AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
The first four members of the homologous series of alkenes are ethene,…
Full specification wording
Structure and formulae of alkenes: 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.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 2 of 2
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
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.
What earns the marks
- 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.
Common mistakes and how to put them right
Writing methene as the first alkene.
Put it right: The first alkene is ethene, C₂H₄, because a C=C needs two carbon atoms.
Using -ane endings for alkenes.
Put it right: Use -ene endings such as ethene and propene.
Giving butene as C₄H₁₀.
Put it right: Butene is C₄H₈ because it is unsaturated.
Exam tips
- 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.
Try the lesson
The start of the Structure and formulae of alkenes: The first four members of the homologous series of alkenes are ethene, propene, butene and pentene. lesson, made from checked teaching notes for this specification point.
Have a go first
Which of these is the first member of the homologous series of alkenes?
Listen
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. A strong answer earns marks for these points. 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. A common slip: Writing methene as the first alkene. Instead, the first alkene is ethene, C₂H₄, because a C=C needs two carbon atoms. Exam tip: Learn the names and formulae of the first four alkenes in order.
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Key words
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Homologous series
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Molecular formulae
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Nomenclature
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Trends in properties
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See every definition
- Homologous series
- 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.
- Molecular formulae
- The formulae C₂H₄, C₃H₆, C₄H₈ and C₅H₁₀ for the first four alkenes, all fitting the general formula CₙH₂ₙ.
- Nomenclature
- The 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.
- Trends in properties
- Boiling point and viscosity increase with chain length.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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