Cracking and Alkenes — AQA GCSE Chemistry

Cracking breaks large hydrocarbon molecules into smaller, more useful ones, including alkenes.

Why crack?

Long-chain hydrocarbons from crude oil are less useful and in lower demand. Shorter molecules (like petrol) are in high demand. Cracking converts the surplus long chains into smaller, more useful molecules — balancing supply and demand.

How cracking is done

Cracking breaks the covalent bonds in large hydrocarbon molecules using heat:

• Catalytic cracking — passing hydrocarbon vapour over a hot catalyst.
• Steam cracking — mixing hydrocarbon vapour with steam and heating to a high temperature.

Cracking produces smaller alkanes (useful as fuels) and alkenes.

Alkenes

Alkenes are hydrocarbons that contain a carbon–carbon double bond (C=C), so they are unsaturated.

• General formula: CₙH₂ₙ.
• They are more reactive than alkanes.
• The first members are ethene (C₂H₄) and propene (C₃H₆).

Test for alkenes

Alkenes turn bromine water from orange to colourless. This is the test for a C=C double bond (alkanes do not decolourise bromine water).

Exam tips

• Cracking makes smaller, more useful molecules and alkenes, balancing supply and demand.
• Methods: catalytic (catalyst) and steam cracking.
• Alkenes are unsaturated (C=C), general formula CₙH₂ₙ, and more reactive than alkanes.
• Bromine water test: orange → colourless indicates an alkene.