Obtaining Useful Substances from Crude Oil (Oxford AQA IGCSE Combined Science Double Award): Revision Note

Cracking

• 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

Demand for short chain hydrocarbon molecules such as petrol, kerosene and diesel is greater than the supply, while demand for long chain hydrocarbons such as fuel oil is less than the supply

• The demand for certain fractions outstrips the supply so cracking is used to break down the hydrocarbons to produce smaller, more useful molecules

• Catalytic cracking involves heating the hydrocarbon molecules to around 470 – 550°C to vaporise them

• The vapours then pass over a hot catalyst

• This process breaks covalent bonds in the molecules as they come into contact with the surface of the catalyst, causing thermal decomposition reactions

• In steam cracking the process is carried out at slightly higher temperatures and the hydrocarbons are mixed with steam 

• The molecules are broken up in a random way which produces a mixture of smaller alkanes and alkenes

  • Alkenes are unsaturated hydrocarbons so contain carbon-carbon double bonds as well as single bonds

    • They have the general formula C_nH_2n

• The smaller alkanes produced from cracking are useful as fuels 

Cracking

Decane is cracked to produce a shorter alkane that could be used as a fuel, and an alkene