AQA A-Level Chemistry AS Paper 2, June 2025: Question 10
1 mark · Easy difficulty · Multiple Choice
Identify which statement regarding catalytic cracking is correct.
Practise this questionQuestion
Question text
10 Which statement about catalytic cracking is correct?
[1 mark]
A A high percentage of alkenes is formed.
B A pressure of about 70 MPa is used.
C C–C bonds are broken.
D A nickel catalyst is used.
Mark scheme
Show the mark scheme
10 C 1 (AO1) C–C bonds are broken.
How to answer it
Catalytic Cracking of Alkanes
This question assesses basic recall of organic industrial chemistry under Section 3.3.1 (Alkanes) of the AQA specification:
- Fundamental nature of cracking: Breaking strong C–C covalent bonds in long-chain alkanes to produce smaller, more useful molecules.
- Conditions & products: Distinguishing clearly between thermal cracking and catalytic cracking (temperature, pressure, catalysts, and primary products).
Analysis of Options
✅ Correct Answer: C
C–C bonds are broken.
By definition, cracking is the thermal or catalytic decomposition of long-chain hydrocarbons into shorter ones. This fundamentally requires the cleavage of carbon–carbon (C–C) single bonds.
❌ Why the Other Options are Incorrect
- A is incorrect: A high percentage of alkenes is the characteristic product of thermal cracking, not catalytic cracking. Catalytic cracking predominantly yields motor fuels containing branched alkanes, cycloalkanes, and aromatic hydrocarbons.
- B is incorrect: Catalytic cracking operates at a slight pressure (just above atmospheric, ~100–200 kPa or 1–2 atm). 70 MPa (700 atm) is extremely high and incorrect even for thermal cracking (which uses up to ~7000 kPa / 7 MPa).
- D is incorrect: Catalytic cracking uses a zeolite catalyst (hydrated aluminosilicate). A nickel catalyst is used for the hydrogenation of alkenes, not cracking.
Key Revision Knowledge: Thermal vs Catalytic Cracking
💡 Direct Comparison Summary
AQA frequently examines the distinct conditions and products of both types of cracking. Keep this table memorised:
| Feature | Thermal Cracking | Catalytic Cracking |
|---|---|---|
| Temperature | High (700 – 1200 K / 400 – 900 °C) | Moderate/Slightly lower (~720 K / 450 °C) |
| Pressure | High (up to 7000 kPa / 70 atm) | Slight / low (just above 1 atm / ~100–200 kPa) |
| Catalyst | None | Zeolite (aluminosilicate with honeycombed/acidic sites) |
| Main Products | High % of alkenes (used to make polymers and chemicals) | Motor fuels (branched alkanes, cycloalkanes, aromatics) |
| Intermediate | Free radicals (homolytic fission) | Carbocations (heterolytic fission) |
Exam Technique & Common Pitfalls
🧠 Exam Technique: Elimination Strategy
- For multiple-choice questions on processes, ask: "What must happen in ALL cracking reactions?"
- Regardless of mechanism (radical or carbocation) or conditions (thermal or catalytic), cracking always breaks C–C bonds to convert larger alkanes into smaller fragments.
- Even if you temporarily blanked on the exact catalyst or pressures, recognizing that C–C bond cleavage is the definition of cracking gives you the answer instantly.
❌ Common Misconceptions
- Confusing the catalysts: Students frequently mix up catalysts across the syllabus. Remember:
• Zeolite = Cracking
• Nickel (Ni) = Hydrogenation of alkenes
• Concentrated H₂SO₄ / H₃PO₄ = Hydration/Dehydration
• Fe = Haber process - Confusing the main products: Thermal = Alkenes (think T-E for Thermal/Ethene); Catalytic = branched/aromatics for fuels.
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.