AQA A-Level Chemistry AS Paper 2, June 2025: Question 9

1 mark · Easy difficulty · Multiple Choice

Identify the correct statement regarding the combustion of petrol in internal combustion engines.

Practise this question

Question

Multiple-choice question 09 asking 'Which statement about the combustion of petrol in vehicles is correct?'. Option A: Exhaust gases can contain a mixture of unburned hydrocarbon molecules. Option B: In the catalytic converter, carbon dioxide is converted into carbon monoxide. Option C: More nitrogen gas is emitted from the catalytic converter than enters the internal combustion engine. Option D: Combustion in a plentiful supply of oxygen often causes an increased amount of black solid to be emitted from the exhaust. Worth 1 mark.
Question text

09 Which statement about the combustion of petrol in vehicles is correct?

[1 mark]

Exhaust gases can contain a mixture of

A

unburned hydrocarbon molecules.

In the catalytic converter, carbon dioxide is converted into

B

carbon monoxide.

More nitrogen gas is emitted from the catalytic converter

C

than enters the internal combustion engine.

Combustion in a plentiful supply of oxygen often causes an

D

increased amount of black solid to be emitted from the exhaust.

Mark scheme

Show the mark scheme Mark scheme for question 09 showing the correct answer is A ('Exhaust gases can contain a mixture of unburned hydrocarbon molecules.'), awarded 1 mark (AO1).

Exhaust gases can contain a mixture of unburned hydrocarbon

09 A 1 (AO1)

molecules.

How to answer it

Combustion of Petrol & Vehicle Emissions

📋 What this question tests

This question assesses your recall and understanding of alkanes as fuels (AQA Chemistry Specification 3.3.1.2), including the pollutants formed during the internal combustion of petrol (complete vs. incomplete combustion, unburned hydrocarbons, nitrogen oxides) and the role of catalytic converters in mitigating these harmful gases.

Question 09 Analysis

Multiple Choice: Statements on Combustion & Vehicle Exhausts

✅ Correct Answer

A: Exhaust gases can contain a mixture of unburned hydrocarbon molecules.

Mark Scheme: 1 Mark (AO1)
Petrol consists of a mixture of liquid hydrocarbons (predominantly C₄ to C₁₂ alkanes). In an engine cylinder, combustion is never 100% efficient; a small fraction of vaporised fuel escapes without reacting and is emitted in the exhaust gas stream.

💡 Key Knowledge

  • Internal Combustion Products: Include CO₂, H₂O, CO, C (soot particulates), unburned hydrocarbons, and NOx (NO and NO₂).
  • Catalytic Converters: Use a ceramic honeycomb coated with a thin layer of precious metals (Pt, Pd, Rh) to catalyse the reaction between harmful pollutants:
    2CO + 2NO → 2CO₂ + N₂
    hydrocarbons + NO → CO₂ + H₂O + N₂
  • Conservation of Nitrogen: Nitrogen gas (N₂) entering the engine can form NO at high temperatures, but NO is converted back to N₂ in the catalytic converter. No new nitrogen atoms are created.

❌ Breakdown of Incorrect Options

  • Option B is incorrect: The converter oxidises poisonous carbon monoxide (CO) into less harmful carbon dioxide (CO₂), not the reverse:
    2CO + O₂ → 2CO₂
  • Option C is incorrect: Nitrogen cannot be created. Air drawn into the engine contains N₂. A small amount reacts to form NOx, which is then reduced back to N₂ in the catalytic converter. The total amount of nitrogen atoms leaving cannot exceed what entered.
  • Option D is incorrect: The "black solid" is carbon soot (C). Soot forms during incomplete combustion (a limited supply of oxygen). A plentiful supply leads to complete combustion, producing CO₂ and H₂O with minimal to no soot.

🧠 Exam Technique & Pitfalls

  • Carefully Read Reaction Directions: Students often rush and confuse "CO converted to CO₂" with "CO₂ converted to CO" (Option B). Catalytic converters aim to produce less toxic substances.
  • Watch for Qualifying Words: Notice "plentiful supply" in Option D. Complete combustion occurs in excess oxygen, which prevents soot formation.
  • Process of Elimination: When in doubt on AO1 recall questions, test the chemical plausibility of each statement against the conservation of mass and the known purpose of emission controls.

Topics

Organic Chemistry · 3.3.2 Alkanes

Question and mark scheme from the AQA A-Level Chemistry examination, AS Paper 2, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.