Butane and octane are both alkanes obtained from crude oil. When octane undergoes complete combustion, sulfur dioxide is also produced as a by-product. Explain why sulfur dioxide is produced during the combustion of octane and describe one environmental consequence of sulfur dioxide being released into the atmosphere.

Pearson Edexcel International GCSE Chemistry (4CH1) — 4.3 Alkanes · Explain · 4 marks · View as Markdown

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

Crude oil contains hydrocarbons as well as small amounts of sulfur impurities. Octane (C₈H₁₈) is a hydrocarbon fraction obtained from crude oil by fractional distillation.

Model answer (4 marks)

Octane is a hydrocarbon but the crude oil from which it is distilled also contains small amounts of sulphur impurities.
During complete combustion the sulphur present in the oil is oxidised by oxygen to give sulphur dioxide (SO₂).
The SO₂ released into the atmosphere reacts with water vapour in clouds to form sulphuric acid.
This acid rain can damage ecosystems, buildings, lakes and trees, and kills aquatic life.

Examiner tips

  • Mention sulphur impurities in crude oil, oxidation to SO₂, reaction with water to form acid rain, and an environmental effect such as damage to ecosystems or buildings.
  • Use the exact phrase "sulphur dioxide" and "acid rain" to match the mark scheme.
  • Keep the answer concise – 4 points, one sentence per point.

Common mistakes

  • Confusing the hydrocarbon itself with the sulphur impurities; students sometimes say the octane *contains* sulphur in its structure. Failing to mention the oxidation step or the formation of acid rain. Using the American spelling "sulfur" instead of the UK spelling "sulphur".

Mark scheme (4 marks)

  1. Octane contains sulfur impurities (not part of the hydrocarbon itself)
  2. During combustion, the sulfur impurities are oxidised / react with oxygen to produce sulfur dioxide
  3. Sulfur dioxide reacts with water vapour in clouds / atmosphere to form acid rain
  4. Acid rain causes damage to ecosystems / buildings / lakes / trees / kills aquatic life

Key terms in this question

combustion · sulfur dioxide

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