Hydrocarbons and Simple Alkanes

When crude oil is pumped out of the ground, it is a viscous black liquid with a strong smell, it looks nothing like the clear petrol you pump at a service station, yet both contain the same family of molecules. Crude oil is a mixture of hydrocarbonsorganic compounds containing only carbon and hydrogen atoms. It formed over 300 million years from the remains of marine microorganisms (phytoplankton and zooplankton) that died, sank to the ocean floor, and were compressed under heat and pressure deep in the Earth's crust. This process takes millions of years, making crude oil a non-renewable resource. Because it formed from ancient living organisms, it is also classified as a fossil fuel.

Crude oil is not a pure substance, it contains hundreds of different hydrocarbon molecules, ranging from methane (CH₄, 1 carbon) to asphalt-like molecules with 50+ carbons. This mixture means crude oil cannot be used directly; it must be separated by refining. The physical process that achieves this separation is fractional distillation, which exploits the fact that different-sized hydrocarbon molecules have different boiling points, directly related to chain length and the strength of intermolecular forces between molecules.

Fractional distillation separates crude oil by exploiting differences in boiling point between different hydrocarbon fractions. The crude oil is first heated to about 400 °C in a furnace, converting most of it to vapour. The vapour enters the bottom of a tall fractionating column, which is hot at the bottom and progressively cooler towards the top. As vapours rise through the column, they cool. Each fraction condenses at the temperature corresponding to its boiling point and is collected at that height. Short-chain molecules (fewer carbons) have weaker intermolecular forces, lower boiling points, and condense high up; long-chain molecules condense lower down.

The column is not perfectly clean, each fraction is a range of hydrocarbon chain lengths, not a single compound. This is acceptable because the fractions are defined by their intended use: petrol-range hydrocarbons (5–10 carbons), kerosene-range (10–16 carbons), diesel-range (14–20 carbons). The consistency required for each application is achieved by further refining and blending after the column. The key principle is: boiling point increases with chain length, because longer chains have more surface area for intermolecular (van der Waals) forces, more energy is needed to separate them.

Each fraction from the fractionating column has a characteristic carbon chain length range, boiling point range, and set of applications. LPG (liquefied petroleum gas, 1–4 carbons): boiling point below 0 °C, stored under pressure as liquid, used for cooking gas and camping stoves. Petrol (5–10 carbons): boiling point 40–200 °C, volatile liquid, used in car engines. Kerosene (10–16 carbons): 150–250 °C, used for jet fuel (aviation turbine fuel, ATF) and heating. Diesel (14–20 carbons): 200–300 °C, higher viscosity, used in trucks, trains, and ships.

The heavier fractions, heavy fuel oil (20–70 carbons), power large ships and industrial boilers. Bitumen (70+ carbons) is the solid-like residue used in road surfacing. Carbon chain length controls viscosity, longer chains make thicker liquids, as well as flash point (minimum temperature for ignition). Petrol ignites below 0 °C (extremely flammable); diesel ignites above 55 °C (less flammable, safer for trucks). These differences are not arbitrary, they arise directly from the molecular chain lengths of each fraction.