Crude oil and Cracking

Crude oil is a fossil fuel that is made from the remains of plankton.

It's structure is made from varying lenghts of alkane chains.

Fractional distilation is the process used to separate crude oil into its varying chain lengths.

This technique relies on the boiling point of each chain.

Smaller chains

• boil at lower temperatures
• separate from the mixture quicker 

The length of a chain changes the properties of the molecule.

Shorter chains have

• low boiling points
• low viscosity
• high flamability

These properties make the smaller chains very useful as fuels and power supplies,

Longer chains have

• high melting and boiling points
• high viscocity
• low flamability

Because of these properties, longer chains are not very useful. Most longer chains undergo a process called cracking to shorten the chain and make them more useful.

Cracking is the process in which long hydrocarbon chains are split into a short alkene and an alkane. For example, passing hexane over a hot catalyst will split it into butane and ethene

C6H14 -> C4H10 + C2H4

The alkene produced can then be used to create polymers and plastics so there are no waste products in cracking.

Cracking a long alkane chain can be done in two ways,

Catalyitic Cracking involves

• passing the chains over hot aluminium silicates
• 500*C and at a low pressure to stop the smaller molecules re-bonding back into a large one.

Thermal Cracking involves

• heating the chains to around 600*C
• a pressure of about 70 atmospheres.

Thermal Cracking produces a lot of alkenes whereas Catalytic prouces more shorter alkanes.

An easy test to identify if an alkene is present is to mix it with bromine water.

The aqueous chemical has an orangey brown colour when unreacted but turns clear if it bonds with another chemical.

For example, Bromine and propene would would react to form dibromopropane, which is clear.

Br2 + C3H6 -> C3H6Br2