Fractional Distillation is baically:
Crude Oil is put into a fractionating column.
It is heated to over 300 degrees C
The crude oil is made up of many different alkanes of differnt boiling points.
When heated, the different parts of the crude oil evaporate.
They all condense at different temperatures.
Long chained hydrocarbons are found at the bottom fraction because they have strong bonds which are hard to break.
Shorter chained hydrocarbons are found at the top fractions because they have weaker bond which are easy to break.
These fractions have now seperated different length hydrocrabons into smaller alkanes.
NOTE: Some of the long chained hydrocarbons which are found at the bottom can be broke down into smaller hydrocarbons to make them more useful. This can be completed through CRACKING: using heat (from steam of a catalyst) to break these long chains into smaller alkanes and alkenes.
Hope that helped and, if you can phrase it better than I have, that would get you top marks. Just make sure you include each stage and this will get you an easy 6 marks!