AQA A-Level Chemistry Paper 3, 2021: Question 21

1 mark · Medium difficulty · Multiple Choice

This question checks your understanding of redox reactions and the role of transition metals. You are asked to identify which chemical equation does not involve the reduction of a transition metal—meaning the oxidation state of the metal does not decrease during the reaction. It requires knowledge of oxidation states and recognising transition metal redox changes.

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Question

AQA A-Level Chemistry Paper 3, 2021: Question 21
Question text

21 Which equation does not show the reduction of a transition metal?

[1 mark]

A TiCl4 + 2 Mg → Ti + 2 MgCl2

B 2 FeCl3 + 2 KI → 2 FeCl2 + 2 KCl + I2

C MnO2 + 4 HCl → MnCl2 + Cl2 + 2 H2O

D CoO + 4 HCl → [CoCl ]2‒ + H O + 2 H+

Mark scheme

Show the mark scheme Mark scheme for AQA A-Level Chemistry Paper 3, 2021: Question 21

21 D 1 CoO + 4 HCl → [CoCl ]2‒ + H O + 2 H+

How to answer it

Multiple Choice Explained: Transition Metal Reduction

Correct Answer: D – CoO + 4 HCl → [CoCl4]2− + H2O + 2 H+

Why? This is not a reduction. The cobalt remains in the +2 oxidation state before and after the reaction. It simply forms a complex ion – there is no change in oxidation state.

Why A is incorrect:

TiCl4 + 2 Mg → Ti + 2 MgCl2 – Titanium is reduced from +4 in TiCl4 to 0 in elemental titanium. This is a reduction of a transition metal.

Why B is incorrect:

2 FeCl3 + 2 KI → 2 FeCl2 + 2 KCl + I2 – Iron is reduced from +3 to +2, so again this is a reduction of a transition metal.

Why C is incorrect:

MnO2 + 4 HCl → MnCl2 + Cl2 + 2 H2O – Manganese is reduced from +4 in MnO2 to +2 in MnCl2. This is also a clear example of reduction.

Topics

Physical Chemistry · Inorganic Chemistry · 3.1.7 Oxidation, Reduction, and Redox Equations · 3.2.5 Transition Metals

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.