Fractional Distillation of Crude Oil (Oxford AQA International A Level Chemistry)
Revision Note
Written by: Richard Boole
Reviewed by: Stewart Hird
Fractional Distillation of Crude Oil
Crude oil, or petroleum, is currently the world's main source of organic chemicals
It is a mixture of mostly alkanes
Alkanes are saturated hydrocarbons, which means that:
They contain only single covalent carbon-carbon bonds
They are compounds made from only hydrogen and carbon
They can be straight chain or branched chain
As a mixture, it isn’t a very useful substance
However, the different hydrocarbons that make up the mixture are useful
The mixture of alkanes from crude oil is separated into fractions, based on boiling points
Each fraction has different uses and 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 the fraction an alkane will be separated into
The size of each molecule is directly related to how many carbon and hydrogen atoms the molecule contains
The fractional distillation of crude oil
Fractional distillation is carried out in a fractionating column which has a temperature gradient
It is very hot at the bottom of the column and cooler at the top
During the process of fractional distillation:
The crude oil is heated and vapourises
The vapours of hydrocarbons enter the column which has a temperature gradient
The vapours of hydrocarbons with high boiling points condense at the bottom of the column
The vapours of hydrocarbons with lower boiling points rise up the column and condense at the top
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
Examiner Tips and Tricks
Fractional distillation is a physical process, 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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