Cracking
- Saturated molecules contain single bonds only whereas unsaturated molecules contain double bonds between their carbon atoms
- Alkanes are saturated compounds and alkenes are unsaturated compounds
- Long chain alkane molecules are further processed to produce other products consisting of smaller chain molecules
- A process called cracking is used to convert them into short chain molecules which are more useful
- Small alkenes and hydrogen are produced using this process
- Kerosene and diesel oil are often cracked to produce petrol, other alkenes and hydrogen
Decane is cracked to produce octane for petrol and ethene for ethanol synthesis
- 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 smaller alkanes and alkenes
- Hydrogen and a higher proportion of alkenes are formed at higher temperatures and higher pressure
Writing Equations for Cracking
- We can use the general formulae for alkanes and alkenes to check that we have correctly balanced equations for cracking
- Hexane for example, can be cracked to form butane and ethene, both of which are very useful molecules
- Ethene as the starting material for the production of alcohol and butane is used as a fuel
- The equation for this cracking reaction is:
C6H14 ⟶ C4H10 + C2H4
- Note that the starting compound for this reaction is an alkane and thus the general formula CnH2n+2 applies
- Butane is also an alkane and so the same rule applies
- Ethene is an alkene and so its formula will follow the CnH2n rule
Examiner Tip
Always check that sum of the carbons and hydrogens adds up on each side of the equation AND that you have made alkanes or alkenes.