CAIE A-Level Chemistry 26.2 Homogeneous and Heterogeneous Catalysis

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: 6]

(a)

Catalysts may be homogeneous or heterogeneous.

[ 6 ]

(i)

Identify two metals that act as heterogeneous catalysts in the removal of $\mathrm{NO}_{2}$ from the exhaust gases of car engines. and

[ 1 ]

(ii)

Iron acts as a heterogeneous catalyst in the Haber process.

Describe the mode of action of this iron catalyst.

[ 2 ]

(iii)

$\mathrm{Fe}^{2+}$ ions act as a homogeneous catalyst in the reaction between $\mathrm{I}^{-}(\mathrm{aq})$ and $\mathrm{S}_{2} \mathrm{O}_{8}{ }^{2-}(\mathrm{aq})$ .

Write equations for the two reactions that occur when $\mathrm{Fe}^{2+}(\mathrm{aq})$ is added to a mixture of $\mathrm{I}^{-}(\mathrm{aq})$ and $\mathrm{S}_{2} \mathrm{O}_{8}{ }^{2-}(\mathrm{aq})$ .
equation $1 \quad \mathrm{~S}_{2} \mathrm{O}_{8}{ }^{2-}+$
equation 2

[ 2 ]

$\mathrm{S}_{2} \mathrm{O}_{8}{ }^{2-}+2 \mathrm{Fe}^{2+} \rightarrow 2 \mathrm{SO}_{4}{ }^{2-}+2 \mathrm{Fe}^{3+}$ $2 \mathrm{Fe}^{3+}+2 \mathrm{I}^{-} \rightarrow 2 \mathrm{Fe}^{2+}+\mathrm{I}_{2}$

[1] any multiple

[1] any multiple

(iv)

Explain the difference between a homogeneous catalyst and a heterogeneous catalyst.

[ 1 ]

[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

26.2 Homogeneous and heterogeneous catalysts question 1

Question (a)

Question (i)

Question (ii)

Question (iii)

Question (iv)

26.2 Homogeneous and heterogeneous catalysts question 2

26.2 Homogeneous and heterogeneous catalysts question 3