Pentane (C₅H₁₂) is a member of the alkane homologous series. Explain what is meant by the term 'homologous series' and describe three features that all alkanes share as members of the same homologous series.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Alkanes are saturated hydrocarbons. The first four members of the alkane homologous series are methane (CH₄), ethane (C₂H₆), propane (C₃H₈) and butane (C₄H₁₀).
Model answer (5 marks)
A homologous series is a group of compounds that have the same general formula and differ by a constant unit, usually a CH₂ group.
All alkanes share the following features:
1. They are hydrocarbons – they contain only carbon and hydrogen atoms.
2. They are saturated – every carbon–carbon bond is a single bond.
3. They have the general formula CₙH₂ₙ₊₂, which gives the correct number of hydrogen atoms for any n.
These common features give alkanes similar physical and chemical properties.
All alkanes share the following features:
1. They are hydrocarbons – they contain only carbon and hydrogen atoms.
2. They are saturated – every carbon–carbon bond is a single bond.
3. They have the general formula CₙH₂ₙ₊₂, which gives the correct number of hydrogen atoms for any n.
These common features give alkanes similar physical and chemical properties.
Examiner tips
- Use the definition of a homologous series first, then list three features in separate points.
- Show the general formula CₙH₂ₙ₊₂ explicitly.
- Mention that the CH₂ increment is the key to the series.
Common mistakes
- Confusing the definition of a homologous series with a functional group.
- Omitting the CH₂ increment or the general formula.
- Listing a feature that is not common to all alkanes, e.g. colour or state of matter.
Mark scheme (5 marks)
- A homologous series is a family/group of compounds that share the same general formula (and differ by CH₂ each time)
- All alkanes contain only carbon and hydrogen atoms (they are hydrocarbons)
- All alkanes are saturated — they contain only single C–C bonds (no double or triple bonds)
- Alkanes share the same general formula CₙH₂ₙ₊₂
- Members of the same homologous series have similar chemical properties (because they share the same functional group / type of bonding)
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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