Crude oil is a mixture of hydrocarbons. Fractional distillation is used to separate crude oil into fractions, each of which contains hydrocarbons with similar chain lengths. Explain how fractional distillation separates crude oil into fractions, and describe how the properties of the fractions change as the chain length of the hydrocarbons increases.

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 finite resource formed over millions of years from the remains of ancient marine organisms. It is a complex mixture of hydrocarbon molecules of different chain lengths.

Model answer (5 marks)

Crude oil is heated until the hydrocarbons evaporate. The vapour rises through a fractionating column where the temperature decreases with height. Hydrocarbons of different chain lengths have different boiling points – the shorter‑chain, lower‑boiling species evaporate first and rise higher up the column, where they condense and are collected at the top. Longer‑chain, higher‑boiling hydrocarbons remain lower in the column and are collected from the bottom. As the chain length increases the boiling point rises, the viscosity of the fraction increases, and the ease of ignition (flammability) decreases.

Examiner tips

  • Use the phrase ‘fractionating column’ and link boiling point to chain length. Mention that lower‑boiling fractions are collected at the top, higher‑boiling at the bottom. Show the change in viscosity and flammability with chain length. Keep the answer concise – 5 marks only.
  • Use the exact terminology: ‘boiling point’, ‘fractionating column’, ‘viscosity’, ‘flammability’.
  • Avoid vague statements such as ‘different properties’ – specify boiling point, viscosity and flammability.

Mark scheme (5 marks)

  1. Crude oil is heated / vaporised so the hydrocarbons evaporate
  2. Hydrocarbons with different chain lengths have different boiling points
  3. Shorter-chain / lower boiling point hydrocarbons rise higher up the fractionating column and condense / are collected at the top
  4. As chain length increases, boiling point increases
  5. As chain length increases, viscosity increases AND/OR flammability (ease of ignition) decreases

Key terms in this question

hydrocarbons · fractional distillation · fractions

Related

More Introduction to organic chemistry questions

▶ Try answering this question with AI marking (free) →