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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 is first separated into simpler, more useful parts called fractions by fractional distillation. But distillation alone is not enough: it produces too much of some fractions and too little of others, so the surplus long-chain molecules are then broken down by cracking into shorter, more useful ones — including the alkenes used to make plastics. This lesson, part of Topic C6 of OCR Gateway Combined Science A, brings the two processes together.
By the end of this lesson you should be able to describe fractional distillation and name the main fractions and their uses, explain that separation depends on boiling point and molecular size, explain why long-chain hydrocarbons are cracked, write balanced equations for cracking, and describe alkenes and the bromine-water test.
This lesson pairs AO1 (recalling the fractions, their uses and why cracking is needed) with AO2, where you write balanced equations for cracking and apply the bromine-water test to distinguish an alkene from an alkane.
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. Fractional distillation is a physical process — it simply sorts the hydrocarbons already present by boiling point, so no new substances are made.
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. A common misconception is that the column is hottest at the top — it is the other way round.
| Fraction (top → bottom) | Chain length / boiling point | Main use |
|---|---|---|
| Refinery gases | Shortest / lowest | Bottled gas (LPG) for heating and cooking |
| Petrol | 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 |
The whole process depends on boiling point, which depends on the size of the molecules and the intermolecular forces between them. Larger molecules (longer chains) have stronger intermolecular forces, so more energy is needed to separate them — they have high boiling points and condense low down. Smaller molecules have weaker forces, lower boiling points, and condense higher up.
Two fractions, A and B, are collected. A comes off 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 has 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.
Exam Tip: Always tie separation back to boiling point, then to molecular size / intermolecular forces. That link connects C6 to earlier bonding work and earns the explanation marks.
Fractional distillation produces fractions in fixed proportions set by the make-up of the crude oil — and these proportions do not match demand. There is usually a surplus of long-chain fractions (such as fuel oil) but a shortage of short-chain fractions, especially petrol, which is needed in enormous quantities for cars. Rather than waste the surplus, refineries crack the long-chain hydrocarbons into the shorter molecules that are in demand.
So cracking solves two problems at once: it uses up less-wanted long-chain hydrocarbons, and it makes more of the valuable short-chain hydrocarbons (and the alkenes needed for plastics).
Exam Tip: The reason for cracking is a supply-and-demand mismatch: distillation gives too much long-chain hydrocarbon and too little petrol. Cracking converts the surplus into more useful, shorter molecules.
Cracking is the breaking of long-chain alkanes into shorter, more useful molecules. It is a form of thermal decomposition — a large molecule broken down by heat. There are two main methods:
| Method | Conditions |
|---|---|
| Catalytic cracking | The long-chain hydrocarbon is heated until it vaporises, then passed over a hot catalyst |
| Steam cracking | The hydrocarbon is vaporised, mixed with steam and heated to a very high temperature |
In both methods, a long-chain alkane is broken into a shorter-chain alkane plus one or more alkenes. The shorter alkane is useful as a fuel (such as petrol); the alkenes are useful for making polymers. For example, decane can crack to give octane (a petrol-sized molecule) and ethene:
C10H22→C8H18+C2H4
Check it balances: carbons 10=8+2 ✓; hydrogens 22=18+4 ✓.
Dodecane, C12H26, is cracked to make octane, C8H18, and ethene, C2H4. Write the balanced equation, adding more ethene molecules as needed.
Step 1 — write what is known: C12H26→C8H18+C2H4.
Step 2 — check carbon: left 12; right 8+2=10. Two carbons short, so add another C2H4 → C8H18+2C2H4.
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