Crude Oil, Hydrocarbons and Cracking
Crude Oil and Hydrocarbons
Crude oil is a finite resource that took millions of years to form from the compressed remains of ancient marine organisms. It is a mixture of many different hydrocarbons — molecules containing only hydrogen and carbon. Because crude oil is a mixture rather than a pure compound, its components can be separated without a chemical reaction.
Fractional Distillation
Crude oil is separated into useful fractions (groups of hydrocarbons with similar boiling points) by fractional distillation. The crude oil is heated in a furnace until most of it vaporises. The vapour enters a tall fractionating column that is hot at the bottom (>350°C) and cool at the top (<25°C). Hydrocarbons condense when they reach their boiling point:
- Top of column: petrol (gasoline) and gases — small molecules, low boiling point.
- Middle: kerosene (aviation fuel), diesel.
- Bottom: heavy fuel oil and bitumen — large molecules, very high boiling point.
In fractional distillation, smaller hydrocarbons have lower boiling points and are collected near the top of the column. They are also less viscous (flow more easily) and more flammable. Larger hydrocarbons have higher boiling points, greater viscosity, and are less flammable.
Alkanes
Most hydrocarbons in crude oil are alkanes. They have the general formula \(C_nH_{2n+2}\) and are described as saturated because they contain only single C—C bonds. The first four alkanes are:
- Methane: \(CH_4\) (natural gas)
- Ethane: \(C_2H_6\)
- Propane: \(C_3H_8\)
- Butane: \(C_4H_{10}\)
As the carbon chain length increases, boiling point and viscosity increase, while flammability decreases. These trends are caused by stronger intermolecular forces between larger molecules.
Combustion of Hydrocarbons
When hydrocarbons burn in a plentiful supply of oxygen, complete combustion occurs, producing only carbon dioxide and water:
$$C_nH_{2n+2} + \frac{3n+1}{2}O_2 \rightarrow nCO_2 + (n+1)H_2O$$
For example, for methane: \(CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O\). This releases large amounts of energy, making hydrocarbons valuable fuels.
When oxygen supply is limited, incomplete combustion occurs, producing toxic carbon monoxide (CO) and/or carbon particles (soot). Carbon monoxide is colourless and odourless but dangerously toxic — it binds to haemoglobin in red blood cells, preventing oxygen transport.
Burning hydrocarbon fuels also releases other pollutants:
- Sulfur dioxide (SO2): produced when sulfur impurities in fuels burn. Dissolves in rainwater to form acid rain, which damages buildings, soils, and aquatic ecosystems.
- Nitrogen oxides (NOₓ): formed when nitrogen and oxygen in the air react at the high temperatures inside engines. Also contribute to acid rain and the formation of photochemical smog.
- Carbon dioxide: a greenhouse gas — its increasing concentration in the atmosphere contributes to global warming.
Incomplete combustion produces carbon monoxide (toxic) and soot. Sulfur impurities produce SO2 (acid rain). High-temperature combustion produces nitrogen oxides (acid rain, smog). Carbon dioxide causes global warming.
Cracking
Cracking is used to break long-chain hydrocarbons (plentiful but less useful) into shorter-chain hydrocarbons that are in higher demand. Cracking always produces at least one alkene — a hydrocarbon with a C=C double bond. Alkenes have the general formula \(C_nH_{2n}\).
Catalytic cracking: long-chain hydrocarbons are vaporised and passed over a heated aluminium oxide (or zeolite) catalyst at around 500°C. The catalyst lowers the activation energy and allows cracking at a lower temperature. Produces mainly branched-chain alkanes and aromatic compounds, useful for petrol.
Thermal cracking (steam cracking): carried out at very high temperatures (around 850°C) and high pressure using steam. Produces a higher proportion of alkenes (especially ethene, \(C_2H_4\)), which are used as monomers to make plastics and polymers.
A simple example of cracking: a long alkane such as decane can be cracked into octane (a petrol component) and ethene: \(C_{10}H_{22} \rightarrow C_8H_{18} + C_2H_4\).
Testing for Alkenes
Alkenes can be distinguished from alkanes using bromine water. Alkenes decolourise bromine water (orange to colourless) because the C=C double bond reacts with bromine in an addition reaction. Alkanes do not react with bromine water under normal conditions, so the orange colour remains.
Exam Focus: Fractions and Cracking
When answering questions on fractional distillation, remember that shorter hydrocarbon chains have lower boiling points, are more volatile, flow more easily, and ignite more readily. Cracking serves the dual purpose of increasing the supply of shorter, more useful fractions and producing alkenes as raw materials for polymer manufacture. Thermal cracking uses very high temperatures and pressures; catalytic cracking uses a zeolite catalyst at lower temperatures — be precise about which conditions apply to each method in your exam answers.