \mathrm{O}_{2}+\mathrm{I}^{-} \rightarrow \mathrm{IO}^{-}+\mathrm{H}_{2} \mathrm{O}$$

$step $2 \mathrm{H}^{+}+\mathrm{IO}^{-} \rightarrow \mathrm{HIO}$$

$step $3 \mathrm{HIO}+\mathrm{I}^{-} \rightarrow \mathrm{I}_{2}+\mathrm{OH}^{-}$$

$step $4 \mathrm{OH}^{-}+\mathrm{H}^{+} \rightarrow \mathrm{H}_{2} \mathrm{O}$$

Step 1 is the rate-determining step.

[ 2 ]

(i)

Use this mechanism to construct a balanced equation for this reaction.

[ 1 ]

$\begin{aligned} \mathrm{H}_{2} \mathrm{O}_{2}+2 \mathrm{H}^{+}+2 \mathrm{I}^{-} \rightarrow \mathrm{I}_{2}+2 \mathrm{H}_{2} \mathrm{O} \mathrm{OR} \mathrm{H}_{2} \mathrm{O}_{2}+2 \mathrm{HI} \rightarrow \mathrm{I}_{2}+2 \mathrm{H}_{2} \mathrm{O} \end{aligned}$

(ii)

Deduce the order of reaction with respect to each of the following.
$\mathrm{H}_{2} \mathrm{O}_{2}=$ $\mathrm{I}^{-}=$

\mathrm{H}^{+}=

[ 1 ]

$\mathrm{H}_{2} \mathrm{O}_{2}=1$ AND $\mathrm{I}^{-}=1$ AND $\mathrm{H}^{+}=0$

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26.1.5—Mechanisms and rate-determining step question 1

Question (a)

Question (i)

Question (ii)