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Learn0620Papers 2, 4 · ExtendedOrganic chemistryAlkenes
Organic chemistry · Subtopic 3 of 5

Alkenes and Cracking

Cracking breaks long-chain alkanes into shorter, more useful alkanes and alkenes (and sometimes hydrogen), using heat and a catalyst. It matches supply to demand: there's more long-chain oil than needed, and not enough petrol. Alkenes (CnH2n\mathrm{C_nH_{2n}}) contain a C=C double bond, so they undergo addition reactions: with bromine, hydrogen (→ alkanes) and steam (→ alcohols). Bromine water turns from orange to colourless with an alkene but not with an alkane.

Cambridge 5070 Paper 1Cambridge 5070 Paper 2Cambridge 0620 Papers 2, 4Cambridge 0620 Papers 1, 3Edexcel 4CH1 Paper 1CEdexcel 4CH1 Paper 2C≈ 40 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Describe cracking and explain why it is needed.
  • Write cracking equations.
  • Describe addition reactions of alkenes.
  • Use bromine water to distinguish alkanes and alkenes.
Pause and think

A refinery produces lots of heavy fuel oil but customers want petrol. What can it do?

Section 2 of 14

Core idea

Start hereBreaking big into small

Refineries have far more heavy oil than anyone wants, and not enough petrol. Cracking snaps big molecules into smaller, more valuable ones, and produces alkenes, the raw material for plastics.

The big idea

Cracking makes short alkanes and alkenes. The C=C bond in alkenes opens up so other atoms can add across it.

1. Cracking

Worked exampleBalancing a cracking equation

C16H34→C8H18+2C3H6+ ?\mathrm{C_{16}H_{34}\rightarrow C_8H_{18}+2C_3H_6+\ ?}

Goal
Missing product.
Strategy
Conserve C and H.
  1. C:16−8−6=2\mathrm{C}:16-8-6=2
    Carbons left.
  2. H:34−18−12=4\mathrm{H}:34-18-12=4
    Hydrogens left.
AnswerC2H4\mathrm{C_2H_4} (ethene).
✓ Check8+6+2=168+6+2=16 C; 18+12+4=3418+12+4=34 H ✓.

2. Addition reactions

What adds to ethene

The double bond opens; nothing else is produced.

Exam tipThe test Shake with bromine water: alkenes turn it from orange to colourless; alkanes leave it orange.
Pause and think

Why is "addition" a good name for these reactions?

Summary

  • Cracking: heat + catalyst.
  • Alkenes: C=C, addition.
  • Bromine water decolourised.
Key words
Cracking
Breaking long-chain hydrocarbons into shorter ones.
Unsaturated
Containing a C=C double bond.
Addition reaction
Two molecules join to form one product.
HistoryThermal cracking. Cracking was patented in 1913 by William Burton at Standard Oil. It doubled the petrol obtained from each barrel just as cars became popular.
Fun factRipening bananas. Ethene is a plant hormone. Supermarkets ripen green bananas by exposing them to ethene gas in sealed rooms.
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