Fractional Distillation of Crude Oil (AQA A Level Chemistry): Revision Note

Fractional Distillation

• Crude oil is currently the world’s main source of organic chemicals

• It is a complex mixture of hydrocarbons, mainly alkanes, which may be straight-chain (unbranched) or branched

• As a mixture, crude oil is not very useful

  • However, it can be separated into fractions, each containing hydrocarbons with similar chain lengths and therefore similar boiling points

  • Hydrocarbons with similar numbers of carbon atoms have similar intermolecular forces and boiling points

• The separation process is called fractional distillation

  • This is carried out in a fractionating column that is hot at the bottom and cooler at the top

• Crude oil is first heated so that most of it vaporises:

  • The vapours enter the fractionating column and rise upwards

  • Hydrocarbons with high boiling points condense lower down in the column, where the temperature is higher, and are drawn off

  • Hydrocarbons with lower boiling points rise further up the column before condensing and are collected higher up

• Smaller hydrocarbon molecules (with lower boiling points) are collected near the top of the column, often as gases

• Larger hydrocarbon molecules (with higher boiling points) are collected lower down.

• Crude oil also contains small amounts of other compounds, including sulfur-containing compounds

• When these are burned, sulfur dioxide can be produced, which contributes to acid rain