Crude oil

• Crude oil is a?mixture?of hydrocarbons containing?alkanes,?cycloalkanes?and?arenes?(compounds with a benzene ring)
• The crude oil is?extracted?from the earth in a?drilling?process?and transported to an?oil?refinery
• At the oil refinery the crude oil is separated into useful fuels by?fractional distillation
• This is a separating technique in which the wide range of different hydrocarbons are separated into fractions based on their boiling points

Crude oil is initially separated into fractions with similar boiling points in a process called fractional distillation

• However, the smaller hydrocarbon fractions (such as gasoline fractions) are in high demand compared to the larger ones
• Therefore, some of the?excess?heavier fractions are broken down into smaller,?more?useful?compounds
• These more useful compounds include?alkanes?and?alkenes?of?lower?relative formula mass (Mr)
• This process is called?cracking

The heavier fractions that are obtained in fractional distillation are further cracked into useful alkane and alkenes with lower Mr?values

Cracking

• The large hydrocarbon molecules are fed into a?steel?chamber?and?heated?to a high temperature and then passed over an?aluminium?oxide?(Al2O3) catalyst
  • The chamber does not contain any?oxygen?to prevent?combustion?of the hydrocarbon to water and carbon dioxide
• When a large hydrocarbon is cracked, a?smaller?alkane?and?alkene?molecules are formed
  • Eg. octane and ethene from decane

Long hydrocarbon fraction is cracked into two smaller ones

• The?low-molecular mass?alkanes?formed make good fuels and are in high demand

• The?low-molecular?mass?alkenes?are more reactive than alkanes due to their double bond
  • This makes them useful for the chemical industry as the starting compounds (feedstock) for making new products
  • Eg. they are used as monomers in polymerisation reactions

Alkenes are reactive molecules and can undergo many different types of reactions making them useful as starting compounds