8.15–8.17Cracking and alkenes
Hydrocarbons can be broken down — cracked — to produce smaller, more useful molecules.
Cracking can be done by various methods including catalytic cracking and steam cracking. You should be able to describe in general terms the conditions used for each:
• catalytic cracking: the vapour is passed over a hot catalyst • steam cracking: the vapour is mixed with steam and heated to a very high temperature.
The products of cracking include alkanes and another type of hydrocarbon called alkenes.
There is a high demand for fuels with small molecules, and so some of the products of cracking are useful as fuels. Alkenes are used to produce polymers and as starting materials for the production of many other chemicals.
You should be able to give examples to illustrate the usefulness of cracking, and to explain how modern life depends on the uses of hydrocarbons. The economic argument is the one to make: fractional distillation produces far more of the long-chain fractions than anyone wants, and far less petrol than is needed. Cracking converts the surplus into what is in demand — and produces the alkenes that polymers, and so most modern plastics, are made from.
You should be able to balance chemical equations as examples of cracking, given the formulae of the reactants and products. Cracking equations balance like any other: the atoms on the left must all appear on the right.
Worked example
Decane, C₁₀H₂₂, is cracked to produce octane, C₈H₁₈, and one other product. Identify the product.
- Count the carbon atoms on the left: 10. Octane accounts for 8, so 2 are left.
- Count the hydrogen atoms on the left: 22. Octane accounts for 18, so 4 are left.
- The missing product is therefore C₂H₄.
- Check it against CnH₂ₙ: for n = 2 that is C₂H₄, so it is an alkene — ethene.
Answer: C₁₀H₂₂ → C₈H₁₈ + C₂H₄. The second product is ethene, an alkene.
Alkenes are more reactive than alkanes and react with bromine water, which is used as a test for alkenes. You should be able to recall the colour change when bromine water reacts with an alkene: the orange bromine water is decolourised, turning colourless. With an alkane there is no change, and the bromine water stays orange.
Common mistake
Decolourised means the colour disappears — the mixture goes colourless, not white and not clear. "Clear" describes transparency, and orange bromine water is already clear.
Before you practise
- Petrol, kerosene and diesel are non-renewable fossil fuels from crude oil; methane comes from natural gas.
- Cracking breaks long saturated alkane molecules into shorter alkanes and alkenes.
- It is done because the supply from the ground does not match demand — there is too much of the long fractions and not enough petrol.
- The alkenes produced are the starting material for polymers, so cracking makes feedstock as well as fuel.
- Two methods: catalytic cracking over a hot catalyst, or steam cracking at a very high temperature.
- Check that a cracking equation balances — the carbons and hydrogens on each side must be equal.
- Alkenes decolourise bromine water, which is how you tell them from alkanes.