AQA A-Level Chemistry Paper 3, 2017: Question 17

1 mark · Easy difficulty · Multiple Choice

Identify the step in the Contact Process in which the vanadium catalyst is oxidised.

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Question

Multiple-choice question 17: 'V2O5 can be used as a catalyst in the Contact Process. Which is a step in the Contact Process in which the vanadium is oxidised?' Four options are given: A: SO2 + V2O5 -> SO3 + 2VO2; B: SO3 + 2VO2 -> SO2 + V2O5; C: 2VO2 + 1/2 O2 -> V2O5; D: V2O5 -> 2VO2 + 1/2 O2.
Question text

17 V2O5 can be used as a catalyst in the Contact Process.

Which is a step in the Contact Process in which the vanadium is oxidised?

[1 mark]

A SO2 + V2O5 → SO3 + 2VO2

B SO3 + 2VO2 → SO2 + V2O5

C 2VO2 + O2 → V2O5

D 1

V2O5 → 2VO2 + O2

Mark scheme

Show the mark scheme Mark scheme table row showing question 17 with the correct answer as option C.

17 C

How to answer it

Vanadium(V) Oxide in the Contact Process

📋 What this question tests

This question assesses knowledge of heterogeneous catalysis and the variable oxidation states of transition metals, specifically:

  • The role and mechanism of V₂O₅ in the Contact Process.
  • The two-step redox cycle involved in the catalytic oxidation of SO₂ to SO₃.
  • Tracking oxidation states of transition metal species during a catalytic cycle.
Question 17 [1 Mark]

Identifying the Step in which Vanadium is Oxidised

AQA A-Level Chemistry • Inorganic Chemistry: Transition Metals

✅ Correct Answer: C

2VO₂ + ½O₂ → V₂O₅

  • In VO₂, the oxidation state of V is +4.
  • In V₂O₅, the oxidation state of V is +5.
  • Vanadium increases in oxidation state from +4 to +5, which represents oxidation.
  • This regeneration step re-forms the original catalyst.

💡 Key Knowledge: The Contact Process Mechanism

The overall reaction is:

SO₂ + ½O₂ → SO₃

This takes place via a two-step heterogeneous catalytic cycle:

  1. Reduction step: V₂O₅ oxidises SO₂ to SO₃ while vanadium is reduced from +5 to +4:
    SO₂ + V₂O₅ → SO₃ + 2VO₂
  2. Oxidation / Regeneration step: Oxygen re-oxidises VO₂ back to V₂O₅ (+4 to +5):
    2VO₂ + ½O₂ → V₂O₅

🧠 Exam Technique: Step-by-Step Option Elimination

  • Option A: While this is a real step in the Contact Process, V changes from +5 in V₂O₅ to +4 in VO₂. This is reduction, not oxidation.
  • Option B: This is the reverse of step 1, not a step in the Contact Process.
  • Option C: V changes from +4 in VO₂ to +5 in V₂O₅. This is an increase in oxidation state (oxidation) and matches the actual regeneration step.
  • Option D: Thermal decomposition of V₂O₅; not part of the Contact Process catalytic cycle.

❌ Common Errors & Pitfalls

  • Confusing the steps: Selecting option A because it contains the main reactant (SO₂) and catalyst (V₂O₅), without reading that the question specifically asks for the step where vanadium is oxidised.
  • Miscalculating oxidation numbers:
    • In VO₂: V + 2(-2) = 0 ⇒ V = +4
    • In V₂O₅: 2V + 5(-2) = 0 ⇒ V = +5
  • Forgetting the definition of a catalyst: A catalyst must be regenerated at the end of the reaction, which is precisely what step C accomplishes.
Mark Scheme Award: [1 mark] for selecting option C.

Topics

Inorganic Chemistry · Physical Chemistry · 3.2.5 Transition Metals · 3.1.7 Oxidation, Reduction and Redox Equations

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.