Crude oil as a mixture of hydrocarbons, alkanes and their general formula, the fractional distillation of crude oil, the uses of the fractions, and the cracking of long-chain hydrocarbons.

A six-mark Level of Response question. Reward describing the process: the crude oil is heated until it evaporates (vapourises) and the vapour enters the bottom of the fractionating column, which is hot at the bottom and cooler at the top (a temperature gradient). As the vapours rise, they cool, and each fraction condenses (turns to liquid) when it reaches a part of the column at its boiling point: fractions with high boiling points condense low down (where it is hot) and fractions with low boiling points rise higher before condensing (near the cooler top). The fractions are collected at different heights. Then explain the trend: going down the column, the hydrocarbon molecules in the fractions get larger (longer chains), so they have higher boiling points, are more viscous (thicker), are less flammable (less easily ignited) and are less volatile. Markers reward the heating and vapourising, the temperature gradient, fractions condensing at their boiling points at different heights, and the trends in molecule size, boiling point, viscosity and flammability. A common error is to say the oil is separated by melting point.

A C6.2 structured question. Reward: cracking is the breaking down (thermal decomposition) of long-chain hydrocarbon molecules into smaller, more useful molecules, using a high temperature and a catalyst (or steam). Long-chain hydrocarbons are cracked because there is a greater demand for shorter-chain hydrocarbons (such as petrol) than is produced by fractional distillation alone, so the less useful long chains are broken into more useful short ones; cracking also produces alkenes, which are used to make polymers (plastics) and other chemicals. The two types of product are shorter-chain alkanes and alkenes. For example $\text{C}_{10}\text{H}_{22} \rightarrow \text{C}_8\text{H}_{18} + \text{C}_2\text{H}_4$. Markers credit cracking as breaking long molecules into smaller ones using heat (and a catalyst), the reason (demand for short chains and for alkenes to make polymers), and the products (a shorter alkane and an alkene). A common error is to say cracking joins molecules together.