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Cracking & Alkenes (AQA GCSE Chemistry: Combined Science)
Revision Note
Cracking hydrocarbons
- 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
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
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 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
Conditions for thermal cracking
- 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
- Hydrogen and a greater proportion of alkenes form when cracking is performed at higher temperatures and higher pressure
Products of cracking
- The molecules are broken up in a random way which produces a mixture of shorter alkanes and alkenes
- Alkanes are saturated molecules containing carbon-carbon single bonds only
- Alkenes are unsaturated molecules containing carbon=carbon double bonds
Example of cracking
Decane is cracked to produce octane for petrol and ethene for polymerisation and ethanol synthesis
Writing equations for cracking
Cracking and alkenes
- By the Law of Conservation of Mass, the reactant hydrocarbon that is being cracked must have the same number of carbon and hydrogen atoms as all of the product hydrocarbons combined
- The reactant hydrocarbon must be an alkane (general formula CnH2n+2)
- The product hydrocarbons are a mixture of alkanes (general formula CnH2n+2) AND alkenes (general formula CnH2n)
- For example, the cracking of hexane to form butane and ethene, which are both useful shorter hydrocarbons
- Ethene is the starting material for the plastic polythene
- Butane is used as a fuel
C6H14 ⟶ C4H10 + C2H4
-
- Hexane contains 6 carbon atoms; butane and ethene contain (4 + 2 =) 6 carbon atoms
- Hexane contains 14 hydrogen atoms; butane and ethene contain (10 + 4 =) 14 hydrogen atoms
- We can also use the general formulae for alkanes and alkenes to check that we have correct equations
- The reactant hexane and butane product are both alkanes and follow the CnH2n+2 general formula
- The other product, ethene, is an alkene and follows the general formula CnH2n
Worked example
a)
Complete the following symbol equation for the cracking of eicosane, C20H42.
C20H42 → .................... + C2H4
b)
Explain whether the unknown product is an alkane or an alkene.
Answer:
a)
The complete symbol equation is:
C20H42 → C18H38 + C2H4
- Eicosane contains 20 carbon atoms while the known product contains 2 carbon atoms
- Therefore, the unknown product must contain 20 - 2 = 18 carbon atoms
- Eicosane contains 42 hydrogen atoms while the known product contains 4 hydrogen atoms
- Therefore, the unknown product must contain 42 - 4 = 38 hydrogen atoms
b)
The unknown product is an alkane because:
- It has the general formula CnH2n+2
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.
Alkenes
- 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:
CnH2n
- 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
Test for alkenes
- 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
- Bromine water is used in the test for alkenes as it is safer and easier to handle than bromine
- 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
- The solution no longer contains free bromine so it loses its colour / decolourises
Test for alkenes
Bromine water decolourises in the presence of an alkene
Examiner Tip
Alkenes are more reactive than alkanes due to the presence of the carbon carbon double bond which contains an area of high electron density.
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