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This lesson covers cracking and introduces alkenes as required by the AQA GCSE Chemistry specification (5.8.1). Cracking is a vital industrial process that converts less useful long-chain hydrocarbons into more useful shorter-chain hydrocarbons and alkenes. Alkenes are the starting materials for making polymers (plastics).
Fractional distillation of crude oil produces fractions in fixed proportions. However, the demand for different fractions does not match the supply:
| Situation | Detail |
|---|---|
| Oversupply | Long-chain hydrocarbons (fuel oil, bitumen) are produced in larger quantities than needed |
| High demand | Short-chain hydrocarbons (petrol, diesel) and alkenes (for plastics) are in greater demand than supply |
Cracking solves this problem by breaking down long-chain hydrocarbons into shorter, more useful molecules.
Exam Tip: Always explain cracking in terms of supply and demand. A common exam question asks "why is cracking carried out?" The answer must include: there is a greater demand for shorter-chain hydrocarbons than the amount produced by fractional distillation.
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