Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 5% of your exam marks.
stable
Rare
Stable5%
Fractional distillation and cracking are standard multi-mark questions.
What crude oil is
- Crude oil (petroleum) is a thick black mixture of many different hydrocarbons formed over millions of years from the remains of ancient marine organisms
- It is a non-renewable finite resource — the supply cannot be replaced on a human timescale
- In its raw state crude oil is not directly useful; it has to be separated into groups of hydrocarbons with similar properties, called fractions
Fractional distillation
- Fractional distillation is the industrial process used to separate crude oil into its fractions
- It is carried out in a tall fractionating column in which the temperature falls continuously from the bottom (very hot, about 350 °C) to the top (cooler, about 25 °C) — a vertical temperature gradient
- The process step-by-step:
- Crude oil is heated until it vaporises
- The hot vapours enter the column near the bottom
- As the vapours rise, they cool steadily
- Each hydrocarbon condenses back to liquid when its temperature reaches its own boiling point — long-chain hydrocarbons condense first, near the hot bottom; short-chain hydrocarbons condense further up, where it is cooler
- The condensed liquids are tapped off at different levels of the column as separate fractions
Main fractions and what each one is used for
| Fraction | Chain length (C atoms) | Approximate boiling range / °C | Main use |
|---|---|---|---|
| Refinery gas | 1–4 | < 25 | Bottled gas; domestic heating and cooking |
| Gasoline (petrol) | 4–12 | 40–100 | Fuel for cars |
| Kerosene (paraffin) | 12–16 | 150–240 | Fuel for jet aircraft |
| Diesel (gas oil) | 14–18 | 220–300 | Fuel for lorries, trains, some cars |
| Fuel oil | 19–25 | 250–320 | Fuel for ships and power stations |
| Bitumen | > 70 | > 350 | Surfacing roads and roofs |
How properties change down the column
- As chain length increases:
- Boiling point rises — bigger molecules have stronger intermolecular forces to overcome
- Viscosity rises — the liquid flows less readily (bitumen is tar-thick; petrol is runny)
- Volatility falls — bigger molecules evaporate less readily
- Colour darkens — the heaviest fractions are deep brown to black
- Flammability falls — long-chain hydrocarbons are harder to ignite