OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2024: Question 6
1 mark · Medium difficulty · Multiple Choice
Identify which given equation does not represent a disproportionation reaction.
Practise this questionQuestion
Question text
6 Which equation does not represent a disproportionation reaction?
A Cl2 + H2O HClO + HCl
B Cl2 + 2NaOH NaClO + NaCl + H2O
C 4KClO3 KCl + 3KClO4
D 4HCl + MnO2 MnCl2 + Cl2 + 2H2O
Your answer [1]
Mark scheme
Show the mark scheme
6 D 1
How to answer it
Identifying Disproportionation Reactions
What this question tests
This question assesses your ability to define, identify, and analyze disproportionation reactions using oxidation states. You must be able to track changes in oxidation numbers across different chemical species in both halogen and transition metal redox equations to determine whether a single element is simultaneously oxidized and reduced.
Exam Breakdown
✅ Correct Answer: D
Equation D ( 4HCl + MnO₂ → MnCl₂ + Cl₂ + 2H₂O ) is not a disproportionation reaction. Instead, it is a standard redox reaction where chlorine is only oxidized (from -1 in HCl to 0 in Cl₂) and manganese is only reduced (from +4 in MnO₂ to +2 in MnCl₂).
💡 Key Knowledge
- Disproportionation Definition: A reaction where the same element is simultaneously oxidized and reduced.
- Halogen Chemistry: Reactions of halogens with water (A) and cold dilute alkali (B), as well as thermal decomposition of chlorates(V) (C), are classic OCR examples of disproportionation.
- Oxidation State Rule: Always assign oxidation numbers to every atom of the key element on both reactant and product sides.
🧠 Exam Technique
Don't waste time checking every single element in every option. Focus immediately on the element undergoing redox changes (chlorine in options A, B, and C; chlorine and manganese in option D). Look for an option where an element appears in two or more different products with both higher and lower oxidation states than in the single reactant.
❌ Common Errors
- Assuming any equation with multiple chlorine-containing products must be disproportionation.
- Confusing standard displacement or redox reactions (like option D, where reactants feature two different elements undergoing separate oxidation and reduction) with disproportionation involving a single reactant species.
Step-by-Step Analysis of All Options
- Option A ( Cl₂ + H₂O → HClO + HCl ): Chlorine goes from oxidation state 0 in Cl₂ to +1 in HClO (oxidation) and -1 in HCl (reduction). This is disproportionation.
- Option B ( Cl₂ + 2NaOH → NaClO + NaCl + H₂O ): Chlorine goes from oxidation state 0 in Cl₂ to +1 in NaClO (oxidation) and -1 in NaCl (reduction). This is disproportionation.
- Option C ( 4KClO₃ → KCl + 3KClO₄ ): Chlorine goes from oxidation state +5 in KClO₃ to -1 in KCl (reduction) and +7 in KClO₄ (oxidation). This is disproportionation.
- Option D ( 4HCl + MnO₂ → MnCl₂ + Cl₂ + 2H₂O ): Chlorine in HCl is -1 and goes to 0 in Cl₂ (oxidation only). Manganese in MnO₂ is +4 and goes to +2 in MnCl₂ (reduction only). Because two separate elements are changing oxidation states (one getting oxidized, one getting reduced), this is NOT disproportionation.
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, Periodic table, elements and physical chemistry (01), June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.