Crude Oil (Oxford AQA IGCSE Chemistry)
Revision Note
Author
Alexandra BrennanExpertise
Chemistry
Crude Oil
Crude oil is a finite resource which we find in the Earth's crust
It is finite because it is used up much quicker than it is produced
It is a complex mixture of compounds, mainly 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)
Where crude oil is found
Crude oil is found under the sea
Exam Tip
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 will 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 crude oil (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
Fractional Distillation
Fractional distillation involves evaporation and condensation
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
Each fraction contains molecules with a similar number of carbon atoms
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
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.
You do not need to know the specific fractions that are produced during fractional distillation.
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