AQA A-Level Chemistry Paper 3, June 2018: Question 23

1 mark · Easy difficulty · Multiple Choice

Identify the role of nitric acid in the reaction generating the nitronium ion for the nitration of benzene.

Practise this question

Question

Multiple-choice question 23 stating: 'The nitration of benzene uses a nitrating mixture of concentrated nitric acid and concentrated sulfuric acid.' Followed by the equation: HNO3 + 2H2SO4 -> NO2+ + H3O+ + 2HSO4-. It asks 'Which statement is correct?' with options: A: HNO3 acts as a base, B: HNO3 acts as a catalyst, C: HNO3 acts as an electrophile, D: HNO3 acts as a reducing agent.
Question text

23 The nitration of benzene uses a nitrating mixture of concentrated nitric acid and

concentrated sulfuric acid.

HNO + 2H SO NO + + H O+ + 2HSO –

32 4 2 3 4

Which statement is correct?

[1 mark]

A HNO3 acts as a base.

B HNO3 acts as a catalyst.

C HNO3 acts as an electrophile.

D HNO3 acts as a reducing agent.

Mark scheme

Show the mark scheme Mark scheme table row showing question 23 with the correct answer given as A.

23 A

How to answer it

Nitration of Benzene: Generation of the Electrophile

📋 What this question tests

This question assesses your understanding of the mechanism of electrophilic aromatic substitution, specifically the initial generation of the nitronium ion ( NO₂⁺ ) electrophile. You must apply Brønsted-Lowry acid-base theory in a non-aqueous mixture of concentrated acids and identify the specific chemical roles of each reactant species.

Question 23 • 1 Mark

Role of Reactants in the Nitrating Mixture

Reaction equation: HNO₃ + 2H₂SO₄ → NO₂⁺ + H₃O⁺ + 2HSO₄⁻

✅ Correct Answer

A: HNO₃ acts as a base.

Mark Scheme: Option A (1 mark)

💡 Key Knowledge

  • Brønsted-Lowry definitions: An acid is a proton ( H⁺ ) donor; a base is a proton ( H⁺ ) acceptor.
  • Acid strength comparison: Concentrated sulfuric acid ( H₂SO₄ ) is a significantly stronger acid than concentrated nitric acid ( HNO₃ ).
  • Because H₂SO₄ is stronger, it forces HNO₃ to accept a proton, forming the intermediate protonated nitric acid ion ( H₂NO₃⁺ ), which rapidly loses water to yield the nitronium ion ( NO₂⁺ ).
  • The produced H₂O is then protonated by a second molecule of H₂SO₄ to give H₃O⁺ and HSO₄⁻ .

🧠 Exam Technique & Step-by-Step Breakdown

Trace the proton transfers occurring in the reaction mechanism:

  1. Protonation: HNO₃ + H₂SO₄ ⇌ H₂NO₃⁺ + HSO₄⁻
    Here, HNO₃ accepts H⁺ → behaves as a base.
  2. Loss of water: H₂NO₃⁺ ⇌ NO₂⁺ + H₂O
  3. Hydronium formation: H₂O + H₂SO₄ ⇌ H₃O⁺ + HSO₄⁻
  4. Overall: Adding these together gives the question's equation. H₂SO₄ donates protons (acid), while HNO₃ accepts a proton (base).

❌ Why the Other Options Are Incorrect

  • B is wrong: HNO₃ is a reactant that is permanently altered and consumed to provide the -NO₂ group. H₂SO₄ is the catalyst because it is reformed when the arenium intermediate loses a proton to HSO₄⁻ .
  • C is wrong: The electrophile is the nitronium ion ( NO₂⁺ ), not uncharged HNO₃ .
  • D is wrong: This is an acid-base and elimination process, not a redox reaction. The oxidation state of nitrogen is +5 in both HNO₃ and NO₂⁺ .

Topics

Organic Chemistry · Physical Chemistry · 3.3.10 Aromatic Chemistry · 3.1.12 Acids and Bases

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.