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

WJEC A-Level Chemistry (Wales) — 2.4 Organic compounds · Explain · 5 marks · View as Markdown

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

Crude oil is a finite resource formed over millions of years from the remains of ancient organisms. It is a complex mixture of hydrocarbon compounds.

Model answer (5 marks)

Crude oil is heated in a distillation column until it vapourises. The vapours rise through the column, which is temperature‑graded from hot at the bottom to cool at the top. Because hydrocarbons have different boiling points, the vapour containing the lowest‑boiling (smallest) molecules rises first and condenses in the upper part of the column where the temperature is lower. As the temperature decreases further up the column, larger molecules condense at progressively higher positions. Each condensed liquid is collected as a separate fraction. The fraction with the smallest molecules is often used as refinery gases or LPG, which is used as fuel for heating or cooking.

Examiner tips

  • Show the temperature gradient in the column and link boiling point to position of condensation
  • Use the words ‘vapourises’, ‘condenses’, ‘fraction’, ‘boiling point’
  • Give a specific use for the lightest fraction, e.g. LPG for heating

Common mistakes

  • Confusing the direction of the column (saying molecules rise to the bottom)
  • Not mentioning that the column is temperature‑graded
  • Giving a use that is not for the lightest fraction, such as diesel or kerosene

Mark scheme (5 marks)

  1. Crude oil is heated / vaporised so the hydrocarbons evaporate
  2. Hydrocarbons have different boiling points
  3. Hydrocarbons with smaller/shorter molecules have lower boiling points and rise higher up the column / condense higher up the column
  4. Hydrocarbons condense / turn back to liquid when the temperature in the column equals their boiling point, and are collected as separate fractions
  5. A correct use for the fraction with the smallest molecules (e.g. refinery gases / LPG used as fuel for heating or cooking / bottled gas)

Key terms in this question

fractional distillation · hydrocarbon · fraction

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