Cracking and alkenes

Cracking

Crude oil contains too many long-chain alkanes (less useful) and not enough short ones (in demand for petrol). Cracking breaks long alkanes into shorter alkanes plus alkenes.

Conditions: high temperature (~600 degrees C) and a catalyst (e.g. silica or alumina).

Example: C_10 H_22 -> C_8 H_18 + C_2 H_4 (octane + ethene).

The shorter alkanes are useful as fuels; the alkenes are feedstocks for polymers and other chemicals.

Alkenes

Alkenes contain a C=C double bond. General formula: C_n H_2n.

• Ethene: C_2 H_4
• Propene: C_3 H_6
• Butene: C_4 H_8

The C=C bond makes alkenes more reactive than alkanes. They are unsaturated — they can take more atoms across the double bond.

Test for alkenes (bromine water)

Bromine water decolourises (orange to colourless) when shaken with an alkene.

Reaction: ethene + Br_2 -> 1,2-dibromoethane. The Br atoms add across the double bond.

Alkanes do not decolourise bromine water under normal conditions.

Addition polymerisation

Many alkene molecules (monomers) join together to form a long chain (polymer). The C=C bond opens and the units link.

n CH_2=CH_2 -> --(CH_2-CH_2)_n-- (poly(ethene))

Other examples: poly(propene), poly(chloroethene) = PVC.

WJEC exam tip

When drawing the repeat unit of a polymer, you must (a) replace the C=C double bond with a single bond, (b) add bonds extending out of each carbon at the start and end of the unit, and (c) put the brackets and 'n' subscript outside. Lose any of these and you lose marks.