Methane, pentane and decane are all alkanes found in crude oil. Describe how the properties of these three alkanes change as the number of carbon atoms increases, and explain why these differences make some fractions more useful as fuels for vehicles than others.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Methane has 1 carbon atom, pentane has 5 carbon atoms and decane has 10 carbon atoms.
Model answer (4 marks)
Boiling point rises as the carbon chain length increases – methane boils at –161 °C, pentane at 36 °C and decane at 174 °C.
Viscosity also rises – the longer alkanes are thicker and flow less easily.
Flammability falls – the larger molecules are harder to ignite.
Because vehicle engines need a fuel that can be pumped easily and ignites readily, the short‑chain petrol fraction (e.g. methane, ethane, propane) is preferred over the heavier fractions such as decane.
Viscosity also rises – the longer alkanes are thicker and flow less easily.
Flammability falls – the larger molecules are harder to ignite.
Because vehicle engines need a fuel that can be pumped easily and ignites readily, the short‑chain petrol fraction (e.g. methane, ethane, propane) is preferred over the heavier fractions such as decane.
Examiner tips
- Use the exact terms ‘boiling point’, ‘viscosity’ and ‘flammability’.
- Show the trend (increase, increase, decrease) clearly.
- Explain the practical reason – easier flow and ignition for engines.
- Keep the answer concise – 4 marks only.
Mark scheme (4 marks)
- Boiling point increases as the number of carbon atoms increases
- Viscosity increases (or the alkanes become less easy to flow / thicker / less runny) as the number of carbon atoms increases
- Flammability decreases (or they are harder to ignite / less flammable) as the number of carbon atoms increases
- Shorter-chain alkanes (e.g. petrol fraction) are more useful as vehicle fuels because they are more flammable / ignite more easily and flow more easily through engines
Key terms in this question
Related
- All Edexcel GCSE Chemistry (1CH0) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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