AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Petroleum is a mixture consisting mainly of alkane hydrocarbons that can…
Full specification wording
Fractional distillation of crude oil: Petroleum is a mixture consisting mainly of alkane hydrocarbons that can be separated by fractional distillation.
Crude oil is a mixture of alkanes of many chain lengths, and because they have different boiling points they can be separated by fractional distillation.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 2 of 2
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Crude oil is a mixture of alkanes of many chain lengths, and because they have different boiling points they can be separated by fractional distillation. The oil is heated until it vaporises and is fed into a tall column that is hot at the bottom and cooler towards the top. Vapours rise until they reach a tray cool enough for them to condense, and the liquid is drawn off there. Long chain molecules have more electrons and a greater surface area of contact, so their London forces are stronger, their boiling points higher, and they condense low down as fuel oil and bitumen. Short chains condense near the top as refinery gases and petrol, with naphtha and kerosene between. Each fraction is still a mixture.
What earns the marks
- Vaporising the crude oil and feeding it into a column with a temperature gradient.
- A fraction condensing at the height where the temperature falls below its boiling point.
- Linking a higher boiling point to a longer chain and stronger London forces.
- Naming fractions in the correct order of boiling point up the column.
Common mistakes and how to put them right
Saying each fraction is a pure compound rather than a mixture collected over a range of boiling points.
Putting the hot end at the top of the column.
Explaining higher boiling points by stronger covalent bonds, when no covalent bonds break on boiling.
Calling the process simple distillation, which separates only two well separated components.
Exam tips
- Name London forces explicitly when explaining the boiling point order, since intermolecular forces alone is weaker.
- Say that forces between molecules are overcome, not that bonds inside molecules break.
- Give the fractions in order and attach a use to each if the question asks for one.
Try the lesson
The start of the Fractional distillation of crude oil: Petroleum is a mixture consisting mainly of alkane hydrocarbons that can be separated by fractional distillation. lesson, made from checked teaching notes for this specification point.
Have a go first
Petroleum (crude oil) is an important raw material. Which statement best describes the main composition of petroleum?
Listen
Crude oil is a mixture of alkanes of many chain lengths, and because they have different boiling points they can be separated by fractional distillation. The oil is heated until it vaporises and is fed into a tall column that is hot at the bottom and cooler towards the top. Vapours rise until they reach a tray cool enough for them to condense, and the liquid is drawn off there. Long chain molecules have more electrons and a greater surface area of contact, so their London forces are stronger, their boiling points higher, and they condense low down as fuel oil and bitumen. Short chains condense near the top as refinery gases and petrol, with naphtha and kerosene between. Each fraction is still a mixture. A strong answer earns marks for these points. vaporising the crude oil and feeding it into a column with a temperature gradient. a fraction condensing at the height where the temperature falls below its boiling point. linking a higher boiling point to a longer chain and stronger London forces. naming fractions in the correct order of boiling point up the column. Exam tip: Name London forces explicitly when explaining the boiling point order, since intermolecular forces alone is weaker.
Key words
Say what each word means before you turn the card.
crude oil composition
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fractional distillation
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temperature gradient
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London forces and chain length
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See every definition
- crude oil composition
- Petroleum is a mixture consisting mainly of alkane hydrocarbons of many chain lengths.
- fractional distillation
- A process in which vaporised crude oil is fed into a tall column and vapours rise until they reach a tray cool enough for them to condense.
- temperature gradient
- The column is hot at the bottom and cooler towards the top.
- London forces and chain length
- Long chain molecules have more electrons and a greater surface area of contact, so their London forces are stronger and their boiling points higher.
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