Explain why shorter-chain alkane fractions obtained from crude oil are more useful as fuels than longer-chain alkane fractions.

Edexcel GCSE Chemistry (1CH0) — 10.1 Crude oil, hydrocarbons and alkanes (Chem only) · Explain · 4 marks · View as Markdown

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

Crude oil is a mixture of hydrocarbon molecules of different chain lengths. After fractional distillation, refineries often have a surplus of longer-chain fractions and a greater demand for shorter-chain fractions.

Model answer (4 marks)

Shorter‑chain alkanes have lower boiling points, so they are more volatile and evaporate quickly.
They ignite and burn more readily, making them more flammable.
They also have lower viscosity, so they flow more easily through fuel systems.
These properties mean shorter‑chain alkanes are more practical and efficient as fuels for engines and combustion devices.

Examiner tips

  • Use the word "volatile" and "flammable" to show understanding of properties.
  • Explain how lower viscosity improves fuel delivery.
  • Link the properties directly to practical use in engines.

Common mistakes

  • Confusing boiling point with melting point.
  • Saying longer‑chain alkanes are better fuels without explaining why.
  • Using vague terms like "good" or "better" without justification.

Mark scheme (4 marks)

  1. Shorter-chain alkanes have lower boiling points / are more volatile
  2. Shorter-chain alkanes ignite / burn more easily (are more flammable)
  3. Shorter-chain alkanes have lower viscosity / flow more easily / are less viscous
  4. These properties make shorter-chain alkanes more practical / easier to use as fuels (e.g. in engines or combustion systems)

Key terms in this question

alkane · fraction

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