Cracking and Alkenes
Practise Cracking and Alkenes for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Cracking breaks long-chain hydrocarbons into shorter, more useful ones and produces alkenes, which decolourise bromine water.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
Cracking breaks large hydrocarbon molecules into smaller, more useful ones. It is a thermal decomposition reaction.
It is done because there is greater demand for short-chain hydrocarbons such as petrol than the fractional distillation of crude oil supplies. Cracking converts surplus long-chain fractions into the shorter molecules that are actually needed.
There are two methods. Catalytic cracking heats the hydrocarbons to vaporise them and passes the vapour over a hot catalyst. Steam cracking mixes the vapour with steam and heats it to a very high temperature. Both produce alkanes and alkenes, and the alkenes are used to make polymers.
Revision notes
Why cracking is needed
There is greater demand for short-chain hydrocarbons such as petrol than crude oil supplies directly.
Cracking converts surplus long-chain fractions into shorter, more useful molecules, matching supply to demand.
The two methods
Catalytic cracking: heat the hydrocarbons to vaporise them, then pass the vapour over a hot catalyst.
Steam cracking: mix the hydrocarbon vapour with steam and heat it to a very high temperature. Both are forms of thermal decomposition.
The products
Cracking produces smaller alkanes, which are useful as fuels, and alkenes.
Alkenes are more reactive than alkanes and are used to produce polymers and as starting materials for other chemicals. Alkenes react with bromine water, turning it from orange to colourless.
Key points
- Cracking breaks large hydrocarbons into smaller ones.
- It is a thermal decomposition reaction.
- Demand for short-chain hydrocarbons exceeds supply.
- Catalytic cracking uses a hot catalyst.
- Steam cracking uses steam at high temperature.
- Products include alkanes and alkenes.
Worked examples
Example 1
Explain why cracking is carried out. [2 marks]
Model answer
There is greater demand for short-chain hydrocarbons such as petrol than can be supplied by fractional distillation alone (1 mark), so cracking converts surplus long-chain molecules into shorter, more useful ones (1 mark).
Example 2
Describe the two methods of cracking. [2 marks]
Model answer
Catalytic cracking: the hydrocarbons are heated to vaporise them and passed over a hot catalyst (1 mark). Steam cracking: the vapour is mixed with steam and heated to a very high temperature (1 mark).
Example 3
Describe the test that distinguishes an alkene from an alkane. [2 marks]
Model answer
Add bromine water and shake (1 mark). An alkene turns the orange bromine water colourless, whereas an alkane leaves it orange (1 mark).
Common mistakes
Saying cracking makes larger molecules.
It breaks large molecules into smaller ones.
Forgetting to mention demand.
The economic reason is why cracking is carried out at all.
Getting the bromine water result backwards.
The alkene decolourises it; the alkane does not.
Saying bromine water turns blue.
It turns from orange to colourless.
Exam tips
- Give the economic reason when asked why cracking is done.
- Learn both cracking methods by name and description.
- Describe the bromine water test with both colours.
- Say alkenes are more reactive than alkanes.
Key terms
- Cracking
- Breaking large hydrocarbons into smaller molecules.
- Thermal decomposition
- Breaking down a substance using heat.
- Alkene
- An unsaturated hydrocarbon with a carbon-carbon double bond.
- Bromine water
- The reagent used to test for alkenes.
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Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.