Ethene is produced by cracking long-chain hydrocarbons. Describe what happens during steam cracking and explain why cracking is an important industrial process.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Long‑chain hydrocarbons are first heated until they vapourise.
The hydrocarbon vapour is mixed with steam and heated to a very high temperature (≈800–900 °C).
At these temperatures the long‑chain molecules undergo thermal decomposition, breaking the C–C bonds and forming smaller molecules.
The main products are alkenes such as ethene, which are then used to produce polymers and other chemicals.
Cracking therefore converts less useful long‑chain hydrocarbons into shorter‑chain molecules that are in greater demand, are more flammable and can be used as fuels or feedstocks for further chemical synthesis.
The hydrocarbon vapour is mixed with steam and heated to a very high temperature (≈800–900 °C).
At these temperatures the long‑chain molecules undergo thermal decomposition, breaking the C–C bonds and forming smaller molecules.
The main products are alkenes such as ethene, which are then used to produce polymers and other chemicals.
Cracking therefore converts less useful long‑chain hydrocarbons into shorter‑chain molecules that are in greater demand, are more flammable and can be used as fuels or feedstocks for further chemical synthesis.
Examiner tips
- Use the exact sequence: vapourise → steam mix → high temperature → thermal decomposition → alkenes
- Mention both the product (ethene) and the industrial benefit (fuel/chemical feedstock)
- Show the link between high temperature and bond breaking
Common mistakes
- Confusing steam cracking with catalytic cracking or not stating the high temperature
- Omitting the role of steam or the production of alkenes
- Using vague terms like "break bonds" without linking to thermal decomposition
Mark scheme (5 marks)
- Long-chain hydrocarbons are heated to vaporise them / turn them into a gas
- The hydrocarbon vapour is mixed with steam and heated to a very high temperature
- Thermal decomposition causes the long-chain molecules to split into smaller molecules
- Cracking produces alkenes (such as ethene), which are used to make polymers / other chemicals
- Cracking converts less useful long-chain hydrocarbons into shorter-chain molecules that are in greater demand / more useful as fuels because they are more flammable
Key terms in this question
steam cracking · cracking · hydrocarbons
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
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