Cracking Hydrocarbons (AQA GCSE Combined Science: Synergy: Physical Sciences): Revision Note

Cracking hydrocarbons

• 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

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

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 470 – 550°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

Conditions for thermal cracking

• In steam or thermal cracking the process is carried out at slightly higher temperatures and produces more unsaturated compounds

• The vaporised hydrocarbons are mixed with steam and heated to a high temperature which induces cracking

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

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

Writing equations for cracking

Cracking and alkenes

• 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 is an alkane (general formula C_nH_2n+2)

• The product hydrocarbons are a mixture of:

  • Smaller alkanes (general formula C_nH_2n+2) 

  • 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 poly(ethene) 

  • Butane is used as a fuel

C₆H₁₄ ⟶ C₄H₁₀ + C₂H₄ 

• Checking the hexane cracking equation is balanced:

  • Carbon:

    • Hexane contains 6 carbon atoms

    • Butane and ethene contain (4 + 2 =) 6 carbon atoms

  • Hydrogen:

    • 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  

Alkenes and polymers

• Cracking produces a mixture of shorter alkanes and alkenes

  • Alkenes are unsaturated hydrocarbons, that contain at least one carbon-carbon double bond (C=C)

• The presence of the C=C double bond makes alkenes more reactive than alkanes,

  • This means that they can be used to make a wider range of useful materials

• Alkenes are used to produce polymers and as starting materials for the production of many other chemicals

• Small ethene molecules join together in a process called addition polymerisation to produce long-chain molecules of poly(ethene)

  • In addition polymerisation, many small monomer molecules react together to form a long polymer chain with no other products