CAIE A-Level Chemistry A2 26.2 Homogeneous and Heterogeneous Catalysts Questions

Practise classifying catalysts by phase and explaining surface pathways or regenerated homogeneous intermediates.

Exam points

classify a catalyst as homogeneous when it shares the reactant phase and heterogeneous when it does not
for a solid surface, describe adsorption, bond weakening/reaction and product desorption
for a homogeneous pathway, write steps showing the catalyst consumed and later regenerated

Covered syllabus points

26.2.1—Catalysts can be homogeneous or heterogeneous 26.2.2—Heterogeneous catalyst action 26.2.3—Mode of action of a homogeneous catalyst

[Maximum number: 2]

(a)

Catalysts can be heterogeneous or homogeneous.

Explain what is meant by a homogeneous catalyst.

[ 1 ]

(b)

Complete the table by placing one tick $(\checkmark)$ in each row to indicate the mode of action of the catalyst in each reaction.

Table for Question (b) — CAIE A-Level Chemistry A2

[ 1 ]

$\mathrm{Rh}$ : heterogeneous; $\mathrm{Fe}^{3+}$ : homogeneous.

[Maximum number: 2]

Hypophosphorous acid is an inorganic acid.
The conjugate base of hypophosphorous acid is $\mathrm{H}_{2} \mathrm{PO}_{2}^{-}$ .

A student suggests that the reaction between $\mathrm{H}_{2} \mathrm{PO}_{2}^{-}(\mathrm{aq})$ and $\mathrm{OH}^{-}(\mathrm{aq})$ might happen more quickly in the presence of a heterogeneous catalyst.

Describe the mode of action of a heterogeneous catalyst.

[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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Covered syllabus points

Question 1

Question (a)

Question (b)

Question 2

Question 3