Cracking Hydrocarbons (WJEC GCSE Science (Double Award)): Revision Note

Cracking Fractions

• 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

Supply & demand of crude oil fractions

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 convert excess unwanted fractions into more useful ones

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

• Cracking involves heating the hydrocarbon molecules to around 600 – 700°C to vaporise them

• The vapours then pass over a hot powdered catalyst of alumina or silica

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

• 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 

• Hydrogen and a greater proportion of alkenes form when cracking is performed at higher temperatures and higher pressure

Example of cracking

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

Useful hydrocarbon molecules

• Shorter alkanes are useful because they have a lower boiling point and are more flammable

  • This makes them more useful fuels

• Shorted alkenes are useful because they contain a carbon=carbon double bond which makes them reactive

  • This makes alkenes, such as ethene, useful as a starting point in the production of polymers / plastics

Equations for Cracking

• By the Law of Conservation of Mass, the reactant hydrocarbon that is being cracked must have the same number of carbon and hydrogen atoms as all of the product hydrocarbons combined

  • The reactant hydrocarbon must be an alkane (general formula C_nH_2n+2)

  • The product hydrocarbons are a mixture of alkanes (general formula C_nH_2n+2) AND alkenes (general formula C_nH_2n)

• For example, the cracking of hexane to form butane and ethene, which are both useful shorter hydrocarbons

  • Ethene is the starting material for the plastic polythene 

  • Butane is used as a fuel

C₆H₁₄ ⟶ C₄H₁₀ + C₂H₄ 

• Hexane contains 6 carbon atoms; butane and ethene contain (4 + 2 =) 6 carbon atoms

  • Hexane contains 14 hydrogen atoms; butane and ethene contain (10 + 4 =) 14 hydrogen atoms 

• We can also use the general formulae for alkanes and alkenes to check that we have correct equations

  • The reactant hexane and butane product are both alkanes and follow the C_nH_2n+2 general formula 

  • The other product, ethene, is an alkene and follows the general formula C_nH_2n