OCR A-Level Chemistry AS Breadth in chemistry (01), June 2024: Question 9

1 mark · Medium difficulty · Multiple Choice

Identify which reaction does not show disproportionation of chlorine from four given chemical equations.

Practise this question

Question

A multiple-choice question numbered 9 asking which reaction does not show disproportionation of chlorine. Four options A, B, C, and D are listed with chemical equations: A is MnO2 + 4HCl -> MnCl2 + Cl2 + 2H2O, B is Cl2 + H2O -> HCl + HClO, C is 2ClO2 + 2NaOH -> NaClO2 + NaClO3 + H2O, and D is 2NaOH + Cl2 -> NaCl + NaClO + H2O. A box for the student's answer and the mark allocation of [1] are at the bottom.
Question text

9 Which reaction does not show disproportionation of chlorine?

A MnO2 + 4HCl MnCl2 + Cl2 + 2H2O

B Cl2 + H2O HCl + HCl O

C 2Cl O2 + 2NaOH NaCl O2 + NaCl O3 + H2O

D 2NaOH + Cl2 NaCl + NaCl O + H2O

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme table shows question number 9 with the correct answer A and 1 mark.

9 A 1

How to answer it

Identifying Non-Disproportionation Reactions

What this question tests

This question tests your understanding of disproportionation—a specific type of redox reaction where a single element is simultaneously oxidized and reduced. To solve it, you must correctly assign oxidation numbers to chlorine across multiple reactions and identify the one exception where disproportionation does not occur.

Question 9 Analysis

Identifying Reactions That Do Not Show Disproportionation

✅ Correct Answer: A

Reaction A ( MnO₂ + 4HCl → MnCl₂ + Cl₂ + 2H₂O ) is not disproportionation. Here, chlorine in HCl has an oxidation state of -1 and remains at 0 in Cl₂ (oxidation only, caused by MnO₂). It is a standard redox reaction, but not disproportionation.

💡 Key Knowledge: Disproportionation

  • Definition: The oxidation and reduction of the same element in a single chemical reaction.
  • What to look for: One reactant containing an element must split into at least two products where that same element has a higher oxidation number in one and a lower oxidation number in the other.

🧠 Exam Technique

  • Quickly check options B, C, and D: In all three, a single chlorine-containing species ( Cl₂ or ClO₂ ) forms products where chlorine has both increased and decreased its oxidation state.
  • In option A, chlorine only increases from -1 in HCl to 0 in Cl₂ .

❌ Common Errors

  • Confusing general redox reactions with disproportionation. Just because an element changes its oxidation state doesn't mean it's disproportionation—it must be both oxidized and reduced simultaneously from the same starting state.
  • Miscalculating oxidation states in chlorine oxides and oxoanions (like ClO₂ or HClO ).

📐 Breakdown of Oxidation States for Chlorine

Let's verify why options B, C, and D are disproportionation:

  • Option B ( Cl₂ + H₂O → HCl + HClO ): Cl₂ starts at 0 , goes to -1 in HCl (reduced) and +1 in HClO (oxidized). (Disproportionation)
  • Option C ( 2ClO₂ + 2NaOH → NaClO₂ + NaClO₃ + H₂O ): Cl in ClO₂ starts at +4 , goes to +3 in NaClO₂ (reduced) and +5 in NaClO₃ (oxidized). (Disproportionation)
  • Option D ( 2NaOH + Cl₂ → NaCl + NaClO + H₂O ): Cl₂ starts at 0 , goes to -1 in NaCl (reduced) and +1 in NaClO (oxidized). (Disproportionation)
Mark Scheme Award: 1 mark for selecting A.

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 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.