Crude Oil, Hydrocarbons and Cracking
Crude Oil and Hydrocarbons
Crude oil is a finite resource formed over millions of years from the remains of ancient marine organisms. It is a mixture of many different hydrocarbons (molecules containing only hydrogen and carbon atoms).
The hydrocarbons in crude oil are separated by fractional distillation. The crude oil is heated until most of it evaporates. The vapours rise up a tall column that is hot at the bottom and cool at the top. Different fractions condense at different heights depending on their boiling points: smaller molecules condense near the top (low boiling point) and larger molecules condense near the bottom (high boiling point).
Crude oil is separated by fractional distillation. Smaller hydrocarbons have lower boiling points, are less viscous, more flammable, and are collected near the top of the column. Larger hydrocarbons have higher boiling points and are collected near the bottom.
Alkanes
Most hydrocarbons in crude oil are alkanes. They have the general formula \(C_nH_{2n+2}\). The first four are:
- Methane: \(CH_4\)
- Ethane: \(C_2H_6\)
- Propane: \(C_3H_8\)
- Butane: \(C_4H_{10}\)
As the carbon chain gets longer: boiling point increases, viscosity increases (thicker), and flammability decreases (harder to ignite). These trends are due to stronger intermolecular forces between the larger molecules.
Hydrocarbons are used as fuels because they release energy when burned (combustion). Complete combustion (plenty of oxygen) produces carbon dioxide and water:
$$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$$
Incomplete combustion (limited oxygen) produces carbon monoxide (toxic) and/or carbon (soot), as well as water. Carbon monoxide is dangerous because it is colourless, odourless, and binds to haemoglobin, preventing oxygen transport.
Cracking
Cracking is the process of breaking down long-chain hydrocarbons into shorter, more useful molecules. There is more demand for shorter-chain hydrocarbons (fuels like petrol) than for longer-chain ones. Cracking also produces alkenes, which are used to make polymers and other chemicals.
Cracking: long alkane ā shorter alkane + alkene. Methods: catalytic cracking (hot catalyst) or steam cracking (high temperature + steam). Alkenes have a C=C double bond and are used to make polymers.
Alkenes contain a carbon-carbon double bond (\(C=C\)) and have the general formula \(C_nH_{2n}\). They are described as unsaturated. You can test for the presence of a \(C=C\) double bond using bromine water: it turns from orange to colourless with an alkene (the bromine adds across the double bond). Alkanes do not decolourise bromine water because they are saturated (no double bonds).