Alkene – IGCSE Chemistry Definition
IGCSE Chemistry definition of alkene: an unsaturated hydrocarbon with general formula CₙH₂ₙ. Covers reactions, the bromine test, and exam tips for Cambridge 0620.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
Alkenes are the most reactive hydrocarbon series at IGCSE. Their C=C double bond gives them a rich set of addition reactions and makes them the monomers for addition polymerisation. The bromine water test to identify alkenes is one of the most frequently examined chemical tests.
The 0620 definition
Alkenes are unsaturated hydrocarbons with the general formula CₙH₂ₙ. They contain at least one carbon–carbon double bond (C=C).
The first three alkenes
| Name | Formula | Structural formula |
|---|---|---|
| Ethene | C₂H₄ | CH₂=CH₂ |
| Propene | C₃H₆ | CH₃CH=CH₂ |
| Butene | C₄H₈ | CH₃CH₂CH=CH₂ (but-1-ene) |
There is no methene — a minimum of two carbons is required for a C=C bond.
Key reactions of alkenes
1. Addition of bromine (test for unsaturation)
CH₂=CH₂ + Br₂ → CH₂BrCH₂Br
Observation: bromine water decolourised (orange to colourless). This is the standard test for a C=C bond.
2. Addition of hydrogen (hydrogenation)
CH₂=CH₂ + H₂ → C₂H₆ (ethane)
Conditions: nickel catalyst, 150 °C.
3. Addition of steam (hydration)
CH₂=CH₂ + H₂O → C₂H₅OH (ethanol)
Conditions: phosphoric acid catalyst, 300 °C, 60 atm. This is the industrial method for making ethanol.
4. Addition polymerisation
nCH₂=CH₂ → −(CH₂CH₂)ₙ−
Many ethene molecules join to form poly(ethene). See addition polymerisation.
5. Combustion
C₂H₄ + 3O₂ → 2CO₂ + 2H₂O
Alkenes burn with a smoky/sooty flame due to higher carbon content relative to hydrogen.
Source of alkenes: cracking
Alkenes are produced by cracking long-chain alkanes from crude oil. They are too reactive and valuable to be used as fuels — they are used to make polymers and other chemicals.
Worked exam question
Ethene reacts with steam. (a) Name the product. [1] (b) State the type of reaction. [1] (c) State the conditions needed. [1]
Mark scheme
(a) Ethanol [1]
(b) Addition [1]
(c) Phosphoric acid catalyst / high temperature (300 °C) / high pressure (60 atm) [1] (one condition for the mark)
Common exam mistakes
- Calling the first alkene “methene” — it does not exist. Ethene is the smallest alkene.
- Writing the general formula as CₙH₂ₙ₊₂ — that is alkanes. Alkenes are CₙH₂ₙ.
- Forgetting that alkenes come from cracking, not directly from crude oil distillation.
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