Crude oil

The oil industry sits at the very heart of modern life, and this chapter explains why. Fuels for cars, aircraft and ships; plastics; the organic chemicals that go into solvents, drugs, dyes and even explosives — every one of these traces back to a single unappealing starting material: crude-oil, a sticky, black liquid that is converted into useful things on an industrial scale. Nothing about the liquid itself looks valuable. Its value comes entirely from what it contains and what can be done to it, and both of those ideas are the business of this chapter.

Here is the route you will travel. First, what crude oil actually is: a fossil-fuel brewed underground over millions of years, and — the statement everything else stands on — a mixture of hydrocarbons, compounds containing carbon and hydrogen only. Second, how the oil industry exploits a simple property trend (bigger molecules boil at higher temperatures) to split crude oil into useful fractions by fractional distillation, and which fraction gets used for which job, from refinery gases at the top of the column to bitumen on the roads. Third, what happens when these hydrocarbons burn: the energy release that makes them fuels, the products of complete and incomplete combustion, and the pollutants — carbon monoxide, sulfur dioxide, oxides of nitrogen — that turn burning fuel into an environmental problem. Finally, cracking: the industrial trick that converts surplus long-chain hydrocarbons into the shorter, more profitable ones the market actually wants.

Every statement in that route is examinable in both Paper 1C and Paper 2C, with no chemistry-only band anywhere in this chapter — like the introduction to organic chemistry before it (u4:c22), this is shared territory for every candidate. Twelve specification statements carry the chapter, numbered 4.7 to 4.18, and they chain together: the mixture idea (4.7) makes the distillation story (4.8–4.10) possible; the fractions feed the combustion story (4.11–4.13) and the pollution story (4.14–4.16); and the supply mismatch between what distillation delivers and what the market wants motivates cracking (4.17–4.18). Keep that chain in view as you work through the sections — each one picks up exactly where the last left off.

One habit is worth building from the first page: always ask why the oil industry does things this way. Why distill instead of using crude oil raw (because raw crude oil has no uses)? Why crack (because distillation delivers the wrong balance of chain lengths)? Why scrub factory gases (because sulfur impurities become acid rain)? Examiners reward answers that connect a process to its reason, and this chapter is full of reason-and-process pairs to practise on.