Cracking and alkenes — AQA GCSE Chemistry
Test yourself on Cracking and alkenes with AQA GCSE practice questions.
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Cracking and alkenes explained
Cracking turns large, less useful alkanes into smaller, more useful molecules.
Read the full explanation
A long-chain fraction such as decane can be cracked to give octane, used in petrol, plus ethene, which is used to make poly(ethene). This matters because demand for petrol and for polymers is high, while long-chain fractions from crude oil are in surplus. Cracking may be thermal or catalytic, and it produces alkanes plus alkenes. The alkenes are the reactive products that feed the polymer industry, and the shorter alkanes are cleaner-burning fuels. A good answer names a starting material, a product and its use, and links the process to matching supply with demand.
Your focus
- State that cracking converts large alkanes into smaller, more useful molecules.
- Give a named example of cracking with at least one product and its use.
- Explain how cracking helps match the supply of fractions to demand for fuels and polymer feedstocks.
Cracking and alkenes exam tips
Marking Points
- Cracking converts large alkane molecules into smaller molecules, including alkenes, which are more useful.
- An example is cracking decane to produce octane for petrol and ethene for making poly(ethene).
- Alkenes from cracking are used to make polymers such as poly(ethene) and poly(propene).
- Shorter-chain alkanes produced by cracking are used as fuels because they burn more cleanly than heavy fractions.
- Cracking helps match supply to demand: heavy fractions are in surplus while fuels and polymer feedstocks are in demand.
- Award credit for a named product and a specific use, not just the word cracking.
Examiner Tips
- 💡Name the starting alkane, the products and one use for each product.
- 💡Link the example to supply and demand to show why cracking is useful.
- 💡Use the terms thermal cracking and catalytic cracking correctly if you mention conditions.
Common Mistakes
- Saying cracking makes only alkenes; correction: cracking produces alkanes and alkenes.
- Giving no named product or use; correction: state a product such as ethene and its use in making poly(ethene).
- Confusing cracking with fractional distillation; correction: fractional distillation separates crude oil without changing molecules, while cracking breaks large molecules into smaller ones.