Cracking (Edexcel IGCSE Chemistry (Modular))
Revision Note
Cracking hydrocarbons
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 600 – 700 °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
Products of cracking
The molecules are broken up in a random way which produces a mixture of shorter alkanes and alkenes
Example of cracking
Decane is cracked to produce octane for petrol and ethene for ethanol synthesis
Fraction supply & demand
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
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
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
Examiner Tip
Remember that cracking is an endothermic reaction.
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