Structure and formulae of alkenes — AQA GCSE Chemistry
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Structure and formulae of alkenes explained
Alkenes are a homologous series of unsaturated hydrocarbons.
Read the full explanation
Their defining feature is a double covalent bond between two carbon atoms, written as C=C. Ethene has the formula C₂H₄, with two carbon atoms joined by a double bond and two hydrogen atoms bonded to each carbon. The double bond means alkenes have two fewer hydrogen atoms than the corresponding alkane, so ethene is C₂H₄ while ethane is C₂H₆. The double bond makes alkenes reactive, so they undergo addition reactions, decolourise bromine water and can join together in addition polymerisation. General formula is CₙH₂ₙ for the chain members studied.
Your focus
- Define an alkene as a hydrocarbon containing a C=C double bond.
- Draw and interpret the structure and formula of ethene.
- Explain how the double bond makes alkenes unsaturated and reactive.
Structure and formulae of alkenes exam tips
Marking Points
- Alkenes contain only carbon and hydrogen, so they are hydrocarbons.
- The defining feature is a double covalent bond between two carbon atoms, shown as C=C.
- Ethene is C₂H₄ and has a C=C double bond with two hydrogen atoms on each carbon.
- Alkenes are unsaturated because they have fewer hydrogen atoms than the corresponding alkane.
- The double bond makes alkenes reactive, so they undergo addition reactions and addition polymerisation.
- The general formula of the alkene homologous series is CₙH₂ₙ.
Examiner Tips
- 💡Show the double bond clearly in displayed and structural formulae.
- 💡Use the word unsaturated and link it to the C=C bond.
- 💡Check hydrogen counts: ethene C₂H₄, propene C₃H₆, butene C₄H₈.
Common Mistakes
- Drawing ethene with a single C-C bond; correction: draw C=C with two shared pairs of electrons.
- Saying alkenes are saturated; correction: alkenes are unsaturated because of the C=C double bond.
- Using the alkane general formula CₙH₂ₙ₊₂ for alkenes; correction: use CₙH₂ₙ for alkenes.