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6.2.8 Crude Oil & Fractional Distillation

Crude Oil

  • Crude oil is a finite resource which we find in the Earth's crust
  • It is also called petroleum and is a complex mixture of hydrocarbons which also contains natural gas
    • Hydrocarbons are compounds that are made of carbon and hydrogen atoms only

  • The hydrocarbon molecules in crude oil consist of a carbon backbone which can be in a ring or chain, with hydrogen atoms attached to the carbon atoms
  • The mixture contains molecules with many different ring sizes and chain lengths
  • It is a thick, sticky, black liquid that is found in porous rock (under the ground and under the sea)
  • Crude oil formed over millions of years from the effects of high pressures and temperatures on the remains of plants and animals
  • It is being used up much faster than it is being formed, which is why we say crude oil is a finite resource

Crude-oil-under-the-sea, IGCSE & GCSE Chemistry revision notes

Crude oil found under the sea

Exam Tip

Crude oil is also sometimes referred to as petroleum. Some fractions may have different names in the UK and the USA, e.g. gasoline is the name used in the USA for petrol. You may be asked to give a definition of the term hydrocarbon - be careful! You must say a compound which contains carbon and hydrogen atoms only. If you do not say only, then you might not get the mark.

Fractional Distillation

  • Crude oil as a mixture is not a very useful substance but the different hydrocarbons that make up the mixture, called fractions, are enormously valuable, with each fraction having many different applications
  • Each fraction consists of groups of hydrocarbons of similar chain lengths
  • The fractions in petroleum are separated from each other in a process called fractional distillation
  • The molecules in each fraction have similar properties and boiling points, which depend on the number of carbon atoms in the chain
  • 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

Diagram showing the process of fractional distillation to separate crude oil in a fractionating column

  • 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
  • As the size of the hydrocarbon increases, the boiling point increases because the intermolecular forces get stronger and require more energy to break

Exam Tip

As you move up a fractionating column the temperature decreases, so the compounds with higher boiling points come off lower down the column.

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