Large hydrocarbon molecules can be broken down into smaller molecules by a process called cracking. Cracking can be done in two ways: by heating a mixture of hydrocarbon vapours and steam to a very high temperature, or by passing hydrocarbon vapours over a hot catalyst. During cracking, thermal decomposition reactions produce a mixture of smaller molecules. Some of the molecules are alkanes, which are saturated hydrocarbons. These alkanes with smaller molecules are more useful as fuels.
Some of the other smaller molecules formed are alkenes. Alkenes are unsaturated hydrocarbons with the general formula CnH2n. They are unsaturated because they contain fewer hydrogen atoms than alkanes with the same number of carbon atoms.
C10H22 ---800oC + catalyst---> C5H12 + C3H6 + C2H4
Decane ---800oC + catalyst---> pentane + propene + ethene
Alkenes have a double bond between two carbon atoms and this makes them more reactive than alkanes. Alkenes react with bromine water turning it from orange to colourless.
MAKING POLYMERS FROM ALKENES
Plastics are made from very large molecules called polymers. Polymers are made from many small molecules joined together, called monomers. The reaction to make a polymer is called polymerisation.
Lots of ethene molecules can join together to form poly(ethene), commonly called polythene. In the polymerisation reaction, the double bond in each ethene molecule becomes a single bond and thousands of ethene molecules join together in long chains.
Other alkenes can polymerise in a similar way. Propene can form polypropene. Many of the plastics we use as bags, bottles, containers and toys are made from alkenes.
NEW AND USEFUL POLYMERS
Polymers can be designed to make materials…