AQA A-Level Chemistry Paper 3, 2017: Question 8
1 mark · Easy difficulty · Multiple Choice
Identify the correct conditions and products for the catalytic cracking of alkanes.
Practise this questionQuestion
Question text
08 The table shows possible conditions and products for the cracking of alkanes.
Which row is correct?
[1 mark]
Type of cracking Conditions Products
A High pressure
Thermal Mainly alkanes
High temperature
B Slight pressure
Thermal Mainly alkenes
High temperature
C Mainly branched
Slight pressure
Catalytic alkanes and
High temperature
aromatics
D Mainly branched
High pressure
Catalytic alkanes and
High temperature
aromatics
Mark scheme
Show the mark scheme
8 C
How to answer it
Cracking of Alkanes: Conditions and Products
This question assesses direct recall of the specific reaction conditions and characteristic product distributions for the two industrial methods of cracking long-chain alkanes:
- Thermal cracking: High temperature (700–1200 K / 400–900 °C) and high pressure (up to 7000 kPa / 70 atm), producing a high percentage of alkenes.
- Catalytic cracking: High temperature (~720 K / 450 °C), slight/moderate pressure (> 1 atm), using a zeolite catalyst, producing mainly motor fuels (branched alkanes, cycloalkanes, and aromatics).
Multiple Choice Analysis
Identifying the correct combination of cracking type, conditions, and products
✅ Correct Answer: C
Row C is correct:
- Type: Catalytic
- Conditions: Slight pressure, High temperature (plus a zeolite catalyst)
- Products: Mainly branched alkanes and aromatics (and cycloalkanes)
🧠 Exam Technique: Elimination Strategy
Approach row-by-row using definitive specification facts:
- Row A is incorrect: Thermal cracking requires high pressure and temperature, but produces mainly alkenes (not alkanes).
- Row B is incorrect: Thermal cracking operates under high pressure, not slight pressure.
- Row D is incorrect: Catalytic cracking operates under slight/low pressure (just above atmospheric), not high pressure. High pressure would be expensive and is unnecessary when a zeolite catalyst is present.
💡 Key Knowledge: Thermal vs Catalytic Cracking Summary
| Feature | Thermal Cracking | Catalytic Cracking |
|---|---|---|
| Temperature | High (700–1200 K / 400–900 °C) | High (~720 K / 450 °C) |
| Pressure | High pressure (up to 7000 kPa / 70 atm) | Slight / low pressure (just above 1 atm) |
| Catalyst | None | Zeolite (hydrated aluminosilicate) |
| Main Products | High percentage of alkenes (e.g. ethene for polymers), plus some alkanes | Motor fuels: branched alkanes, cycloalkanes, and aromatic hydrocarbons |
| Intermediate | Free radicals (homolytic fission) | Carbocations (heterolytic fission) |
❌ Common Misconceptions & Traps
- Confusing the pressures: Students frequently forget that catalytic cracking uses only slight pressure. Remember: using a catalyst speeds up the reaction at lower, safer, and cheaper pressures.
- Confusing product distributions:
• Thermal = makes double bonds (alkenes) useful for making plastics/polymers.
• Catalytic = makes branched and ring structures which burn much more smoothly in car engines (higher octane rating). - Assuming "alkanes crack to form only alkanes": C–C bonds break, requiring at least one fragment to form a C=C double bond (or release H₂) to balance the hydrogen stoichiometry. Thermal cracking maximises alkenes.
Topics
Organic Chemistry · 3.3.2 Alkanes
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.