CAIE A-Level Chemistry 26.2.3 Homogeneous Catalyst Mechanism

Practise writing homogeneous catalytic steps in which the catalyst reacts in one step and is regenerated later.

Exam points

write one step that consumes the catalyst and forms a new intermediate or oxidation state
write a later step that reforms the original catalyst while converting the remaining reactant
sum the steps to recover the uncatalysed overall equation and cancel the catalyst/intermediate

[Maximum number: 2]

Iodine is found naturally in compounds in many different oxidation states.

The reaction of $\mathrm{I}^{-}$ ions with persulfate ions, $\mathrm{S}_{2} \mathrm{O}_{8}{ }^{2-}$ , can be catalysed by $\mathrm{Fe}^{3+}$ ions.

2 \mathrm{I}^{-}+\mathrm{S}_{2} \mathrm{O}_{8}^{2-} \rightarrow \mathrm{I}_{2}+2 \mathrm{SO}_{4}^{2-}

Write equations to show how $\mathrm{Fe}^{3+}$ catalyses this reaction.

$\begin{aligned} \text { Reduction of } \mathrm{Fe}^{3+}: \quad 2 \mathrm{Fe}^{3+}+2 \mathrm{I}^{-} \rightarrow 2 \mathrm{Fe}^{2+}+\mathrm{I}_{2}[1] \text { Regeneration of } \mathrm{Fe}^{3+}: 2 \mathrm{Fe}^{2+}+\mathrm{S}_{2} \mathrm{O}_{8}^{2-} \rightarrow 2 \mathrm{Fe}^{3+}+2 \mathrm{SO}_{4}^{2-}[1] \end{aligned}$

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26.2.3—Mode of action of a homogeneous catalyst question 1