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

Fractional distillation produces lots of long chain hydrocarbons and not many short chain hydrocarbons in relative. The short chain hydrocarbons are useful but long chain hydrocarbons are not. This means we have to break the long chain hydrocarbons into short chain hydrocarbons as demand for short chain hydrocarbons is high. This is done in a process known as cracking. This process is a form of thermal decomposition, which just means breaking down the molecules into simpler molecules by heating them.