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 600–700ºC

Passing long-chain hydrocarbons over a hot catalyst (in this case silica or alumina at roughly 600-700ºC) will cause them to break down into smaller chains of hydrocarbons

Some of the atoms are lost from the molecules, making them unsaturated and able to form a double bond. This is how you can get alkenes from cracking, not just short-chain hydrocarbons