Cracking involves hydrocarbons can be produce smaller, more useful molecules. This process involves heating the hydrocarbons to vapourise them. The vapours are either passed over a hot catalyst or mixed with steam and heated to a very high temperature so that thermal decomposition reactions than occur.
Alkenes are called unsaturated hydrocarbons because alkene molecules always contain a carbon-carbon double covalent bond. Unsaturated alkenes are always more reactive than saturated alkanes due to the presence of the double covalent bond.
Because alkenes are more reactive than alkanes, they are much more useful to make other substances e.g. polymers (plastics).
Alkenes react with bromine water…