Knowledge Organiser: Alkenes
Key Terms
- Alkene — unsaturated hydrocarbon, CₙH₂ₙ
- Unsaturated — contains C=C double bond
- Addition reaction — atoms add across C=C
- Decolourisation — bromine water turns colourless
First Four Alkenes
- Ethene — C₂H₄
- Propene — C₃H₆
- Butene — C₄H₈
- Pentene — C₅H₁₀
Addition Reactions
- + H₂ → alkane (Ni catalyst, 150°C)
- + Br₂ → dibromoalkane (test)
- + H₂O → alcohol (H₃PO₄, 300°C, 70 atm)
Exam Checklist
- CₙH₂ₙ formula (not CₙH₂ₙ₊₂)
- Bromine water: say "decolourised"
- Unsaturated = C=C double bond
- Source of alkenes: cracking
Key Equations
- General formula: CₙH₂ₙ
- C₂H₄ + Br₂ → C₂H₄Br₂ (addition with bromine — decolourises)
- C₂H₄ + H₂O → C₂H₅OH (hydration → ethanol; H₃PO₄ catalyst)
- n(CH₂=CH₂) → (−CH₂−CH₂−)ₙ (addition polymerisation)
Common Mistakes
- Using CₙH₂ₙ₊₂ for alkenes: That is the alkane formula — alkenes are CₙH₂ₙ because the double bond removes two hydrogen atoms
- Saying alkenes are saturated: Alkenes are UNSATURATED — they contain a C=C double bond which can be opened in addition reactions
- Not saying bromine water is "decolourised": The correct observation is orange → colourless (not "turns clear" — clear and colourless are different)
- Saying alkanes decolourise bromine water too: Only alkenes decolourise bromine water — alkanes have no double bond so cannot undergo addition reactions
Memory Aid
- ENE — contains a C=C doublE bond (the E in ENE)
- E — Extremely reactive compared to alkanes
- General formula: CₙH₂ₙ — just 2n hydrogens (count: 2 × carbons)