Crude oil is separated into fractions by fractional distillation. Each fraction contains hydrocarbons with different chain lengths. Explain why shorter-chain hydrocarbons are more useful as fuels than longer-chain hydrocarbons.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Crude oil contains hydrocarbons ranging from very short carbon chains to very long carbon chains. Short-chain hydrocarbons are found in fractions collected near the top of the fractionating column, while long-chain hydrocarbons are collected near the bottom.
Model answer (5 marks)
Short‑chain hydrocarbons have lower boiling points than long‑chain hydrocarbons, so they evaporate more readily and can be used as fuels that ignite easily. Their lower molecular weight also means they are more flammable and give a higher energy output per unit mass. Because they are gases or low‑viscosity liquids at room temperature, they flow easily, are easier to store and transport, and can be pumped through pipelines or used in engines with less energy loss. In contrast, long‑chain hydrocarbons have higher boiling points, higher viscosity and are less flammable; they are harder to ignite and tend to produce incomplete combustion and more soot.
Examiner tips
- Use the word ‘lower boiling point’ to link chain length to volatility. Mention flammability and energy density. Explain ease of flow/transport. Contrast with incomplete combustion of long chains.
Common mistakes
- Confusing viscosity with boiling point. Forgetting to mention energy output per mass. Saying long chains are better fuels without justification.
Mark scheme (5 marks)
- Short-chain hydrocarbons have lower boiling points (than long-chain hydrocarbons)
- Short-chain hydrocarbons are more flammable / ignite more easily
- Short-chain hydrocarbons flow more easily / have lower viscosity
- Short-chain hydrocarbons are gases or liquids at room temperature, making them easier to store / transport / use as fuels
- Long-chain hydrocarbons are more difficult to burn completely / produce more incomplete combustion / more soot
Key terms in this question
hydrocarbon · fractional distillation
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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