Obtaining Useful Substances from Crude OilOxford AQA IGCSE Chemistry: Revision notes

Fractional distillation of crude oil produces more of the longer-chain fractions than there is demand for, but not enough of the shorter, more useful ones (such as petrol). Cracking solves this by breaking down larger, less useful hydrocarbon molecules into smaller, more useful ones.

• Cracking is a form of thermal decomposition
• The smaller molecules produced are more useful as fuels and as feedstock for the chemical industry

Cracking is carried out by:

1. Heating the long-chain hydrocarbon to vaporise it
2. Then either:
   • passing the vapour over a hot catalyst, or
   • mixing the vapour with steam and heating it to a very high temperature

Both methods break carbon-carbon bonds in the long-chain molecule.

Cracking a long-chain alkane produces a mixture of smaller molecules, always including at least one shorter alkane and one alkene.

• Alkanes produced are saturated, general formula CnH2n+2
• Alkenes produced are unsaturated hydrocarbons, general formula CnH2n, containing a carbon-carbon double bond (C=C)
• Some of the smaller molecules produced are useful as fuels (e.g. petrol), while the alkenes are useful as feedstock for making other chemicals, such as polymers

The required practical tests for the presence of a carbon-carbon double bond (an alkene) in an unknown hydrocarbon:

1. Add a small sample of the unknown hydrocarbon to a test tube of orange bromine water
2. Shake gently
3. If the bromine water turns from orange to colourless, the hydrocarbon contains a C=C double bond and is an alkene
4. If the bromine water stays orange, the hydrocarbon is saturated (an alkane) and contains no double bond