Modification of Alkanes By Cracking (Oxford AQA International A Level Chemistry)

Thermal & Catalytic Cracking

The economics of cracking alkanes

• The fractional distillation of crude oil produces several fractions

• In general, the demand for certain fractions outstrips the supply

  • Supply is how much of a particular fraction can be produced from fractional distillation

  • Demand is how much customers want to buy

Supply and demand graph for crude oil fractions

• Petrol, kerosene and diesel are in high demand but short supply

  • These are more useful fractions

• Fuel oil and bitumen are in low demand but have a high supply

  • These can be described as surplus fractions

• Therefore, some of the surplus fractions are broken down into smaller, more useful compounds

What is cracking?

• Cracking is an endothermic process that breaks down surplus fractions into smaller, more useful compounds

• Cracking involves breaking the single, covalent carbon-carbon bonds in long chain alkanes

Example of cracking

• Large/ long chain hydrocarbons are cracked into smaller, more useful compounds

• A smaller alkane and one (or more) alkene molecules are formed

  • These more useful compounds include alkanes and alkenes of lower relative formula mass (M_r)

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

Cracking decane to form octane and ethene

• There are two types of cracking:

  • Thermal cracking 

  • Catalytic cracking 

Comparing thermal and catalytic cracking