IGCSE Chemistry Alkanes and Alkenes Notes
Alkanes are saturated hydrocarbons; alkenes are unsaturated. Both are important as fuels and feedstocks, but their reactions are very different.
Key concepts
- Alkanes
- General formula CₙH₂ₙ₊₂; only single C–C bonds; relatively unreactive but burn well.
- Alkenes
- General formula CₙH₂ₙ; contain at least one C=C; react by addition.
- Isomers
- Same molecular formula but different arrangement of atoms.
- First four alkanes: methane, ethane, propane, butane.
- First three alkenes: ethene, propene, butene.
- Alkenes decolourise bromine water; alkanes do not.
- Alkenes undergo addition polymerisation to form plastics.
Worked example
Explain, with an equation, how bromine water is used to distinguish ethane and ethene.
- 1Add bromine water to each gas.
- 2Ethane: no change (stays orange).
- 3Ethene: goes colourless — bromine adds across the C=C.
- 4Equation: C₂H₄ + Br₂ → C₂H₄Br₂
Answer: Ethene decolourises bromine water: C₂H₄ + Br₂ → C₂H₄Br₂. Ethane does not react.
Common mistakes
✗ Saying 'goes clear' instead of 'goes colourless'.
✓ 'Colourless' is the mark-scheme term for a solution that has lost its colour.
✗ Writing C₂H₆ for ethene.
✓ Ethene is C₂H₄ (one C=C); ethane is C₂H₆.
✗ Drawing polymers with a double bond inside the repeat unit.
✓ The C=C opens during polymerisation, so the repeat unit has only single bonds.
Quick check
Try these, then reveal the answers.
What is the molecular formula of propene?Reveal answer
C₃H₆
Why are alkanes fairly unreactive?Reveal answer
Only strong C–C and C–H single bonds; no functional group and no C=C.
Name the product formed when ethene reacts with bromine.Reveal answer
1,2-dibromoethane (C₂H₄Br₂).
Exam tips
- Learn general formulae — they let you write any member of the series.
- For displayed formulae, show every bond and every H atom.
- For polymerisation, draw n monomers on the left and (repeat unit)ₙ on the right.
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Frequently asked questions
Why are alkenes described as unsaturated?+
They contain at least one C=C double bond, so they can add more atoms across it.
What is the difference between an addition and a combustion reaction?+
Addition joins molecules across a C=C; combustion burns a hydrocarbon in oxygen to give CO₂ and water.
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