14.1.1—Preparation of alkanes

An alkene can be hydrogenated to an alkane with H₂ over a nickel or platinum catalyst on heating. Long-chain alkanes can be cracked over hot aluminium oxide to form shorter hydrocarbons.

Hydrogenation adds H across C=C and increases saturation. Cracking breaks C–C bonds and usually gives a mixture, often including an alkane and an alkene, so balance atoms rather than guessing one product.

Ethene + H₂ → ethane. A long-chain alkane can crack to octane plus ethene if the carbon and hydrogen totals balance.

A catalyst speeds hydrogenation but is not a reactant, and cracking is not simply “breaking every bond” into identical fragments.