Which two features of transition metals explain their catalytic activity?

They have variable oxidation states, and their 3d and 4s electrons can form weak bonds to reactant molecules at a surface.

Explain how a variable oxidation state allows a transition metal ion to catalyse a reaction in solution.

The ion is oxidised by one reactant and then reduced back by the other, so it takes part in two faster steps and is regenerated unchanged.

Which transition metal ion catalyses the reaction between peroxodisulfate ions and iodide ions?

Iron(II) ions catalyse it, cycling between Fe²⁺ and Fe³⁺.

Write the equation for the first step of that catalysed reaction.

The equation is S₂O₈²⁻ + 2Fe²⁺ → 2SO₄²⁻ + 2Fe³⁺.

Write the equation for the second step of that catalysed reaction.

The equation is 2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂.

What is the overall equation, and what has happened to the catalyst by the end?

The overall equation is S₂O₈²⁻ + 2I⁻ → 2SO₄²⁻ + I₂, and the iron(II) is regenerated in the second step so it is not used up.

OCR A-Level Chemistry B H433

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Developing Metals

Catalytic activity of transition metals

3 things to know
Components 01, 02 and 03

Overview

Recall questions on how a variable oxidation state allows catalysis, the two steps of the iodide and peroxodisulfate route, and adsorption strength.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Numbers

How does a variable oxidation state let a transition metal ion catalyse a reaction, and what are the two steps when iron ions catalyse peroxodisulfate reacting with iodide?

Process

Why is the peroxodisulfate and iodide reaction slow uncatalysed, and what happens if adsorption is too strong or too weak?

Key term

Which transition metals are used as catalysts in a car exhaust catalytic converter?

Questions from this subtopic

Each one with the model answer underneath.

1
Which two features of transition metals explain their catalytic activity?
Model answer
They have variable oxidation states, and their 3d and 4s electrons can form weak bonds to reactant molecules at a surface.
2
Explain how a variable oxidation state allows a transition metal ion to catalyse a reaction in solution.
Model answer
The ion is oxidised by one reactant and then reduced back by the other, so it takes part in two faster steps and is regenerated unchanged.
3
Which transition metal ion catalyses the reaction between peroxodisulfate ions and iodide ions?
Model answer
Iron(II) ions catalyse it, cycling between Fe²⁺ and Fe³⁺.
4
Write the equation for the first step of that catalysed reaction.
Model answer
The equation is S₂O₈²⁻ + 2Fe²⁺ → 2SO₄²⁻ + 2Fe³⁺.
5
Write the equation for the second step of that catalysed reaction.
Model answer
The equation is 2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂.
6
What is the overall equation, and what has happened to the catalyst by the end?
Model answer
The overall equation is S₂O₈²⁻ + 2I⁻ → 2SO₄²⁻ + I₂, and the iron(II) is regenerated in the second step so it is not used up.

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