Knowledge Organiser: Alkenes
Part of Alkenes · GCSE GCSE Chemistry revision
This topic summary covers Knowledge Organiser: Alkenes within Alkenes for GCSE Chemistry. Revise Alkenes in Organic Chemistry for GCSE Chemistry with 20 exam-style questions and 15 flashcards. This is a high-frequency topic, so it is worth revising until the explanation feels precise and repeatable. It is section 14 of 14 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Topic position
Section 14 of 14
Practice
20 questions
Recall
15 flashcards
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
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Practice Questions for Alkenes
What is the general formula for alkenes?
Explain what is meant by the term 'unsaturated' when applied to alkenes.
Quick Recall Flashcards
20 questions on Alkenes — practise free
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