AQA AS Chemistry

Revision Notes

3.2.1 Fractional Distillation of Crude Oil

Fractional Distillation

  • Crude oil is currently the world’s main source of organic chemicals
  • As a mixture, it isn’t a very useful substance, but the different hydrocarbons that make up the mixture, called fractions, are useful, with each fraction having different applications
  • It is a mixture of mostly alkanes, unbranched and branched chain
    • The mixture is separated into fractions, based on boiling points
  • Each fraction consists of groups of hydrocarbons of similar chain lengths
    • Hydrocarbons of similar chain length have similar boiling points
  • The fractions are separated from each other in a process called fractional distillation
  • The size and length of each hydrocarbon molecule determines in which fraction it will be separated into
  • The size of each molecule is directly related to how many carbon and hydrogen atoms the molecule contains
  • Most fractions contain mainly alkanes, which are compounds of carbon and hydrogen with only single bonds between them

Fractional-Distillation, IGCSE & GCSE Chemistry revision notes

The Process of Fractional Distillation of Crude Oil

  • Fractional distillation is carried out in a fractionating column which is very hot at the bottom and cool at the top
  • Crude oil enters the fractionating column and is heated so vapours rise
  • Vapours of hydrocarbons with very high boiling points will immediately condense into liquid at the higher temperatures lower down and are tapped off at the bottom of the column
  • Vapours of hydrocarbons with low boiling points will rise up the column and condense at the top to be tapped off
  • The different fractions condense at different heights according to their boiling points and are tapped off as liquids
  • The fractions containing smaller hydrocarbons are collected at the top of the fractionating column as gases
  • The fractions containing bigger hydrocarbons are collected at the lower sections of the fractionating column
  • The crude oil contains small amounts of other compounds
  • Some of these compounds may contain sulfur, and when burned sulfur dioxide can be produced
    • This is one of the causes of acid rain

 

Exam Tip

Fractional distillation is a physical process and not a chemical one, so no covalent bonds are broken. The intermolecular forces between molecules are broken as the crude oil is heated.

You do not need to memorise the fractions, you just need to understand the principle of what is happening during the process, and why it is used.

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