Crude Oil & Fractional Distillation (OCR Gateway GCSE Chemistry: Combined Science)

• 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 found under the sea

• 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

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