What is cracking?

Cracking is the process by which longer chain hydrocarbons are split into shorter, more useful hydrocarbons.


Generally, cracking results in an alkane and an alkene.

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Why are the products of cracking useful?

  1. Shorter hydrocarbons make better fuels.
  2. Shorter alkenes can also be used as feedstock for the petrochemical industry to make polymers and detergents.

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What type of reaction is cracking an example of?

Cracking is a thermal decomposition reaction - i.e. breaking down molecules by heating them.

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What catalyst is used in catalytic cracking?

Hot powdered aluminium oxide.

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Outline how catalytic cracking works.

  1. Long chain alkanes are heated until they vaporise into a gas.
  2. The gaseous alkanes are passed over a hot aluminium oxide catalyst.
  3. This breaks the long chain alkanes into a shorter chain alkane and an alkene.

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How is steam cracking different to catalytic cracking?

Steam cracking is different because there is no catalyst involved. Instead the vaporised long chain alkane is mixed with steam at very high temperatures.

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What are alkenes?

Hydrocarbons with a double bond between the two carbon atoms.

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What are the first 4 alkenes?

2 carbons - ethene

3 carbons - propene

4 carbons - butene

5 carbons - pentene

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Why is there not an alkene called methene?

Methene would indicate there is only 1 carbon atom, and so there could be no carbon-carbon double bond, which means the molecule wouldn't be an alkene.

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Are alkenes saturated?

No, they are unsaturated as they have a carbon-carbon double bond.

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What is the general formula for alkenes?

CnH2n

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What colour change occurs when you add bromine water to alkenes?

Orange ➔ colourless

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