Organic Chemistry
Edexcel IGCSE Chemistry 4CH0 — 4.2 Crude Oil — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions
5 min read 4 parts Lesson 8 of 18 Unit quiz
Lesson summary
Crude oil — cracking: explain why long-chain hydrocarbons are cracked, describe catalytic cracking, and identify the shorter alkane and alkene products. Fractional distillation separates crude oil into fractions but does not change the molecules.
- decane → octane + ethene
Part 1 of 4Why cracking is used
Fractional distillation separates crude oil into fractions but does not change the molecules. The amount of each fraction produced may not match demand: long-chain fractions can be in surplus while shorter-chain fuels and alkene feedstocks are more useful.
Cracking
Cracking breaks large hydrocarbon molecules into smaller ones. It helps convert lower-demand fractions into products used as fuels or as starting materials for other chemicals. Cracking requires an energy input, so it is endothermic.
Part 2 of 4Catalytic cracking
Catalytic cracking
Heat a long-chain hydrocarbon until it vaporises, then pass its vapour over a hot solid catalyst such as powdered aluminium oxide. The high temperature and catalyst help break carbon-carbon bonds in the molecules.
The exact split of a chain can vary, so industrial cracking produces a mixture rather than just one fixed pair of products. The shortened carbon chains can then be separated and used for different purposes.
Part 3 of 4Products and equations
1Alkane
Cracking commonly makes a shorter alkane and an alkene. Alkanes are saturated and have carbon-carbon single bonds. Alkenes are unsaturated and contain a carbon-carbon double bond.
2Decane → octane + ethene
For example, decane → octane + ethene, or C10H22 → C8H18 + C2H4. Check that carbon and hydrogen atoms balance. Octane can be used in petrol, while ethene can be used to make other chemicals.
Part 4 of 4Supply, demand and exam checks
Shorter-chain hydrocarbons may be more in demand than the amounts obtained directly from crude oil. Cracking surplus long-chain material increases the supply of those shorter molecules and valuable alkenes.
Check:
Check How can you tell that ethene, rather than octane, is an alkene? Answer: Ethene has a carbon-carbon double bond and fits the unsaturated alkene family; octane is a saturated alkane.
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