After crude oil has been distilled into fractions you still have some long and short hydrocarbons. Some hydrocarbons are more sought after than other though and so some of the longer molecule fractions are “cracked” into smaller molecules.
Long chain hydrocarbons come out from the fractional distillation as thick tar like substances so are cracked into smaller molecules.
Cracking can produce not only petrol, paraffin and other fuels but also substances like ethene which is used to make plastics.
Cracking is a thermal decomposition reaction i.e. breaking down molecules using heat.
First the long-chain hydrocarbon is vaporised and then passed over a powdered catalyst of aluminium oxide- this is done between 400ºC and 700ºC! The long chain molecules crack apart on the surface of the catalyst.
Usually when it cracks, two things are formed:
- A shorter alkane (i.e. a saturated hydrocarbon)
- An alkene (i.e. an unsaturated hydrocarbon)
So let’s take an example of hexane:
As you can see the long chain hydrocarbon hexane is cracked into butane (a shorter alkane), which has four carbon atoms, and ethene (an alkene) which has two carbon atoms.