"5.12 recall that fractional distillation of crude oil produces more long-chain hydrocarbons than can be used directly and fewer short-chain hydrocarbons than required"

a)When crude oil undergoes fractional distillation there are too many long chain hydrocarbons (e.g. bitumen) produced and not enough short chain hydrocarbon (e.g. refinery gases)
b) To solve these problems the long chain hydrocarbons undergo a chemical reaction called crackling.