Crude oil is a mixture of hydrocarbons. Fractional distillation is used to separate crude oil into fractions, and some fractions are then cracked to produce more useful products. Explain how fractional distillation separates crude oil into fractions, and state one reason why cracking is carried out.
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 organisms. It is the source of many important fuels and chemical feedstocks.
Model answer (5 marks)
Crude oil is heated in a furnace so that the hydrocarbons evaporate. The vapour rises into a fractionating column where the temperature decreases with height. Because different hydrocarbons have different boiling points, the vapour condenses at different levels of the column. Short‑chain hydrocarbons, which have lower boiling points, condense higher up and are collected as lighter fractions such as petrol; longer‑chain hydrocarbons, with higher boiling points, condense lower down and are collected as heavier fractions such as diesel or lubricating oils.
Cracking is carried out to increase the yield of useful lighter products – for example, to produce more petrol or to generate alkenes that can be used as monomers for plastics.
Cracking is carried out to increase the yield of useful lighter products – for example, to produce more petrol or to generate alkenes that can be used as monomers for plastics.
Examiner tips
- Use the word ‘boiling point’ and ‘condense’ to show understanding of separation; mention the temperature gradient in the column; state the purpose of cracking clearly; keep the answer concise and to the point.
Common mistakes
- Confusing evaporation with condensation; not mentioning the temperature gradient; failing to state the reason for cracking (e.g. demand for lighter fuels).
Mark scheme (5 marks)
- Crude oil is heated / vaporised so the hydrocarbons evaporate
- Different hydrocarbons / fractions have different boiling points
- Hydrocarbons condense / are collected at different levels / temperatures in the fractionating column
- Longer chain hydrocarbons have higher boiling points / condense lower in the column; shorter chain hydrocarbons have lower boiling points / condense higher in the column
- Cracking is carried out because there is greater demand for shorter chain / lighter fractions (such as petrol / fuels) than for longer chain / heavier fractions, OR to produce alkenes which can be used as monomers / in making plastics
Key terms in this question
fractional distillation · fraction · cracking · hydrocarbon
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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