Structure and formulae of alkenes — AQA GCSE Chemistry
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Structure and formulae of alkenes explained
Structure and formulae of alkenes covers 2 specification points, including Alkenes are hydrocarbons with a double… and The first four members of the homologous….
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Alkenes are hydrocarbons with a double carbon-carbon bond.
Alkenes are a homologous series of unsaturated hydrocarbons. 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.
The first four members of the homologous series of alkenes are ethene, propene, butene and pentene.
A homologous series is a family of compounds with the same general formula and similar chemical properties, in which each member differs from the next by a CH₂ unit. The first four alkenes are ethene C₂H₄, propene C₃H₆, butene C₄H₈ and pentene C₅H₁₀. Their names use the stem for the number of carbon atoms, eth-, prop-, but- and pent-, with the ending -ene showing the C=C double bond. Butene and pentene have structural isomers because the C=C can be in different positions, but all members fit CₙH₂ₙ. Trends in boiling point and viscosity increase with chain length.
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.
Show all 6 objectives
- Name and write the formulae of the first four alkenes.
- Explain what is meant by a homologous series using alkenes as the example.
- Use the general formula CₙH₂ₙ to deduce the formula of an alkene from its carbon count.
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.
- The first four alkenes are ethene, propene, butene and pentene, in order of increasing carbon chain length.
- Their molecular formulae are C₂H₄, C₃H₆, C₄H₈ and C₅H₁₀.
- Each member differs from the next by a CH₂ unit, which defines a homologous series.
- All members fit the general formula CₙH₂ₙ and contain a C=C double bond.
- The names use the stem for the number of carbon atoms and the ending -ene for the double bond.
- Physical properties such as boiling point increase as the chain length increases.
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.
- 💡Learn the names and formulae of the first four alkenes in order.
- 💡Use the general formula CₙH₂ₙ to check any alkene formula.
- 💡Remember that each member differs by CH₂, not by CH₃ or C₂H₄.
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.
- Writing methene as the first alkene; correction: the first alkene is ethene, C₂H₄, because a C=C needs two carbon atoms.
- Using -ane endings for alkenes; correction: use -ene endings such as ethene and propene.
- Giving butene as C₄H₁₀; correction: butene is C₄H₈ because it is unsaturated.