AQA A Level Chemistry复习笔记3.2.2 Modification of Alkanes by 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 (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

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