Cracking and AlkenesAQA GCSE Chemistry: Revision notes

Fractional distillation of crude oil produces more of some fractions (like heavy, long-chain hydrocarbons) than there is demand for, and less of others (like short-chain fuels) than is needed. Cracking solves this by breaking down larger, less useful hydrocarbon molecules into smaller, more useful ones.

• Cracking can be carried out using catalytic cracking (a catalyst and a high temperature) or steam cracking (very high temperature and steam, no catalyst)
• The products of cracking include smaller alkanes and reactive alkenes

When a long-chain alkane is cracked, it breaks into one shorter alkane and one alkene, and the atoms must balance exactly as in any equation:

longer alkane → shorter alkane + alkene

For example: C₁₀H₂₂ → C₈H₁₈ + C₂H₄

To balance a cracking equation, count the carbon and hydrogen atoms on the reactant side and check they match the total on the product side.

Cracking is extremely useful because it:

• Converts excess, less-in-demand long-chain hydrocarbons into shorter, more useful hydrocarbons that better match fuel demand (e.g. for petrol)
• Produces alkenes, which are far more reactive than alkanes and are essential as the starting materials (monomers) for the plastics and petrochemical industry

Modern life depends heavily on the uses of hydrocarbons made possible by cracking, from transport fuels to the raw materials for polymers.

Alkenes are unsaturated hydrocarbons — they contain a carbon-carbon double bond (C=C) — with the general formula CₙH₂ₙ.

The first four alkenes are ethene, propene, butene and pentene.

Because of their reactive double bond, alkenes will decolourise bromine water (turning it from orange to colourless), while alkanes do not react with bromine water. This is a standard chemical test used to distinguish alkenes from alkanes.