Crude Oil

Crude Oil & Hydrocarbons

• Crude oil is a finite, non-renewable mixture of hydrocarbons (compounds containing hydrogen and carbon only).

• Formation:​ Formed over millions of years from the remains of tiny marine organisms/plankton buried in mud and silt; high temperature and pressure, with no oxygen, changed them into hydrocarbons. It is trapped in porous rocks beneath impermeable rock and extracted by drilling.

Fractional Distillation

• Purpose:​ A physical separation method that splits crude oil into fractions — mixtures of hydrocarbons with similar boiling points, not pure compounds. No chemical reaction occurs.

• Structure:​ The fractionating column is hot at the bottom and cooler at the top, with trays/bubble caps. Crude oil is heated and vaporised, then fed near the bottom. Vapours rise; at each tray, hydrocarbons condense when the temperature is below their boiling point and are tapped off. Hydrocarbons with lower boiling points remain gaseous and rise higher.

• Principle:​ Separation depends on boiling point and chain length. Larger molecules have stronger intermolecular forces, higher boiling points, and condense lower down. Smaller molecules have weaker intermolecular forces, lower boiling points, and rise higher. Only weak intermolecular forces are overcome — covalent bonds are not broken.

• Products (top → bottom):​ Refinery gases (LPG, cooking/heating), petrol/gasoline (cars), kerosene (jet fuel/paraffin heaters), diesel (lorries/buses/cars), fuel oil (ships/power stations/heating), bitumen (roads/roofs). Some specifications also include lubricating oil/waxes between diesel and fuel oil.

• Trends:​ Going up the column, fractions have shorter chains, lower boiling points, are more volatile, more flammable, less viscous, and lighter in colour. Going down, the opposite is true.

Combustion

• Fuel:​ A substance that releases useful heat energy when burned.

• Complete combustion:​ Occurs with excess oxygen. Hydrocarbon → carbon dioxide + water. Example: CH₄ + 2O₂ → CO₂ + 2H₂O. Clean blue flame.

• Incomplete combustion:​ Occurs with limited oxygen. Hydrocarbon → carbon monoxide and/or carbon (soot) + water. Example: CH₄ + 1.5O₂ → CO + 2H₂O. Yellow/smoky flame, soot deposits.

• Reasons for incomplete combustion in an experiment:​ Insufficient oxygen/air supply (e.g. air hole closed, lid restricting air, burning in a confined jar); fuel not mixed thoroughly with air; too much fuel for the available oxygen; poor ventilation/enclosed space.

• Carbon monoxide:​ Poisonous because it reduces the blood’s capacity to transport oxygen.

Pollutants from Combustion

• Sulphur impurities​ burn to form SO₂ or SO₃ , which dissolves in water in the atmosphere to form acid rain (H₂SO₃/H₂SO₄).

• Nitrogen oxides (NOₓ):​ Formed at high engine temperatures when nitrogen and oxygen react; contribute to acid rain and smog.

• Carbon monoxide:​ Toxic gas from incomplete combustion.

• Soot/particulates:​ Cause breathing problems and dark smoke.

Cracking

• Purpose:​ To match supply and demand by converting excess long-chain hydrocarbons into shorter, more useful molecules (e.g. petrol, ethene for polymers).

• Method:​ Thermal decomposition using a catalyst (silica or alumina) at 600–700°C.

• Example:​ Decane → octane + ethene.