Cracking and Alkenes

This lesson covers cracking — the process of breaking long-chain hydrocarbons into shorter, more useful molecules — as required by the Edexcel GCSE Chemistry specification (1CH0). You need to know why cracking is necessary, the two main methods (thermal and catalytic), and how to write balanced equations for cracking reactions.

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Why Is Cracking Needed?

As you learned in previous lessons, fractional distillation of crude oil produces a mismatch between supply and demand:

• Too much of the long-chain, heavy fractions (fuel oil, bitumen) — supply exceeds demand.
• Not enough of the short-chain, lighter fractions (petrol, kerosene, LPG) — demand exceeds supply.

Cracking solves this problem by converting long-chain hydrocarbons (for which there is low demand) into shorter-chain hydrocarbons (for which there is high demand), along with useful alkenes.

Products of Cracking

Cracking always produces a mixture of:

1. Shorter alkanes — used as fuels (e.g. petrol-range hydrocarbons).
2. Alkenes — unsaturated hydrocarbons used as raw materials to make polymers (plastics), alcohols and other chemicals.