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This lesson covers alkenes, the C=C double bond, testing with bromine water, and addition polymerisation as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand the properties of alkenes, how to test for unsaturation, and how polymers such as polyethene and PVC are formed.
Alkenes are a homologous series of hydrocarbons that contain at least one carbon–carbon double bond (C=C). This makes them unsaturated hydrocarbons.
The general formula for alkenes is:
CₙH₂ₙ
| Name | Molecular Formula | Structure |
|---|---|---|
| Ethene | C₂H₄ | H₂C=CH₂ |
| Propene | C₃H₆ | CH₃CH=CH₂ |
| Butene | C₄H₈ | CH₃CH₂CH=CH₂ (but-1-ene) |
| Feature | Alkanes | Alkenes |
|---|---|---|
| General formula | CₙH₂ₙ₊₂ | CₙH₂ₙ |
| Bonding | Single C–C bonds only | Contains C=C double bond |
| Saturated / Unsaturated | Saturated | Unsaturated |
| Reactivity | Relatively unreactive | More reactive (due to C=C) |
| Bromine water test | Stays orange | Turns colourless |
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