Crude oil, hydrocarbons and cracking · GCSE Chemistry

Crude oil and alkanes

Crude oil is a finite resource found in rocks. It is the remains of ancient biomass, mainly plankton, that was buried in mud millions of years ago.
Crude oil is a mixture of a very large number of compounds. Most of them are hydrocarbons: compounds of hydrogen and carbon only.
Most hydrocarbons in crude oil are alkanes, with the general formula CₙH₂ₙ₊₂.
The first four alkanes are methane (CH₄), ethane (C₂H₆), propane (C₃H₈) and butane (C₄H₁₀).

The hydrocarbons are separated into fractions by fractional distillation. Each fraction contains molecules with a similar number of carbon atoms.
The crude oil is heated until it evaporates, and the vapour enters a column that is hot at the bottom and cooler at the top.
Each fraction condenses at the level where the temperature is just below its boiling point. Large molecules condense near the bottom; small ones rise to the top.
Fractions are used as fuels (liquefied petroleum gases, petrol, kerosene, diesel oil and heavy fuel oil) and as feedstock for the petrochemical industry, which makes solvents, lubricants, polymers and detergents.

As hydrocarbon molecules get bigger: the boiling point increases, the viscosity increases (they flow less easily), and the flammability decreases.
Complete combustion of a hydrocarbon releases energy and produces carbon dioxide and water: CH₄ + 2O₂ → CO₂ + 2H₂O.
Propane: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.

Cracking breaks long hydrocarbon molecules into smaller, more useful molecules.
Catalytic cracking: the vapour is passed over a hot catalyst. Steam cracking: the vapour is mixed with steam at a very high temperature.
The products are shorter alkanes and alkenes, for example C₁₀H₂₂ → C₈H₁₈ + C₂H₄.
Cracking is needed because there is more demand for short-chain fuels such as petrol than crude oil supplies. Alkenes are used to make polymers and other chemicals.
Alkenes are more reactive than alkanes. Bromine water turns from orange to colourless with alkenes, but not with alkanes.

Crude oil: A finite mixture of hydrocarbons formed from ancient plankton buried in mud.
Hydrocarbon: A compound made of hydrogen and carbon atoms only.
Alkane: A saturated hydrocarbon with the general formula CₙH₂ₙ₊₂.
Fraction: A group of hydrocarbons with a similar number of carbon atoms and similar boiling points.
Fractional distillation: Separating crude oil into fractions using differences in boiling point.
Viscosity: How thick a liquid is, or how easily it flows.
Flammability: How easily a substance catches fire.
Cracking: Breaking large hydrocarbon molecules into smaller, more useful ones.
Alkene: An unsaturated hydrocarbon with a C=C double bond.
Feedstock: A raw material used to make other chemicals.