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

The main problem is that long-chain hydrocarbons are more viscous and less flammable, whereas the short-chain hydrocarbons flow and burn well, which makes them more useful.

However, as the title said, more long-chain hydrocarbons are produced then short-chain ones.

TADA! We have cracking, which breaks up the longer, less useful hydrocarbons into shorter, more useful hydrocarbon