Natural gas is mainly methane, a hydrocarbon that belongs to the alkane homologous series. Propane and butane are also alkanes used as fuels. Explain what is meant by the term 'hydrocarbon' and describe two features that all alkanes share as members of the same homologous series, including the general formula for alkanes.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Natural gas, bottled camping gas, and liquefied petroleum gas (LPG) all contain alkanes. These fuels combust in air to release energy, making them useful in homes and industry.
Model answer (5 marks)
A hydrocarbon is a compound made only of hydrogen and carbon atoms.
1. All alkanes obey the general formula CₙH₂ₙ₊₂.
2. Each successive alkane differs from the previous by one CH₂ unit – one extra carbon and two extra hydrogens.
3. Alkanes are saturated: they contain only single C–C bonds.
4. Because of this, all alkanes show similar chemical behaviour, reacting in the same ways (e.g. combustion, substitution).
1. All alkanes obey the general formula CₙH₂ₙ₊₂.
2. Each successive alkane differs from the previous by one CH₂ unit – one extra carbon and two extra hydrogens.
3. Alkanes are saturated: they contain only single C–C bonds.
4. Because of this, all alkanes show similar chemical behaviour, reacting in the same ways (e.g. combustion, substitution).
Examiner tips
- Define ‘hydrocarbon’ first, then list the two homologous‑series features in order: formula, CH₂ difference, saturation, and similar reactivity.
- Use the exact formula CₙH₂ₙ₊₂ and mention the CH₂ unit explicitly.
Mark scheme (5 marks)
- A hydrocarbon contains hydrogen and carbon only
- All alkanes have the general formula CₙH₂ₙ₊₂
- Alkanes differ from one another by one CH₂ unit (each successive member has one more carbon and two more hydrogen atoms)
- All alkanes have similar chemical properties (they react in similar ways / undergo the same types of reactions)
- All alkanes contain only single carbon–carbon bonds (they are saturated)
Key terms in this question
hydrocarbon · alkane · homologous series · general formula
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →