Crude oil is a finite resource formed from the remains of ancient organisms. It contains many different hydrocarbons that can be separated by fractional distillation. Explain how fractional distillation separates crude oil into fractions, and describe how the properties of the fractions change from the bottom to the top of the fractionating column.

Edexcel A-Level Chemistry (9CH0) — 6.1 Introduction to organic chemistry · Explain · 5 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. It is heated and the vapours rise through a fractionating column, which has a temperature gradient — hottest at the bottom and coolest at the top.

Model answer (5 marks)

Crude oil is heated until the hydrocarbons are vapourised. The vapour rises through a fractionating column that has a temperature gradient – the bottom is hot and the top is cool. As the vapour ascends, each hydrocarbon condenses when it reaches the part of the column that is at its boiling point and is collected as a fraction. Fractions taken from the bottom of the column contain hydrocarbons with longer carbon chains, higher molecular mass and higher boiling points. Fractions taken from the top contain hydrocarbons with shorter chains, lower boiling points, are more volatile, more flammable and less viscous than those at the bottom.

Examiner tips

  • Use the word ‘condense’ to link temperature to collection point
  • Show the temperature gradient explicitly
  • Mention chain length and boiling point changes
  • Keep the answer concise and use exam terminology

Common mistakes

  • Confusing boiling point with melting point
  • Saying the column is ‘uniformly hot’ instead of having a gradient
  • Failing to mention that top fractions are more volatile and less viscous

Mark scheme (5 marks)

  1. Crude oil is heated and the hydrocarbons are vaporised
  2. The fractionating column has a temperature gradient — hottest at the bottom and coolest at the top
  3. Hydrocarbons condense when they reach the part of the column that is at their boiling point / temperature, and are collected as fractions
  4. Fractions collected at the bottom have longer carbon chains / higher molecular mass and higher boiling points
  5. Fractions collected at the top have shorter carbon chains and are more flammable / more volatile / less viscous than fractions at the bottom

Key terms in this question

fractional distillation · fraction · hydrocarbon

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