Explain why cracking is carried out in the petrochemical industry, and describe the conditions used in catalytic cracking.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Crude oil contains many long-chain hydrocarbons. There is a greater demand for shorter-chain hydrocarbons than for longer-chain ones.
Model answer (5 marks)
Cracking is carried out because the demand for shorter‑chain hydrocarbons is higher than the amount produced by simple fractional distillation. The process breaks down large, long‑chain molecules into smaller, shorter‑chain molecules, producing alkenes that are used to make polymers and other useful chemicals.
In catalytic cracking the long‑chain hydrocarbon is heated until it vapourises. The vapour is then passed over a hot powdered aluminium oxide catalyst, which promotes the breaking of C–C bonds and the formation of the desired products.
In catalytic cracking the long‑chain hydrocarbon is heated until it vapourises. The vapour is then passed over a hot powdered aluminium oxide catalyst, which promotes the breaking of C–C bonds and the formation of the desired products.
Examiner tips
- Mention the demand mismatch first to justify cracking
- Explain bond breaking and product type (alkenes)
- Describe heating and catalyst clearly
- Use UK spelling and correct terminology
Common mistakes
- Failing to state the demand reason for cracking
- Omitting the catalyst material or its temperature
- Using the wrong type of catalyst (e.g. zeolite instead of aluminium oxide)
Mark scheme (5 marks)
- There is greater demand for shorter-chain hydrocarbons / shorter-chain fractions than are produced by fractional distillation
- Cracking breaks down larger/longer-chain hydrocarbons into smaller/shorter-chain molecules
- Products of cracking include alkenes, which are used to make polymers / other useful chemicals
- In catalytic cracking, the long-chain hydrocarbon is heated to vaporise it
- The vapour is passed over a hot powdered aluminium oxide catalyst
Key terms in this question
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Chromatography (A-Level only) questions
- Ethene is produced by cracking long-chain hydrocarbons. Describe what happens du…
- A student cracks decane (C₁₀H₂₂) in the laboratory. The products include octene …
- A student uses bromine water to distinguish between two colourless liquids. One …
- Crude oil is separated by fractional distillation. Explain how fractional distil…