Chemistry: Useful substances from crude oil

Alkenes

Fractions can go through a process called cracking, a chemical reaction that produces smaller hydrocarbons including alkanes and alkenes. Ethene and other alkenes are unsaturated hydrocarbons and can be used to create polymers.

Fuels containing large hydrocarbon molecules are not useful because they do not flow easily and are difficult to ignite. 

Cracking allows large hydrocarbon molecules to be broken down into smaller hydrocarbon molecules. 

Fractions containing large molecules are heated to vaporise them and are then either: passed over a hot catalyst or mixed with steam and heated to a very high temperature. 

This breaks chemical bonds in the molecules, causing thermal decomposition reactions. 

The products of cracking contain alkenes which are a family of hydrocarbons that contain the same general formula: C2H2n which means that the amount of hydrogen atoms in an alkene is double the number of carbon atoms. 

Alkenes are unsaturated hydrocarbons and contain a double covalent bond (two lines). Presence of this double bond allows alkenes to react in a way that alkanes can't. They can react with oxygen in the air so they can be used as fuels. They can be used to make ethanol and polymers (plastics). 

Test for unsaturation: Bromine water is normally orange-brown in colour and becomes colourless when shaken with an alkene but stays the same when shaken with an alkane. 

Polymers

Alkenes can be used to make polymers. Polymers are very large molecules…