OCR A-Level Chemistry AS Breadth in chemistry (01), June 2019: Question 9

1 mark · Medium difficulty · Multiple Choice

Identify which given chemical reaction shows chlorine only being oxidised

Practise this question

Question

Multiple choice question 9 asking which reaction shows chlorine only being oxidised, with four options A, B, C, D showing chemical equations involving chlorine species, followed by an answer box.
Question text

9 Which reaction shows chlorine only being oxidised?

A Cl2 + H2O → HCl + HClO

B 2ClO2 + 2NaOH → NaClO2 + NaClO3 + H2O

C 4KClO3 → 3KClO4 + KCl

D MnO2 + 4HCl → MnCl2 + Cl2 + 2H2O

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme table shows the correct answer for question 9 is D, worth 1 mark.

9 D 1 AO2.2

How to answer it

Identifying Oxidation of Chlorine

What this question tests

This question assesses your ability to assign oxidation numbers to elements in different compounds, recognise disproportionation reactions, and specifically identify when a single element undergoes only oxidation (an increase in oxidation number) across a chemical reaction.

Question 9

Exam Breakdown & Analysis

✅ Correct Answer: D

Reaction D shows chlorine going exclusively from an oxidation state of -1 in HCl to 0 in Cl₂ . There is no reduction occurring to any other chlorine atom.

💡 Key Knowledge

  • Oxidation is an increase in oxidation number (loss of electrons).
  • Reduction is a decrease in oxidation number (gain of electrons).
  • Uncombined elements (like Cl₂ ) always have an oxidation number of 0 .
  • In most compounds, chlorine has an oxidation number of -1 (e.g. HCl ).

🧠 Exam Technique

Do not try to guess! Systematically write out the oxidation numbers of the target element (chlorine) for every single species in the given options before making your final choice.

❌ Common Errors

Students often confuse disproportionation (where an element is simultaneously oxidised and reduced, such as in options A, B, and C) with a reaction where an element only undergoes a single redox process.

Mark Scheme Allocation: 1 mark available for selecting option D (AO2.2 - Applying knowledge of redox principles to unfamiliar equations).

Step-by-Step Analysis of Options

Option A: Cl₂ + H₂O → HCl + HClO

Chlorine starts at 0 in Cl₂ . In HCl it is -1 (reduced), and in HClO it is +1 (oxidised). This is a disproportionation reaction, not solely oxidation.

Option B: 2ClO₂ + 2NaOH → NaClO₂ + NaClO₃ + H₂O

Chlorine starts at +4 in ClO₂ . It changes to +3 in NaClO₂ (reduced) and +5 in NaClO₃ (oxidised). Another disproportionation example.

Option C: 4KClO₃ → 3KClO₄ + KCl

Chlorine starts at +5 in KClO₃ . It is reduced to -1 in KCl and oxidised to +7 in KClO₄ . Again, simultaneous oxidation and reduction occurs.

Option D: MnO₂ + 4HCl → MnCl₂ + Cl₂ + 2H₂O

The chlorine in HCl has an oxidation number of -1 . In the products, it forms Cl₂ where its oxidation number is 0 . Since the oxidation number increases from -1 to 0 and no other chlorine is reduced, chlorine is only oxidised.

Topics

Module 2: Foundations in chemistry · Module 3: Periodic table and energy · 2.1 Atoms and reactions · 3.1 The periodic table

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.