AQA AS CHEMISTRY - FRACTIONAL DISTILATION

Fractional Distilation:

This involves different hydrocarbons in crude oil that have different boiling points due to their chain length variation and whether they are branched or unbranched.

More carbon atoms = greater chain length

The greater length means there are more VDW’s forces meaning a greater intermolecular attraction, thus more energy is needed to separate these molecules, this in turn increses their boiling point. This difference in boiling point is what separates the crude oil into the different hydrocarbons

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• Crude oil passed into a fractionating column
• Starts at the base where it is the hottest and is cooler at the top
•  Mixture starts to boil and rise passing up the tower via a series of trays.

the tower contains bubble caps - the mixture rises up until it is suffieciently cool enough and allows for the hydrocarbon chains to condense to a liquid(this is because the temperature is lower than there boiling point)

• They  are then tapped off
• The shorter chain hydrocarbons then condense off nearer the top where the temperature is lower(cooler) because of their lower boiling points

Large HydroCarbon= higher bp so they condense first, tapped near base
Small HydroCarbon= smaller boiling point, condense later, tapped off near top of fractionating column
This process breaks intermolecular forces only