5.13      understand that fractional distillation of crude oil produces more long-chain hydrocarbons than can be used directly and fewer short-chain hydrocarbons than required and explain why this makes cracking necessary.

You just have to remember that the crude oil that comes out of the ground tends to have more of the heavier molecules in it and that these are not so useful as the smaller, lighter fractions.  There is a high demand for the lighter fractions like petrol (gasoline) which is used for car fuel.  There is much less demand for the heavy fractions like fuel oil.  Cracking is a process where big molecules are broken up into smaller ones.  This is economically better, as the lighter products can be sold for more profit.