Crude oil is a mixture of hydrocarbons. Explain how fractional distillation separates crude oil into useful fractions, and state one use of the fraction with the smallest molecules.

WJEC A-Level Chemistry (Wales) — 2.5 Hydrocarbons · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Crude oil is heated until it vapourises. The vapour rises through a fractionating column that has a temperature gradient – hot at the bottom, cool at the top. Because hydrocarbons have different boiling points, each component condenses at a different height in the column. The condensates are collected as separate fractions.

The fraction with the smallest molecules is used as a fuel, e.g. petrol for cars or LPG for heating and cooking.

Examiner tips

  • Use the word ‘vapourises’ and mention the temperature gradient; link boiling point differences to separation; state a specific use of the lightest fraction.
  • Show the sequence: heating → vapour → column → condensation at different heights → collection.

Common mistakes

  • Failing to mention the temperature gradient or boiling point differences; giving a use that is not the lightest fraction (e.g. diesel).

Mark scheme (5 marks)

  1. Crude oil is heated / vaporised
  2. Hydrocarbons have different boiling points
  3. The vapour rises up the fractionating column where there is a temperature gradient (hotter at the bottom, cooler at the top)
  4. Fractions condense and are collected at different levels / heights in the column
  5. A correct use of the fraction with the smallest molecules, e.g. petroleum gas / LPG used as fuel for heating or cooking; petrol used as fuel for cars

Key terms in this question

hydrocarbon · fractional distillation · fraction

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