Modification of Alkanes by Cracking (AQA A Level Chemistry)

Cracking

• Alkanes are hydrocarbons that can be produced by the addition reaction of hydrogen to an alkene or by cracking of longer alkane chains

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

• 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 (M_r)

• This process is called cracking

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

Production of alkanes from cracking

• In cracking these large, less useful hydrocarbon molecules found in crude oil are broken down into smaller, more useful molecules

• 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 one (or more) alkene molecules are formed

  • E.g. octane and ethene from decane

Long hydrocarbons are cracked by heating them and using aluminium oxide catalyst into a smaller alkane and an alkene

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

• There are two types of cracking:

  • Thermal cracking requires high temperatures (up to 1000 ^oC) and high pressure (up to 70 atmospheres) and produces alkanes and a lot of alkenes

  • Catalytic cracking uses a lower temperature (around 450 ^oC) and slight pressure in the presence of a catalyst such as a zeolite or aluminium oxide to produce mainly aromatic hydrocarbons