Crude oil contains hydrocarbons of different chain lengths. During fractional distillation, short-chain alkanes and long-chain alkanes are separated into different fractions. Compare the properties of short-chain alkanes with those of long-chain alkanes, and explain why short-chain fractions are more useful as fuels.

Pearson Edexcel International GCSE Chemistry (4CH1) — 4.3 Alkanes · Compare · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Short‑chain alkanes have lower boiling points than long‑chain alkanes, so they evaporate more readily. They are more volatile and flow more easily, giving them lower viscosity. Because they ignite more readily and are more flammable, they release energy quickly when burned, making them useful as fuels.

Examiner tips

  • Use the word ‘compare’ – list a property for each type and then explain the fuel advantage.
  • Mention boiling point, volatility, viscosity and flammability in separate points.
  • Show the link between lower boiling point and easier evaporation.
  • Explain why easier ignition gives a useful fuel.

Mark scheme (4 marks)

  1. Short-chain alkanes have lower boiling points than long-chain alkanes
  2. Short-chain alkanes are more volatile / evaporate more easily than long-chain alkanes
  3. Short-chain alkanes flow more easily / are less viscous than long-chain alkanes
  4. Short-chain alkanes ignite more easily / are more flammable, making them more useful as fuels because they release energy when burned

Key terms in this question

alkanes · fractional distillation · fractions · crude oil · hydrocarbons · fuel

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