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Crude oil straight from the ground is a dark, sticky mixture of hundreds of different hydrocarbons and is not much use as it is. To turn it into the fuels and feedstocks we rely on, it must first be separated into simpler, more useful parts called fractions. This is done at an oil refinery by fractional distillation in a tall fractionating column. The separation works because the different hydrocarbons have different boiling points, which in turn depend on their chain length. This lesson, part of Topic C6 of OCR Gateway Science A, describes how fractional distillation works, names the main fractions and their uses, and explains why the fractions separate where they do.
By the end of this lesson you should be able to describe fractional distillation as a way of separating crude oil into fractions, explain how the temperature gradient in the column separates the fractions, name the main fractions and their uses, and explain that separation depends on boiling point and so on molecular size and intermolecular forces.
A fraction is a group of hydrocarbons that have similar chain lengths and therefore similar boiling points. Crude oil is far too complex to separate into individual pure compounds on a large scale, so instead it is split into a handful of useful fractions, each still a mixture but one whose molecules are within a similar size range. Each fraction has properties suited to a particular use.
Fractional distillation is a physical process — it separates the hydrocarbons that are already present in the crude oil. No new substances are made; the molecules are simply sorted by boiling point.
Exam Tip: A fraction = hydrocarbons of similar chain length / boiling point. Stress that fractional distillation is a physical separation of substances already in the oil — it does not create new molecules (that is cracking, a later lesson).
Fractional distillation is carried out in a fractionating column that is hot at the bottom and cooler at the top. The steps are:
Exam Tip: The column is hot at the bottom, cool at the top. Each fraction condenses where the temperature equals its boiling point. Saying the vapours "rise and condense as they cool" is the heart of the method.
The fractions, listed from the top of the column (shortest chains, lowest boiling points) to the bottom (longest chains, highest boiling points), are:
| Fraction (top → bottom) | Chain length / boiling point | Main use |
|---|---|---|
| Refinery (petroleum) gases | Shortest / lowest | Bottled gas (LPG) for heating and cooking |
| Petrol (gasoline) | Short / low | Fuel for cars |
| Kerosene | Medium | Fuel for aircraft (jet fuel) |
| Diesel oil | Longer | Fuel for lorries, buses and trains |
| Fuel oil | Long | Fuel for ships and for heating |
| Bitumen | Longest / highest | Surfacing roads and roofs |
Notice the pattern: the fractions at the top are the most volatile and flammable, which makes them good fuels for vehicles; the fractions at the bottom are thick and used for heavy fuels and for surfacing.
Exam Tip: A useful way to remember the order (top to bottom) is by what they fuel, getting heavier as you go down: gases → cars (petrol) → aircraft (kerosene) → lorries (diesel) → ships (fuel oil) → roads (bitumen).
A hydrocarbon fraction is used as jet fuel for aircraft. Where in the column is it collected, and how do its chain length and boiling point compare with petrol?
Step 1 — identify the fraction: jet fuel is kerosene.
Step 2 — recall its position: kerosene comes off in the middle of the column — lower (hotter) than petrol but higher (cooler) than diesel.
Step 3 — compare with petrol: because kerosene condenses lower (in a hotter part), it has a higher boiling point and longer chains than petrol.
Answer: kerosene is collected in the middle of the column; it has longer chains and a higher boiling point than petrol.
The whole process depends on boiling point, and boiling point depends on the size of the molecules and the intermolecular forces between them (an idea from Topic C2). Larger hydrocarbon molecules (longer chains) have more, stronger intermolecular forces, so more energy is needed to separate them — they have high boiling points and condense low down. Smaller molecules (shorter chains) have weaker intermolecular forces, lower boiling points, and condense higher up (or stay as gases).
So the column works because it sets up a temperature gradient: every hydrocarbon condenses at the height where the temperature has fallen to its own boiling point. Hydrocarbons with similar boiling points (and so similar chain lengths) condense together, which is exactly what makes them a fraction.
Two fractions, A and B, are collected. A is collected near the bottom of the column and B near the top. Compare their chain lengths, boiling points and volatility.
Step 1 — use the column's temperature gradient: bottom = hot, top = cool.
Step 2 — A condenses near the hot bottom, so it must have a high boiling point → long chains, and it is less volatile.
Step 3 — B condenses near the cool top, so it has a low boiling point → short chains, and it is more volatile.
Answer: A has longer chains, a higher boiling point and is less volatile; B has shorter chains, a lower boiling point and is more volatile.
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