Crude Oil and Hydrocarbons
Crude Oil — A Finite Resource
Crude oil is a non-renewable resource formed over millions of years from the compressed, heated remains of marine organisms. It is not a single substance but a complex mixture of many different hydrocarbons — molecules containing only carbon and hydrogen. Because it is a mixture, crude oil has no single fixed melting or boiling point. It must be separated before it can be used as a fuel or chemical feedstock.
Crude oil is a finite, non-renewable mixture of hydrocarbons (molecules containing only C and H). Because it is a mixture, not a pure substance, it must be separated by fractional distillation before use.
Fractional Distillation
Fractional distillation separates crude oil at an oil refinery. The oil is heated until most of it vaporises, then the vapours pass up a tall fractionating column. The column is very hot at the bottom and progressively cooler towards the top. Different hydrocarbons condense at different heights, depending on their boiling points.
The key trend: as the length of the hydrocarbon chain increases, the boiling point increases, viscosity increases (it becomes thicker), and flammability decreases. Shorter-chain hydrocarbons are collected from near the top of the column (lower boiling point); longer-chain hydrocarbons condense near the bottom (higher boiling point). The fractions collected include, from top to bottom: petroleum gases, petrol (gasoline), naphtha, kerosene (jet fuel), diesel, fuel oil, and bitumen (residue).
Fractional distillation: shorter chains → lower boiling point → collected higher up the column. Longer chains → higher boiling point → collected lower down. As chain length increases: boiling point ↑, viscosity ↑, flammability ↓.
Alkanes
Most hydrocarbons in crude oil are alkanes. They are described as saturated because all C–C bonds are single bonds — no double bonds are present. The first four members are:
- Methane \(CH_4\) — natural gas, single carbon
- Ethane \(C_2H_6\) — two carbons
- Propane \(C_3H_8\) — three carbons (LPG)
- Butane \(C_4H_{10}\) — four carbons (camping gas)
The general formula \(C_nH_{2n+2}\) applies to all unbranched alkanes. For example, if \(n = 5\) (pentane): \(C_5H_{12}\). Alkanes burn readily in air, which is why they make useful fuels.
Combustion of Hydrocarbons
Hydrocarbons release energy when burned in oxygen — this is combustion. The type of product depends on how much oxygen is available:
Complete combustion (excess oxygen): \(C_xH_y + O_2 \rightarrow CO_2 + H_2O\). For methane: \(CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O\). The only products are carbon dioxide and water. The flame burns cleanly with a blue colour.
Incomplete combustion (limited oxygen): produces carbon monoxide (CO) and/or soot (carbon particles) instead of carbon dioxide. Carbon monoxide is a toxic, colourless, odourless gas that binds to haemoglobin in the blood, preventing oxygen transport — it can be fatal in enclosed spaces. Soot particles cause respiratory problems and contribute to air pollution. The flame turns yellow or orange.
Complete combustion (excess O₂): CO₂ + H₂O only. Incomplete combustion (limited O₂): CO and/or soot (carbon particles). CO is dangerous — colourless, odourless, binds to haemoglobin and prevents oxygen transport.
Cracking
Crude oil contains more long-chain fractions than the market demands, and fewer short-chain fractions (such as petrol) than are needed. Cracking converts these heavier fractions into more valuable products. There are two methods:
- Catalytic cracking: long-chain alkane vapour is passed over a hot catalyst (zeolite / aluminium oxide), which causes the chains to break. Lower temperatures can be used than without a catalyst.
- Steam cracking: the hydrocarbon is mixed with steam and heated to a very high temperature (around 800–900 °C) without a catalyst.
Cracking always produces a shorter-chain alkane and an alkene. For example: \(C_{12}H_{26} \rightarrow C_6H_{14} + C_6H_{12}\) (a hexane and a hexene).
Alkenes are unsaturated — they contain at least one \(C=C\) double bond. Their general formula is \(C_nH_{2n}\). The simplest alkene is ethene (\(C_2H_4\)). Because of the double bond, alkenes are more reactive than alkanes and are valuable as starting materials for making polymers and other chemicals.
You can distinguish alkenes from alkanes using bromine water (an orange solution of bromine dissolved in water). Alkenes decolourise bromine water, turning it from orange to colourless — the bromine adds across the double bond (an addition reaction). Alkanes do not react with bromine water at room temperature; the orange colour remains.
Burning hydrocarbon fuels releases carbon dioxide, a greenhouse gas, into the atmosphere. The rising concentration of greenhouse gases is linked to an enhanced greenhouse effect and global warming. This is not a separate topic — combustion chemistry in this lesson directly connects to the climate change content covered in Earth science. Whenever hydrocarbons burn, the carbon that was locked away for millions of years as crude oil is released into the atmosphere in a matter of seconds.
Burning a fuel can also release sulfur dioxide (if the fuel contains sulfur impurities) and nitrogen oxides (formed at high combustion temperatures when N₂ and O₂ react), both of which contribute to acid rain. These links between fuel chemistry and environmental impact are central to the Ideas about Science thread — they illustrate how scientific understanding of molecules helps us evaluate societal decisions about energy use.