• 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

• Crude oils vary considerably in their composition and some need more refining than others
• Supply?is how much of a particular fraction can be produced from refining the crude oil
• Demand?is how much customers want to buy
• General the demand for certain fractions outstrips the supply so this is why?cracking?is necessary to convert surplus unwanted fractions into more useful ones
• This is mostly larger, heavier fractions that are cracked into smaller lighter fractions

Supply & demand graph for crude oil fractions

• You can see from the chart that fuel oil and bitumen are surplus fractions so they are cracked and modified to produce petrol, kerosene and diesel