OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2019: Question 6

1 mark · Easy difficulty · Multiple Choice

Identify which of the four given chemical equations represents a redox reaction.

Practise this question

Question

Multiple-choice question 6 asking 'Which reaction is a redox reaction?'. Four options are given: A: NaCl + AgNO3 -> AgCl + NaNO3, B: NaNO2 + HCl -> NaCl + HNO2, C: CaSO3 + 2HCl -> CaCl2 + H2O + SO2, D: 3CuO + 2NH3 -> 3Cu + 3H2O + N2. A blank box for the answer is shown at the bottom left with '[1]' on the right.
Question text

6 Which reaction is a redox reaction?

A NaCl + AgNO3 AgCl + NaNO3

B NaNO2 + HCl NaCl + HNO2

C CaSO3 + 2HCl CaCl2 + H2O + SO2

D 3CuO + 2NH3 3Cu + 3H2O + N2

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme table shows question number 6 with the correct answer option D, awarding 1 mark under assessment objective AO1.2.

6 D 1 AO1.2

How to answer it

Identifying Redox Reactions

What this question tests

This question assesses your ability to apply rules for assigning oxidation states and using them to identify oxidation and reduction (redox) processes in chemical equations. Specifically, it tests whether you can distinguish between precipitation, acid-base, thermal decomposition/displacement, and true redox reactions by tracking changes in oxidation numbers.

Question 6 Multiple Choice

Core Solution & Analysis

✅ Correct Answer: D

Reaction D ( 3CuO + 2NH₃ → 3Cu + 3H₂O + N₂ ) is a redox reaction because copper is reduced (oxidation state decreases from +2 to 0) and nitrogen is oxidized (oxidation state increases from -3 to 0).

💡 Key Knowledge

  • Redox definition: A reaction in which both reduction (decrease in oxidation number) and oxidation (increase in oxidation number) occur simultaneously.
  • Elements in standard states: Uncombined elements like Cu and N₂ always have an oxidation state of 0.
  • Compound checks: In CuO , Cu is +2 and O is -2. In NH₃ , N is -3 and H is +1.

🧠 Exam Technique

Scan the multiple-choice options for uncombined elements on either the reactant or product side (e.g., Cu and N₂ in option D). Since elements in their standard states have an oxidation state of 0, formation or consumption of free elements is a massive shortcut and a dead giveaway for a redox reaction!

❌ Common Errors

Students often waste time fully working out every single oxidation state for options A, B, and C when looking for a quick elimination. Remember that neutralisation reactions (like option B forming an acid from a salt) and precipitation reactions (like option A) never involve changes in oxidation states.

Mark Scheme Allocation: 1 mark awarded for selecting D (AO1.2 demonstration of knowledge and understanding).

Topics

Module 2: Foundations in chemistry · 2.1 Atoms and reactions

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