Alkenes can be manufactured by

Analyze the question: It asks which process is used to manufacture alkenes (unsaturated hydrocarbons with the general formula CₙH₂ₙ and a C=C double bond).

Explain the concept: Industrially, alkenes such as ethene and propene are made by cracking. In cracking, large, less useful hydrocarbon molecules (e.g. from crude oil fractions) are broken down by heat (thermal cracking) or with a catalyst (catalytic cracking) into smaller molecules, usually a mixture of an alkane and an alkene.

Example: C₁₀H₂₂ → C₈H₁₈ + C₂H₄ (decane → octane + ethene)

Check each option:

• A. Combustion of alkanes: This produces CO₂ and H₂O (or CO and soot if oxygen is limited), not alkenes. ❌
• B. Cracking of hydrocarbons: This breaks long-chain hydrocarbons into smaller alkanes and alkenes. ✅
• C. Polymerization reactions: This joins alkenes together to form polymers like polythene. It uses up alkenes rather than making them. ❌
• D. Addition of hydrogen to unsaturated vegetable oils: This is hydrogenation, used to make margarine. It converts C=C bonds to C–C bonds, so it removes unsaturation. ❌

Answer: The correct option is B, the cracking of hydrocarbons.

Exam tip: Remember that cracking makes alkenes, polymerization uses alkenes, and hydrogenation removes the double bond. A simple chain is: crack → alkene → polymer.