Methane and ethane are both alkanes. Describe the similarities and differences in their molecular structures, and explain what is meant by calling them a homologous series.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Methane has the molecular formula CH₄. Ethane has the molecular formula C₂H₆.
Model answer (5 marks)
Both methane and ethane are hydrocarbons containing only C and H atoms.
Both molecules are saturated, having only single C–C or C–H bonds.
Methane has one carbon atom, whereas ethane has two carbon atoms – they differ by one CH₂ unit.
A homologous series is a family of compounds that share the same general formula; for alkanes this is CₙH₂ₙ₊₂.
Members of a homologous series have similar chemical properties but their physical properties change gradually, e.g. boiling point rises with increasing chain length.
Both molecules are saturated, having only single C–C or C–H bonds.
Methane has one carbon atom, whereas ethane has two carbon atoms – they differ by one CH₂ unit.
A homologous series is a family of compounds that share the same general formula; for alkanes this is CₙH₂ₙ₊₂.
Members of a homologous series have similar chemical properties but their physical properties change gradually, e.g. boiling point rises with increasing chain length.
Examiner tips
- Use the word ‘saturated’ to describe single bonds. Show the CH₂ difference explicitly. Mention the general formula CₙH₂ₙ₊₂ for alkanes. State the gradual change in physical properties, e.g. boiling point.
Common mistakes
- Confusing unsaturated bonds or forgetting to mention single bonds. Failing to state the CH₂ unit difference. Using the wrong general formula for alkanes (e.g. CₙH₂ₙ).
Mark scheme (5 marks)
- Both methane and ethane contain only carbon and hydrogen atoms (are hydrocarbons)
- Both molecules contain only single (C–C or C–H) bonds / are saturated
- Methane has one carbon atom whereas ethane has two carbon atoms (they differ by one CH₂ unit)
- A homologous series is a family of compounds that share the same general formula (CₙH₂ₙ₊₂ for alkanes)
- Members of a homologous series have similar chemical properties but gradually changing physical properties (e.g. boiling point increases as chain length increases)
Key terms in this question
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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