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.

The heavier fractions are boiled, and the vapours passed over a hot catalyst mixture of aluminium oxide and silicon dioxide at about 650 °C. This causes the longer chains to break up and form shorter chains and alkenes.  Hydrogen can sometimes be produced as well.  An example is given below:

Notice that the C–C bonds in the original hydrocarbon get broken to make smaller molecules.