How is crude oil separated, and why is cracking useful?
Fractional distillation of crude oil; the fractions and their uses; the petrochemical industry; and cracking longer hydrocarbons into useful shorter ones.
A focused answer to AQA GCSE Chemistry 4.7.1 and 4.7.2, covering the fractional distillation of crude oil into fractions and their uses, the petrochemical industry, and how cracking turns longer hydrocarbons into useful shorter molecules and alkenes.
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What this dot point is asking
AQA wants you to describe how crude oil is separated by fractional distillation, name the fractions and their uses, explain the petrochemical industry, and describe cracking, including why it is done and the products it makes. The boiling-point trend from the previous dot point is the mechanism that makes fractional distillation work.
Fractional distillation
Shorter hydrocarbons (lower boiling point) rise higher and leave near the top; longer hydrocarbons (higher boiling point) condense lower and leave near the bottom. Fractional distillation is a physical separation: the hydrocarbon molecules are not changed, only sorted by boiling point.
The fractions and their uses
Common fractions, from the top (lightest) to the bottom (heaviest), include:
- Petrol (gasoline): fuel for cars.
- Kerosene: fuel for aircraft.
- Diesel oil: fuel for some cars, lorries and trains.
- Fuel oil: for ships and heating.
- Bitumen: for surfacing roads and roofs.
The petrochemical industry
The hydrocarbons from crude oil are used as fuels and as a feedstock for the petrochemical industry, which makes solvents, lubricants, polymers and detergents. A vast range of everyday materials begins as a hydrocarbon fraction from crude oil.
Cracking
Cracking is done because there is more demand for short-chain hydrocarbons (such as petrol) than the supply from distillation alone, and because it produces alkenes, which are used to make polymers. Unlike fractional distillation, cracking is a chemical change: bonds are broken and new, smaller molecules form.
Matching supply to demand
Fractional distillation of crude oil produces more of the longer fractions (such as heavy oils) than the market wants, and less of the shorter, more valuable fractions (such as petrol) than is demanded. Cracking solves this mismatch by converting some of the surplus long-chain molecules into the short-chain molecules that are in high demand, so refineries can make their output match what customers need. The alkenes produced at the same time are a bonus: they are the raw material for addition polymers such as poly(ethene), so cracking supplies both better fuels and the building blocks for plastics. This dual purpose, more useful fuels plus alkenes for polymers, is the standard two-mark reason an exam expects for why cracking is carried out.
Try this
Q1. Explain how fractional distillation separates the fractions of crude oil. [2 marks]
- Cue. The column is hot at the bottom and cool at the top, so each fraction condenses at the level matching its boiling point.
Q2. Give two reasons why cracking is carried out. [2 marks]
- Cue. To make more short-chain hydrocarbons for fuels, and to make alkenes for polymers.
Q3. State whether cracking is a physical or chemical change, and why. [1 mark]
- Cue. Chemical, because bonds are broken and new, smaller molecules are formed.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20194 marksCrude oil is separated into fractions by fractional distillation. Describe how the column separates the hydrocarbons, and explain why fractions that leave near the top of the column have lower boiling points than those leaving near the bottom.Show worked answer →
A 4-mark Paper 2 question on the process and the link to boiling point.
Process (2 marks): the crude oil is heated until it vaporises and passes into the fractionating column, which is hot at the bottom and cooler at the top; as the vapours rise they cool, and each hydrocarbon condenses and is collected at the height where the temperature matches its boiling point. Top versus bottom (2 marks): shorter hydrocarbons have lower boiling points, so they rise higher up the cooler column before condensing; longer hydrocarbons have higher boiling points and condense lower down near the hot bottom.
Markers reward the temperature-gradient-along-the-column idea and the boiling-point link to chain length.
AQA 20214 marksCracking is carried out on long-chain hydrocarbons. Explain why cracking is needed, name the two products it makes, and write a balanced symbol equation for the cracking of decane () into octane () and one other product.Show worked answer →
A 4-mark question combining the reasons for cracking with an equation.
Why cracking (2 marks): there is greater demand for short-chain hydrocarbons (such as petrol) than the amount produced directly, so long-chain fractions are cracked to make more; cracking also produces alkenes, which are needed to make polymers. Products (1 mark): shorter alkanes and alkenes. Equation (1 mark): (the other product being ethene, an alkene).
Markers reward the demand and the alkene-for-polymers reasons, and a balanced equation where carbons and hydrogens match.
Related dot points
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A focused answer to AQA GCSE Chemistry 4.7.2, covering alkenes as unsaturated hydrocarbons, their general formula, the bromine water test, addition polymerisation, and how alkenes differ from alkanes.
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A focused answer to AQA GCSE Chemistry 4.9.3, covering the pollutants released when fuels burn, including carbon monoxide, soot, sulfur dioxide and oxides of nitrogen, where they come from, and the harm they cause to health and the environment.
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Sources & how we know this
- AQA GCSE Chemistry (8462) specification — AQA (2016)