Cracking is an important industrial process. Describe what cracking is and explain why the petrochemical industry carries out cracking on fractions obtained from crude oil.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Crude oil is separated into fractions by fractional distillation. Some fractions are in much greater supply than demand requires, whilst other fractions are in very high demand but short supply.
Model answer (4 marks)
Cracking is a process that breaks down large, long‑chain hydrocarbons into smaller, shorter‑chain molecules.
It can be carried out by catalytic cracking or by steam cracking.
The fractions that are heavy and in excess are converted by cracking into products that are in higher demand.
Cracking therefore produces useful shorter‑chain molecules such as fuels (e.g. petrol, diesel) and alkenes that are needed by the petrochemical industry.
It can be carried out by catalytic cracking or by steam cracking.
The fractions that are heavy and in excess are converted by cracking into products that are in higher demand.
Cracking therefore produces useful shorter‑chain molecules such as fuels (e.g. petrol, diesel) and alkenes that are needed by the petrochemical industry.
Examiner tips
- Use the exact terms ‘cracking’, ‘catalytic cracking’, ‘steam cracking’, ‘long‑chain hydrocarbons’, ‘shorter‑chain molecules’, ‘fuels’, ‘alkenes’.
- Show the causal link: heavy fractions → cracking → useful products.
- Include both types of cracking to gain full marks.
Common mistakes
- Confusing cracking with distillation or saying it only makes lighter fuels.
- Omitting the two main types of cracking (catalytic and steam).
- Failing to mention the demand/supply imbalance of heavy fractions.
Mark scheme (4 marks)
- Cracking breaks down larger/longer-chain hydrocarbons into smaller/shorter-chain molecules
- Cracking can be done by catalytic cracking or steam cracking
- Some fractions (longer-chain/heavier) are in excess / low demand and need to be converted into more useful/shorter-chain products
- Cracking produces shorter-chain, more useful molecules such as fuels (e.g. petrol/diesel) and/or alkenes which are in high demand
Key terms in this question
Related
- All Edexcel GCSE Chemistry (1CH0) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
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