OCR A-Level Chemistry AS Breadth in chemistry (01), June 2025: Question 7

1 mark · Easy difficulty · Multiple Choice

Identify which reaction from a given list of chemical equations is not a redox reaction.

Practise this question

Question

Question 7 asks: 'Which of the following is not a redox reaction?' followed by four options: A: CH4(g) + 2O2(g) arrow CO2(g) + 2H2O(l); B: NaOH(aq) + HCl(aq) arrow NaCl(aq) + H2O(l); C: 2KClO3(s) arrow 2KCl(s) + 3O2(g); D: Fe(s) + H2SO4(aq) arrow FeSO4(aq) + H2(g). Below is an answer box and the mark allocation of [1].
Question text

7 Which of the following is not a redox reaction?

A CH4(g) + 2O2(g) CO2(g) + 2H2O(l)

B NaOH(aq) + HCl(aq) NaCl(aq) + H2O(l)

C 2KClO3(s) 2KCl(s) + 3O2(g)

D Fe(s) + H2SO4(aq) FeSO4(aq) + H2(g)

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme snippet showing the correct answer for question 7 is 'B', worth 1 mark.

7 B 1

How to answer it

Identifying Redox vs Non-Redox Reactions

📌 What this question tests

This question assesses your understanding of oxidation numbers and ability to differentiate between redox processes and neutralisation reactions:

  • Assigning oxidation numbers to elements in neutral compounds, elemental states, and ions.
  • Recognising that a reaction is only redox if both oxidation (increase in oxidation number) and reduction (decrease in oxidation number) take place.
  • Rapidly identifying acid-base neutralisations as non-redox processes involving proton transfer rather than electron transfer.

Question 7

Multiple Choice: Identification of a Non-Redox Reaction

✅ Correct Answer

B: NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l) [1 mark]

This is an acid-base neutralisation reaction. There is no change in oxidation state for any element involved:

  • Na: +1 → +1 (no change)
  • O: -2 → -2 (no change)
  • H: +1 → +1 (no change)
  • Cl: -1 → -1 (no change)

📐 Oxidation State Analysis for All Options

Option & Equation Oxidation State Changes Reaction Type
A: CH₄ + 2O₂ → CO₂ + 2H₂O C changes from -4 to +4 (oxidised)
O changes from 0 to -2 (reduced)		 Redox (Combustion)
B: NaOH + HCl → NaCl + H₂O Na (+1), O (-2), H (+1), Cl (-1) throughout.
Zero changes in oxidation states.		 NOT Redox (Neutralisation)
C: 2KClO₃ → 2KCl + 3O₂ Cl changes from +5 to -1 (reduced)
O changes from -2 to 0 (oxidised)		 Redox (Thermal decomposition / disproportionation-like)
D: Fe + H₂SO₄ → FeSO₄ + H₂ Fe changes from 0 to +2 (oxidised)
H changes from +1 to 0 (reduced)		 Redox (Metal-acid displacement)

💡 Key Knowledge: Quick Redox Rules

  • Uncombined elements: Any uncombined element (e.g. O₂, Fe, H₂) has an oxidation number of 0.
  • Formation or consumption of elements: If an uncombined element appears on one side and is in a compound on the other side, the reaction is guaranteed to be redox!
  • Neutralisation: Standard Arrhenius/Brønsted-Lowry neutralisations ( H⁺ + OH⁻ → H₂O ) never involve electron transfer, only proton transfer.

🧠 Exam Technique: The "Element Elimination" Shortcut

You have only around 60–90 seconds per multiple-choice question. Do not calculate every single oxidation state if you don't need to:

  • Scan for free elements with an oxidation state of 0:
    • Option A has O₂ (0) → Redox!
    • Option C has O₂ (0) → Redox!
    • Option D has Fe (0) and H₂ (0) → Redox!
  • By quick elimination in under 10 seconds, only B has no uncombined elements and is clearly an acid-base neutralisation.

❌ Common Errors & Misconceptions

  • Misreading the question: Overlooking the word "not" in bold. Students often choose A because it is a very obvious, familiar redox reaction.
  • Confusion over Option C: Many students struggle to assign the oxidation number of Cl in potassium chlorate(V), KClO₃ :
    (+1) + (Cl) + 3(-2) = 0 ⇒ Cl = +5 . Because both products come from one reactant, some incorrectly assume it is not a redox reaction.
  • Confusing proton transfer with electron transfer: Forgetting that acid-base reactions involve the movement of H⁺ ions, not electron transfer.
Examiner Insight: Multiple choice questions testing redox frequently feature a standard acid + base or precipitation reaction as the distractor/non-redox example. Spotting familiar non-redox types (neutralisation and precipitation) is a high-yield exam skill.

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), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.