Alkanes are a homologous series of hydrocarbons. Explain what is meant by the term 'homologous series' and describe the general trend in the physical properties of alkanes as the number of carbon atoms in the chain increases. Include in your answer a description of the complete combustion of methane.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Methane (CH₄), ethane (C₂H₆), propane (C₃H₈), and butane (C₄H₁₀) are the first four members of the alkane homologous series.
Model answer (5 marks)
A homologous series is a group of compounds that share the same general formula and differ from one another by a constant unit, in this case a CH₂ group.
Alkanes have the general formula CₙH₂ₙ₊₂.
As the number of carbon atoms increases, the boiling and melting points rise because the molecules become larger and have greater London dispersion forces.
The complete combustion of methane is CH₄ + 2O₂ → CO₂ + 2H₂O.
Complete combustion requires an excess or sufficient supply of oxygen.
Alkanes have the general formula CₙH₂ₙ₊₂.
As the number of carbon atoms increases, the boiling and melting points rise because the molecules become larger and have greater London dispersion forces.
The complete combustion of methane is CH₄ + 2O₂ → CO₂ + 2H₂O.
Complete combustion requires an excess or sufficient supply of oxygen.
Examiner tips
- Define homologous series and give the CH₂ difference. State the alkane formula. Mention the rise in boiling/melting points. Write the balanced combustion equation. Note the need for excess oxygen.
Common mistakes
- Writing the wrong combustion products (e.g., CO instead of CO₂). Forgetting that the series differs by CH₂. Using the wrong alkane formula. Not stating that oxygen must be in excess.
Mark scheme (5 marks)
- A homologous series is a series of compounds that share the same general formula / differ by CH₂ between successive members
- Alkanes have the general formula CₙH₂ₙ₊₂
- Boiling point / melting point increases as the number of carbon atoms increases
- Complete combustion of methane produces carbon dioxide and water
- Complete combustion requires excess / sufficient oxygen (or occurs in a plentiful supply of oxygen)
Key terms in this question
homologous series · alkanes · complete combustion
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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