Describe how steam cracking is used to produce smaller, more useful molecules from crude oil fractions, and explain why alkenes produced during cracking are described as unsaturated.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Crude oil is a finite resource that contains a mixture of hydrocarbons. Many of the larger hydrocarbon molecules obtained from crude oil have limited uses, so they are processed to produce more useful products.
Model answer (4 marks)
Steam cracking heats long‑chain hydrocarbons from crude oil fractions with steam to very high temperatures (≈800–900 °C). The heat breaks the C–C bonds, splitting the large molecules into smaller ones. The products include alkenes, which have a carbon–carbon double bond (C=C). Because a double bond means each carbon has only three bonds to other atoms, the molecule contains fewer hydrogen atoms than the maximum possible for that carbon skeleton – it is therefore unsaturated.
Examiner tips
- State the role of steam and high temperature in breaking C–C bonds
- Explain that alkenes have a C=C double bond
- Mention that the double bond reduces the number of H atoms, making the molecule unsaturated
Common mistakes
- Forgetting to mention steam or the high temperature
- Describing alkenes as saturated instead of unsaturated
- Not linking the double bond to the reduced hydrogen count
Mark scheme (4 marks)
- Steam cracking involves heating long-chain hydrocarbons to high temperatures (with steam)
- The large/long-chain hydrocarbon molecules are broken down / split into smaller hydrocarbon molecules
- Alkenes contain a carbon–carbon double bond (C=C)
- Unsaturated means the molecule contains fewer hydrogen atoms than the maximum possible / does not contain the maximum number of hydrogen atoms (due to the double bond)
Key terms in this question
cracking · steam cracking · alkenes · unsaturated · crude oil
Related
- All OCR GCSE Chemistry A: Gateway Science (J248) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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