5.14 describe how long-chain alkanes are converted to alkenes and shorter-chain alkanes by catalytic cracking, using silica or alumina as the catalyst and a temperature in the range of 600–700C.

Long chain hydrocarbons are passed over a hot catalyst (silica or alumina at 600-700 degrees) this causes them to break down into smaller molecules.

As some atoms are lost from molecules, they become unsaturated and can therefore form a double bond. This is how you get alkenes from the process as well as shorter chain alkanes.

The animation on this page is helpful:
http://www.bbc.co.uk/schools/gcsebitesize/science/aqa_pre_2011/oils/polymersrev1.shtml