Methane, ethane, propane and butane are all members of the alkane homologous series. Explain what is meant by a homologous series and describe three properties that all alkanes share as a result of belonging to the same homologous series.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Alkanes are the simplest organic compounds and are found in crude oil. Their general formula is CₙH₂ₙ₊₂.
Model answer (5 marks)
A homologous series is a family of compounds that share the same general formula. Members of a homologous series differ by one CH₂ unit (a methylene group) between successive members.
All alkanes share:
1. They are saturated hydrocarbons – only single C–C and C–H bonds.
2. They undergo the same type of reactions, e.g. complete combustion to CO₂ and H₂O.
3. Their physical properties change gradually – boiling point, melting point and viscosity increase as the number of carbon atoms increases.
All alkanes share:
1. They are saturated hydrocarbons – only single C–C and C–H bonds.
2. They undergo the same type of reactions, e.g. complete combustion to CO₂ and H₂O.
3. Their physical properties change gradually – boiling point, melting point and viscosity increase as the number of carbon atoms increases.
Examiner tips
- Define homologous series and state the CH₂ difference; list three properties with specific examples; use correct terminology (saturated, single bonds, combustion).
Common mistakes
- Confusing alkanes with alkenes or alkynes; forgetting the CH₂ unit difference; giving properties that are not common to all alkanes (e.g. colour).
Mark scheme (5 marks)
- A homologous series is a family/group of compounds that share the same general formula
- Members of a homologous series differ by one CH₂ unit (a methylene group) between successive members
- All alkanes have similar chemical properties / react in similar ways (e.g. undergo combustion)
- Alkanes are saturated hydrocarbons / contain only single C–C and C–H bonds / contain no carbon-carbon double bonds
- Alkanes show a gradual trend in physical properties such as boiling point / melting point / viscosity increasing as the number of carbon atoms increases
Key terms in this question
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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