Cracking oil Fractions

Question 1

What is cracking?

Answer 1

Cracking is a chemical reaction that converts large alkane molecules into smaller alkane molecules and alkene molecules
C8H18 -> C6H14 + C2H4
Octane -> hexane + ethene

Question 2

Conditions for cracking

Answer 2

Heating oil fractions to a high temperature (600-700) and passing them over a hot alumina or silica catalyst ; as a result of these conditions, covalent bonds between atoms in the large alkane molecules break
Can also be done in the laboratory by using pieces of broken pot as the catalyst

Question 3

Why is cracking carried out?

Answer 3

Large alkane molecule fractions have high BPs and therefore do not flow easily and are hard to ignite (not useful as fuels) - on the other hand, fractions with small alkane molecules have low BPs and flow more easily (easier to ignite) - useful as fuels

Question 4

Supply and demand?

Answer 4

Fractional distillation produces
1) A lot of LARGE MOLECULE fractions that can’t be sold (high BP)
2) Not enough of the small molecule fractions with low BPs that are in high demand
Cracking helps oil refineries match its supply for useful products (such as petrol) with its customers demand for them - alkenes are also useful for making polymers

Question 5

Why is crude oil important?

Answer 5

90% of it is used to produce fuels such as petrol and diesel and paraffin (airplane fuel) - remainder is used as feedstock for the chemical industry for making polymer products and thus cannot be replaced as a raw material