OCR A-Level Chemistry AS Breadth in chemistry (01), November 2020: Question 6

1 mark · Medium difficulty · Multiple Choice

Identify which species is oxidised and reduced in the given redox equation involving chlorine-containing compounds.

Practise this question

Question

Multiple choice question 6 shows the equation: 2HClO3 + 2HCl -> 2ClO2 + Cl2 + 2H2O. Four options A, B, C, D describe whether Cl and O are oxidised or reduced.
Question text

6 The equation for a redox reaction is shown below.

2HClO3 + 2HCl → 2ClO2 + Cl2 + 2H2O

Which statement is correct?

A Cl is both oxidised and reduced.

B Cl is oxidised and O is reduced.

C O is both oxidised and reduced.

D O is oxidised and Cl is reduced.

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme indicates the correct answer is A.

6 A 1 1.2

How to answer it

Identifying Oxidation States in Redox Reactions

📌 What this question tests

This question assesses your ability to assign oxidation numbers to elements in different compounds and ions within a chemical equation, and to use those changes to identify which species has been oxidised, reduced, or both (disproportionation).

Question 6: Multiple Choice Analysis

Equation: 2HClO₃ + 2HCl → 2ClO₂ + Cl₂ + 2H₂O

✅ Correct Answer: A

Cl is both oxidised and reduced.

Chlorine starts in two different oxidation states on the left-hand side and changes into products where its oxidation state both increases and decreases.

💡 Key Knowledge (Oxidation States)

  • Hydrogen is almost always +1 when bonded to non-metals.
  • Oxygen is almost always -2 (except in peroxides or fluorine compounds).
  • Apply the rule: Sum of oxidation states in a neutral compound equals 0 .
  • Elemental substances (like Cl₂ ) have an oxidation state of 0 .

🧠 Exam Technique & Step-by-Step Breakdown

  1. Analyze HClO₃: H = +1, O = 3 × (-2) = -6. Therefore, Cl must be +5 to balance the neutral molecule.
  2. Analyze HCl: H = +1, so Cl must be -1 .
  3. Analyze ClO₂: O = 2 × (-2) = -4. Therefore, Cl must be +4 .
  4. Analyze Cl₂: It is an uncombined element, so Cl is 0 .
  5. Compare: Cl goes from +5 to +4 (reduction, decrease) AND from -1 to 0 (oxidation, increase). Thus, chlorine is both oxidised and reduced.

❌ Common Errors & Misconceptions

  • Ignoring Hydrogen and Oxygen: Assuming oxygen changes its oxidation state. Here, oxygen remains at -2 throughout the reaction (in HClO₃ , ClO₂ , and H₂O ), ruling out options C and D.
  • Misidentifying Element States: Forgetting that Cl₂ is an element and giving it an oxidation state based on its chloride ion value ( -1 ) instead of 0 .
Mark Allocation: [1 mark] awarded for selecting option A. Examiner reports indicate this tests core synoptic capability in redox identification without numerical calculation.

Topics

Module 2: Foundations in chemistry · 2.1 Atoms and reactions

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.