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3.3.2 Alkanes summary AQA AS level
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3.2 Alkanes3.3.2.1 Fractional distillation of crude oil
Crude oil is a mixture of different hydrocarbons. It can be separated into the separate molecules by
fractional distillation as the different chain lengths of molecules result in them having different boiling
points.
How is crude oil separated?
⎯ The mixture is vapourised and fed into the fractionating column.
⎯ Vapours rise, cool, and condense.
⎯ Products are siphoned off for different uses.
SHORT CARBON CHAINS LONG CARBON CHAINS
Lower boiling points so rise higher up the Higher boiling points so they don’t have to rise
column before reaching their boiling point. very far up the column before reaching their
Therefore they are collected at the top of the boiling point. They condense and are collected
column. at the bottom of the fractionating column
The compounds collected from the fractionating column are then broken down further via the
method of cracking.
3.3.2.2 Modification of alkanes by cracking
Longer carbon chains are broken down to form smaller, more useful molecules. The carbon – carbon
bonds are broken in order to this which requires harsh reaction conditions.
Types of cracking:
Thermal cracking
⎯ Produces a high proportion of alkanes and alkenes.
⎯ High temperatures around 1200K and pressures around 7000kPA are used to crack the
carbon chains.
Catalytic cracking
⎯ Produces aromatic compounds with carbon rings.
⎯ Lower temperatures around 720K are used along with normal pressure.
⎯ Zeolite catalyst is needed to compensate for these less harsh conditions.
Explain the main economic reason why alkanes are cracked.
⎯ To produce substances which are (more) in demand.
3.3.2.3 Combustion of alkanes
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