Cracking is used in the petrochemical industry to break down larger hydrocarbon molecules. Explain why cracking is necessary and describe what happens during catalytic cracking.

Cracking is necessary because the market demand for shorter‑chain hydrocarbons (such as gasoline and diesel) is higher than that for the long‑chain feedstocks produced by crude oil refining. The process converts these long‑chain hydrocarbons into smaller, more useful molecules.

During catalytic cracking the long‑chain hydrocarbon is first heated until it vapourises. The vapour is then passed over a hot powdered aluminium oxide catalyst. The catalyst promotes the breaking of C–C bonds, producing a mixture of alkanes and alkenes that are the desired products.