Crude oil is a mixture of hydrocarbons. Fractional distillation is used to separate crude oil into fractions. Explain how fractional distillation separates crude oil into fractions, and describe one property that differs between a fraction collected near the top of the fractionating column and a fraction collected near the bottom.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Crude oil is formed over millions of years from the remains of ancient marine organisms. It is a finite resource and an important source of fuels and chemical feedstocks.
Model answer (5 marks)
Crude oil is heated until it vapourises; the vapours rise into a fractionating column.
The column is hotter at the bottom and cooler at the top. Hydrocarbons have different boiling points, so as the vapours ascend they reach a temperature equal to their boiling point and condense. Each hydrocarbon therefore condenses at a different height in the column.
The condensate that collects at the top of the column consists of hydrocarbons with lower boiling points (short‑chain alkanes, for example). The condensate that collects at the bottom contains hydrocarbons with higher boiling points (long‑chain alkanes). Thus a top fraction is less viscous and more flammable than a bottom fraction, which is more viscous and less flammable.
The column is hotter at the bottom and cooler at the top. Hydrocarbons have different boiling points, so as the vapours ascend they reach a temperature equal to their boiling point and condense. Each hydrocarbon therefore condenses at a different height in the column.
The condensate that collects at the top of the column consists of hydrocarbons with lower boiling points (short‑chain alkanes, for example). The condensate that collects at the bottom contains hydrocarbons with higher boiling points (long‑chain alkanes). Thus a top fraction is less viscous and more flammable than a bottom fraction, which is more viscous and less flammable.
Examiner tips
- Show the heating and vapourisation step first. Mention the temperature gradient in the column. Explain condensation at the boiling point. Give a clear property difference (e.g. boiling point, chain length, viscosity).
Common mistakes
- Forgetting that the column is hotter at the bottom. Confusing the direction of the temperature gradient. Giving a property that does not relate to boiling point or chain length.
Mark scheme (5 marks)
- Crude oil is heated / vaporised and the vapours enter the fractionating column
- Different hydrocarbons have different boiling points
- Hydrocarbons condense when the temperature in the column falls to their boiling point / condense at different levels
- The column is hotter at the bottom and cooler at the top, so fractions with higher boiling points are collected lower down
- A named property that correctly differs between a top fraction and a bottom fraction — e.g. fractions near the top have shorter carbon chains / lower boiling points / are more flammable / less viscous compared to fractions near the bottom which have longer carbon chains / higher boiling points / are less flammable / more viscous
Key terms in this question
hydrocarbon · fraction · fractional distillation
Related
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