OCR A-Level Chemistry AS Breadth in chemistry (01), June 2018: Question 7

1 mark · Medium difficulty · Multiple Choice

Identify which given chemical reaction shows the oxidation of sulfur.

Practise this question

Question

Multiple choice question 7 asking 'Which reaction shows oxidation of sulfur?' with four options A, B, C, and D showing various chemical equations involving sulfur-containing compounds and halogens.
Question text

7 Which reaction shows oxidation of sulfur?

A 2HBr + H2SO4 → SO2 + 2H2O + Br2

B SO2 + 2NaOH → Na2SO3 + H2O

C 8HI + H2SO4 → 4I2 + H2S + 4H2O

D H2S + Cl2 → 2HCl + S

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme indicating the correct answer is D for question 7, worth 1 mark.

7 D 1

How to answer it

Identifying Oxidation States in Sulfur Reactions

What this question tests

This question assesses your ability to determine and apply oxidation states to identify redox changes—specifically oxidation (an increase in oxidation number)—in sulfur-containing chemical species.

Question 7

Multiple Choice Analysis

✅ Correct Answer: D

Reaction D shows hydrogen sulfide reacting with chlorine: H₂S + Cl₂ → 2HCl + S .

The sulfur in H₂S has an oxidation state of -2 , and it turns into elemental sulfur ( S ) with an oxidation state of 0 . Because the oxidation number increases from -2 to 0, sulfur is oxidised.

💡 Key Knowledge

  • Oxidation is defined as an increase in oxidation number (loss of electrons).
  • Elements in their standard state (like S or Cl₂ ) always have an oxidation state of 0 .
  • Hydrogen usually has a +1 oxidation state when bonded to non-metals.
  • Oxygen usually has a -2 oxidation state in compounds.

🧠 Exam Technique

Do not guess! Quickly calculate the oxidation state of sulfur in both the reactants and the products for each option. Eliminate options where sulfur's oxidation number stays the same or decreases (reduction).

❌ Common Errors

  • Confusing oxidation with reduction (looking for a decrease instead of an increase in oxidation number).
  • Miscalculating starting oxidation states, especially in complex molecules like H₂SO₄ where sulfur is already at its maximum oxidation state of +6 (so it can only be reduced, never oxidised).

🔍 Breakdown of All Options (Why A, B, and C are incorrect):

  • A: 2HBr + H₂SO₄ → SO₂ + 2H₂O + Br₂ — Sulfur goes from +6 in H₂SO₄ to +4 in SO₂ . This is reduction.
  • B: SO₂ + 2NaOH → Na₂SO₃ + H₂O — Sulfur is +4 in SO₂ and remains +4 in Na₂SO₃ . This is an acid-base/neutralisation reaction, not a redox reaction.
  • C: 8HI + H₂SO₄ → 4I₂ + H₂S + 4H₂O — Sulfur goes from +6 in H₂SO₄ down to -2 in H₂S . This is reduction.
Mark Scheme Allocation: 1 mark awarded for selecting D.

Topics

Module 3: Periodic table and energy · 3.1 The periodic table

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.