Cracking (OCR Gateway GCSE Chemistry: Combined Science)

• 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 C_nH_2n+2 applies
• Butane is also an alkane and so the same rule applies
• Ethene is an alkene and so its formula will follow the C_nH_2n rule