Edexcel IAL Chemistry 11.7 Rate-determining steps

Match rate-law species and powers to reactants involved up to the slow step, then add all steps to confirm the correct overall stoichiometric equation.

Exam points

Reject a proposed slow step when it contains a reactant absent from the measured rate equation.
Derive a rate equation from the molecularity and species in a stated slow elementary step.
Check a mechanism by matching both its rate-determining step and summed overall equation.

[Maximum number: 1]

Bromate (V) ions, $\mathrm{BrO}_{3}^{-}$ , react with bromide ions, $\mathrm{Br}^{-}$ , in aqueous acid.

\mathrm{BrO}_{3}^{-}+5 \mathrm{Br}^{-}+6 \mathrm{H}^{+} \rightarrow 3 \mathrm{Br}_{2}+3 \mathrm{H}_{2} \mathrm{O}

The rate equation for the reaction is shown.

\text { rate }=k\left[\mathrm{BrO}_{3}^{-1}\right]\left[\mathrm{Br}^{-}\right]\left[\mathrm{H}^{+}\right]^{2}

The first four steps in the reaction mechanism are shown.

$Step $\mathbf{1} \mathrm{BrO}_{3}^{-}+\mathrm{H}^{+} \rightarrow \mathrm{BrO}_{3} \mathrm{H}$$

$Step $2 \quad \mathrm{BrO}_{3} \mathrm{H}+\mathrm{H}^{+} \rightarrow \mathrm{BrO}_{3} \mathrm{H}_{2}^{+}$$

$Step $3 \mathrm{BrO}_{3} \mathrm{H}_{2}^{+}+\mathrm{Br}^{-} \rightarrow \mathrm{Br}_{2} \mathrm{O}_{2}+\mathrm{H}_{2} \mathrm{O}$$

$Step $4 \quad \mathrm{Br}_{2} \mathrm{O}_{2}+\mathrm{Br}^{-} \rightarrow \mathrm{BrO}_{2}^{-}+\mathrm{Br}_{2}$$

Which is the rate-determining step?

Step 1

Step 2

Step 3

Step 4

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11.7—Deduce the rate-determining step from a rate equation and vice versa question 1