AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
The many hydrocarbons in crude oil may be separated into fractions, each…
Full specification wording
Fractional distillation and petrochemicals: The many hydrocarbons in crude oil may be separated into fractions, each of which contains molecules with a similar number of carbon atoms, by fractional distillation.
Crude oil is a mixture of many hydrocarbons, compounds of carbon and hydrogen only.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 1 of 6
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Crude oil is a mixture of many hydrocarbons, compounds of carbon and hydrogen only. Because the molecules differ in size, they have different boiling points, so the mixture can be separated by fractional distillation. The crude oil is heated until most of it vaporises, and the vapour enters a tall fractionating column that is hot at the bottom and cooler at the top. As the vapour rises it cools; molecules with many carbon atoms condense at lower, hotter levels and are tapped off, while smaller molecules travel higher before condensing. Each collected fraction is a mixture of hydrocarbons with a similar number of carbon atoms and therefore a similar boiling point range, not a single pure compound.
What earns the marks
- Crude oil is a mixture of many different hydrocarbons, so it has a range of boiling points rather than one sharp boiling point.
- Fractional distillation separates the mixture into fractions, each containing molecules with a similar number of carbon atoms.
- The fractionating column has a temperature gradient: hot near the bottom and cooler towards the top.
- Hydrocarbons with larger molecules have higher boiling points and condense lower down the column.
- Hydrocarbons with smaller molecules have lower boiling points and rise further before condensing.
- Each fraction is still a mixture of hydrocarbons with similar boiling points, not a pure substance.
Common mistakes and how to put them right
Saying fractional distillation separates crude oil into pure hydrocarbons.
Put it right by stating that each fraction is a mixture of hydrocarbons with a similar number of carbon atoms.
Reversing the temperature gradient by saying the column is cool at the bottom and hot at the top.
Put it right by stating that the bottom is hottest and the top is coolest.
Confusing condensation with boiling when explaining where a fraction leaves the column.
Put it right by saying larger molecules condense lower down and smaller molecules condense higher up.
Exam tips
- Link molecular size to boiling point and then to the height at which a fraction is collected, so your explanation forms a clear chain of reasoning.
- Use the term fraction precisely: a group of hydrocarbons with a similar number of carbon atoms and a similar boiling point range.
- When describing the column, state the temperature gradient first, then explain how rising vapour cools and condenses at different heights.
Try the lesson
The start of the Fractional distillation and petrochemicals: The many hydrocarbons in crude oil may be separated into fractions, each of which contains molecules with a similar number of carbon atoms, by fractional distillation. lesson, made from checked teaching notes for this specification point.
Have a go first
In fractional distillation, the many hydrocarbons in crude oil are separated into fractions. What is similar about the molecules within one fraction?
Listen
Crude oil is a mixture of many hydrocarbons, compounds of carbon and hydrogen only. Because the molecules differ in size, they have different boiling points, so the mixture can be separated by fractional distillation. The crude oil is heated until most of it vaporises, and the vapour enters a tall fractionating column that is hot at the bottom and cooler at the top. As the vapour rises it cools; molecules with many carbon atoms condense at lower, hotter levels and are tapped off, while smaller molecules travel higher before condensing. Each collected fraction is a mixture of hydrocarbons with a similar number of carbon atoms and therefore a similar boiling point range, not a single pure compound. A strong answer earns marks for these points. Crude oil is a mixture of many different hydrocarbons, so it has a range of boiling points rather than one sharp boiling point. Fractional distillation separates the mixture into fractions, each containing molecules with a similar number of carbon atoms. The fractionating column has a temperature gradient: hot near the bottom and cooler towards the top. Hydrocarbons with larger molecules have higher boiling points and condense lower down the column. A common slip: Saying fractional distillation separates crude oil into pure hydrocarbons. Instead, stating that each fraction is a mixture of hydrocarbons with a similar number of carbon atoms. Exam tip: Link molecular size to boiling point and then to the height at which a fraction is collected, so your explanation forms a clear chain of reasoning.
See it
Key words
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Fractional distillation
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Fraction
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Hydrocarbons
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See every definition
- Fractional distillation
- The process by which the many hydrocarbons in crude oil are separated into fractions, each containing molecules with a similar number of carbon atoms.
- Fraction
- A mixture of hydrocarbons with a similar number of carbon atoms and therefore a similar boiling point range, not a single pure compound.
- Hydrocarbons
- Compounds of carbon and hydrogen only, many of which make up crude oil.
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