Cracking & Alkenes (AQA 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
• There are two methods used to crack alkanes: catalytic cracking and steam cracking
• As the names suggest, one method uses a catalyst and the other uses steam

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

• 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
• 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
• In steam or thermal cracking the process is carried out at slightly higher temperatures and produces more ring structures and unsaturated compounds
• The vaporised hydrocarbons are mixed with steam and heated to a high temperature which induces 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 is 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 C₂H_2n rule

• Alkenes are a homologous series of hydrocarbon compounds with at least one double bond between two of the carbon atoms on the chain
• The double bond can be written as carbon carbon double bond or as C=C
• The general formula for alkenes is:

C_nH_2n

• Alkenes are generally more desirable than alkanes as they are more reactive due to the presence of the carbon-carbon double bond, so they can take part in reactions in which alkanes cannot, making them more useful than alkanes
• They are used to make polymers and are the starting materials for the production of many other chemicals
• Two useful reactions are the bromination of alkenes and polymerisation

Bromination of Ethene

• Alkenes undergo addition reactions in which atoms of a simple molecule add across the C=C double bond
• The reaction between bromine and ethene is an example of an addition reaction
• The same process works for any halogen and any alkene in which the halogen atoms always add to the carbon atoms across the C=C double bond

Bromine atoms add across the C=C in the addition reaction of ethene and bromine

Bromine Water Test

• Alkanes and alkenes have different molecular structures
• All alkanes are saturated and alkenes are unsaturated
• The presence of the C=C double bond allows alkenes to react in ways that alkanes cannot
• This allows us to tell alkenes apart from alkanes using a simple chemical test called the bromine water test

Diagram showing the result of the test using bromine water with alkanes and alkenes

• Bromine water is an orange coloured solution
• When bromine water is added to an alkane, it will remain as an orange solution as alkanes do not have double carbon bonds (C=C) so the bromine remains in solution
• But when bromine water is added to an alkene, the bromine atoms add across the C=C bond, hence the solution no longer contains free bromine so it loses its colour