Methane, ethane, propane and butane are all members of the alkane homologous series. Describe four properties that all members of a homologous series share, and explain why alkanes are described as saturated hydrocarbons.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
1. All members of a homologous series contain the same functional group – for alkanes this is the –CH3/–CH2– groups.
2. They have the same general formula, CnH2n+2.
3. They show similar chemical properties, reacting only by substitution or combustion.
4. They differ by one CH2 unit, giving a gradual trend in physical properties such as boiling point.
5. Alkanes are saturated hydrocarbons because they contain only single C–C bonds and no double or triple bonds, and they consist solely of carbon and hydrogen atoms.
2. They have the same general formula, CnH2n+2.
3. They show similar chemical properties, reacting only by substitution or combustion.
4. They differ by one CH2 unit, giving a gradual trend in physical properties such as boiling point.
5. Alkanes are saturated hydrocarbons because they contain only single C–C bonds and no double or triple bonds, and they consist solely of carbon and hydrogen atoms.
Examiner tips
- Use the exact phrase "general formula CnH2n+2". Mention the gradual trend in boiling point. Explain saturation as absence of double bonds. Keep answer concise and point‑wise.
Common mistakes
- Confusing alkanes with alkenes or alkynes. Omitting the term "saturated" or the reason for it. Listing properties that are not common to all homologues (e.g. colour).
Mark scheme (5 marks)
- Members of a homologous series have the same functional group / same general formula (CnH2n+2 for alkanes)
- Members of a homologous series have similar chemical properties
- Members of a homologous series differ by one CH2 unit / show a gradual trend in physical properties
- Alkanes are saturated because they contain only single bonds between carbon atoms / no carbon–carbon double bonds
- Alkanes are hydrocarbons because they contain hydrogen and carbon atoms only
Key terms in this question
homologous series · saturated · hydrocarbons · alkanes
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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