Alkanes and alkenes as homologous series, the test for unsaturation, combustion of hydrocarbons, and cracking to make smaller alkanes and alkenes.

A topic 2.5 test question. Add bromine water (orange/brown) to each hydrocarbon and shake (1 mark). An alkene decolourises the bromine water from orange to colourless, because it is unsaturated and adds across the double bond (1 mark). An alkane is saturated and does not decolourise it, so the bromine water stays orange/brown (1 mark). Markers reward the reagent, the colour change with the alkene and the no-change with the alkane. A common error is to get the two results the wrong way round.

A topic 2.5 Explain question. Cracking breaks large, less useful hydrocarbon molecules into smaller, more useful ones, using heat and a catalyst (or steam) (2 marks). It is carried out because there is more demand for short-chain fuels such as petrol than the amount that distillation provides, and to make alkenes for plastics (1 mark). The products are smaller alkanes (useful as fuels) and alkenes (used to make polymers) (1 mark). Markers reward the breaking of large molecules, the reason (demand and alkenes), and the products. A common error is to say cracking joins molecules together.