Crude oil is a mixture of hydrocarbons. When crude oil is fractionally distilled, it is separated into fractions, each containing molecules with similar chain lengths. Explain why the fractions collected at the top of a fractionating column have lower boiling points than those collected at the bottom, and describe one use for a short-chain hydrocarbon fraction.
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 that must be separated before the components can be used as fuels or chemical feedstocks.
Model answer (5 marks)
Short‑chain hydrocarbons contain fewer carbon atoms, so they are smaller molecules. They have weaker London dispersion forces between molecules. Less energy is required to overcome these forces, giving a lower boiling point. In a fractionating column the temperature decreases from bottom to top; only vapour of the low‑boiling, short‑chain hydrocarbons rises and is collected at the top, whereas the higher‑boiling, long‑chain hydrocarbons condense and are collected at the bottom.
A short‑chain fraction is used as petrol (gasoline) for car engines, or as LPG for cooking and heating, or as naphtha as a chemical feedstock.
A short‑chain fraction is used as petrol (gasoline) for car engines, or as LPG for cooking and heating, or as naphtha as a chemical feedstock.
Examiner tips
- Mention chain length and weaker London forces to justify lower boiling point. State the temperature gradient in the column. Give a specific, named use of a short‑chain fraction.
Common mistakes
- Confusing boiling point with melting point. Saying all hydrocarbons boil at the same temperature. Failing to link column temperature to fraction collection.
Mark scheme (5 marks)
- Short-chain hydrocarbons have fewer carbon atoms (and smaller molecules) than long-chain hydrocarbons
- Short-chain hydrocarbons have weaker intermolecular forces between molecules
- Less energy is needed to overcome these forces, so boiling point is lower
- The fractionating column is cooler at the top, so only short-chain (low boiling point) hydrocarbons remain as vapour and are collected there
- A named use of a short-chain hydrocarbon fraction, e.g. petrol/gasoline used as fuel for cars; LPG used as fuel for cooking/heating; naphtha used as a chemical feedstock
Key terms in this question
hydrocarbon · fraction · boiling point · fractionating column
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →