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

5.13 •fractional distillation of crude oil produces more long-chain hydrocarbons than can be used and fewer short ones than required 

                   •cracking is a form of thermal decomposition 

                               •they also produce alkenes, used to make polymers

                                          •C2H4 will always be the alkene