Describe how fractional distillation separates crude oil into fractions, and explain why fractions collected near the top of the fractionating column have lower boiling points than those collected near the bottom.

AQA A-Level Chemistry (7405) — 3.3.10 Aromatic chemistry (A-Level only) · Describe and 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 evaporates.
The vapour rises up the fractionating column.
Different fractions condense at different levels of the column, where the temperature decreases, and are tapped off.
Fractions collected near the top contain molecules with shorter or fewer carbon atoms.
Shorter hydrocarbon molecules have lower boiling points, so they condense higher up the column where it is cooler.

Examiner tips

  • Use the word "evaporates" for heating crude oil. Mention the temperature gradient in the column. Explain the relation between chain length and boiling point. Show the sequence: top = low BP, bottom = high BP.

Common mistakes

  • Saying the column is heated instead of the crude oil. Forgetting that vapour rises. Confusing higher fractions with higher boiling points. Not linking chain length to boiling point.

Mark scheme (5 marks)

  1. Crude oil is heated so that it evaporates / vaporises
  2. The vapour rises up the fractionating column
  3. Different fractions condense at different levels / temperatures in the column and are tapped off
  4. Fractions near the top contain molecules with shorter / fewer carbon atom chains
  5. Shorter hydrocarbon molecules have lower boiling points (so they condense higher up the column where it is cooler)

Key terms in this question

fractional distillation · fractions · fractionating column · boiling point

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