7.1.4Organic Chemistry
Cracking and Alkenes — AQA GCSE Combined Science Trilogy 8464 - Chemistry Revision Notes & Topical Questions
5 min read 8 parts Lesson 5 of 5
Part 1 of 6Why crack hydrocarbons?
Supply and demand
- Supply and demand Crude oil contains more long-chain hydrocarbons than industry needs, while demand for short-chain fuels such as petrol is higher.
- Cracking breaks big molecules into smaller, more useful ones.
Useful products
- Useful products Cracking yields shorter alkanes for fuel and alkenes, which are starting materials for polymers and other chemicals.
Part 2 of 6Methods of cracking
| Method | Conditions | Notes |
|---|---|---|
| Catalytic cracking | Vaporised hydrocarbons passed over a hot catalyst such as aluminium oxide at about 550 °C | Lower energy cost because of the catalyst |
| Steam cracking | Hydrocarbons mixed with steam and heated to a very high temperature | Produces a high proportion of alkenes |
Part 3 of 6Cracking is thermal decomposition
1Long alkane
- Long alkane A large molecule such as decane, C10H22, is heated.
2Bonds break
- Bonds break Heat and the catalyst split carbon-carbon bonds.
3Smaller molecules
- Smaller molecules An alkane and an alkene are made: C10H22 → C8H18 + C2H4.
4Check the balance
- Check the balance Atoms of each element are equal on both sides; one product is always an alkene.
Part 4 of 6Alkenes
Alkene family
Structure
- Structure Hydrocarbons with a carbon-carbon double bond (C=C), making them unsaturated.
General formula
- General formula CnH2n: ethene is C2H4, propene C3H6, butene C4H8.
Reactivity
- Reactivity More reactive than alkanes because the double bond can open to let other atoms add on.
Combustion
- Combustion They burn with a smoky flame in air because incomplete combustion is common.
Part 5 of 6Test for unsaturation
1
Add a few drops of bromine water, which is orange-brown, to the hydrocarbon sample.
2
Shake the mixture gently.
3
With an alkene the bromine adds across the double bond and the mixture is decolourised, turning colourless.
4
With an alkane there is no reaction and the solution stays orange.
Part 6 of 6Uses of alkenes
Polymers
- Polymers Many small alkene molecules join to form long chains such as poly(ethene).
Other chemicals
- Other chemicals Alkenes are used to make alcohols, solvents and other products.
Key terms
| Term | Meaning |
|---|---|
| Cracking | Breaking long-chain hydrocarbons into smaller, more useful molecules. |
| Thermal decomposition | A reaction where heat breaks down a compound. |
| Alkene | An unsaturated hydrocarbon containing a carbon-carbon double bond. |
| Unsaturated | Containing at least one carbon-carbon double bond. |
| Bromine water | An orange-brown solution used to test for alkenes. |
| Catalyst | A substance that speeds up a reaction and is not used up. |
Avoid these mistakes
1
Saying bromine water turns 'clear'; the correct word is colourless, as clear means transparent.
2
Writing cracking equations that do not balance, or that give two alkanes and no alkene.
3
Calling cracking a physical separation, when it is a chemical reaction.
4
Forgetting that alkenes are unsaturated because of the double bond, not because they contain no hydrogen.
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