A car manufacturer is comparing two fuels for use in a new engine: petrol and liquefied petroleum gas (LPG), which is mainly propane. Explain the advantages and disadvantages of using LPG instead of petrol as a fuel for cars.

OCR A-Level Chemistry B: Salters (H433) — DF Developing fuels · Explain · 5 marks · View as Markdown

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

Petrol is a liquid fuel obtained from crude oil and is widely used in car engines. LPG (liquefied petroleum gas) is also obtained from crude oil but contains shorter-chain hydrocarbons such as propane. Both fuels release energy by combustion.

Model answer (5 marks)

1. LPG gives cleaner combustion – complete combustion of propane produces only CO₂ and H₂O, so less CO and other toxic gases are released compared with petrol.
2. Shorter hydrocarbon chains in LPG mean fewer particulates and less soot are formed, improving air quality.
3. LPG is still a fossil fuel; it will eventually be exhausted, so it is not a sustainable long‑term energy source.
4. LPG must be stored under pressure as a liquid, requiring a stronger and heavier tank, which reduces convenience and increases vehicle weight.
5. Burning LPG still emits CO₂, contributing to the greenhouse effect and climate change, so it does not solve global warming.

Examiner tips

  • Use the exact phrases from the mark scheme (e.g., "complete combustion", "fewer particulates", "non‑renewable fossil fuel").
  • Show the five points clearly, each matching one mark.
  • Mention both advantages and disadvantages as requested.

Common mistakes

  • Confusing LPG with natural gas or ignoring the pressure storage requirement.
  • Claiming LPG is renewable or that it eliminates CO₂ emissions.

Mark scheme (5 marks)

  1. Complete combustion of LPG/propane produces carbon dioxide and water only, so fewer toxic pollutants such as carbon monoxide are produced compared to petrol
  2. LPG produces fewer particulates / less soot than petrol because the hydrocarbon chains are shorter
  3. LPG is still a non-renewable fossil fuel / it will eventually run out, so it is not a long-term sustainable solution
  4. LPG must be stored under pressure as a liquid, which requires a stronger/heavier fuel tank, making it less convenient than petrol
  5. LPG still releases carbon dioxide when burned, contributing to the greenhouse effect / climate change, so it does not eliminate the problem of global warming

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