Cracking (Edexcel IGCSE Chemistry: Double Science)

What is cracking?

• Cracking is an industrial process used to break low demand, long chain hydrocarbon molecules into more useful, small chain hydrocarbon molecules
  • Any long chain hydrocarbon can be cracked into smaller chain hydrocarbons
  • For example, kerosene and diesel oil are often cracked to produce petrol, alkenes and hydrogen

Conditions for catalytic cracking

• Catalytic cracking involves heating the hydrocarbon molecules to around 600 – 700 °C to vaporise them
• The vapours then pass over a hot powdered catalyst of aluminium oxide
• This process breaks covalent bonds in the molecules as they come into contact with the surface of the catalyst, causing thermal decomposition reactions

Products of cracking

• The molecules are broken up in a random way which produces a mixture of shorter alkanes and alkenes
  • Alkanes are saturated molecules containing carbon-carbon single bonds only
  • Alkenes are unsaturated molecules containing carbon=carbon double bonds

Example of cracking

Decane is cracked to produce octane for petrol and ethene for ethanol synthesis

• Fractional distillation separates crude oil into fractions containing hydrocarbons of similar chain lengths
• Each fraction has different values for its supply and demand
  • Supply is how much of a particular fraction can be produced from refining the crude oil
  • Demand is how much customers want to buy
• The demand for certain fractions outstrips the supply so cracking is used to convert excess unwanted fractions into more useful ones
• You can see from the chart that fuel oil and bitumen are surplus fractions so they are cracked and modified to produce petrol, kerosene and diesel