A mixture consisting mainly of alkane hydrocarbons that can be separated by fractional distillation.

How does the temperature change up a fractionating column?

It is hottest at the bottom and cooler at the top.

Why do long-chain alkanes condense lower in the column?

Stronger London forces between longer molecules give higher boiling points, so they condense at lower, hotter positions.

A mixture of hydrocarbons with similar chain lengths and a similar boiling range.

Which fraction condenses nearest the top of the column?

Refinery gas, the shortest chains.

What happens in cracking?

C–C bonds in long-chain alkanes are broken to form shorter alkanes and alkenes.

What conditions are used in thermal cracking?

High pressure and high temperature.

What conditions are used in catalytic cracking?

Slight pressure, high temperature and a zeolite catalyst.

What does catalytic cracking mainly produce?

Motor fuels and aromatic hydrocarbons.

Give the economic reasons for cracking alkanes.

Demand for short-chain alkanes exceeds supply, long-chain fractions are in surplus, and alkenes are valuable feedstock for polymers.

Fractional distillation and crackingAQA A-Level Chemistry: Flashcards

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Why are alkanes called saturated hydrocarbons?

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Why are alkanes called saturated hydrocarbons?: They contain only carbon and hydrogen, with only single C–C bonds.
What is petroleum?: A mixture consisting mainly of alkane hydrocarbons that can be separated by fractional distillation.
How does the temperature change up a fractionating column?: It is hottest at the bottom and cooler at the top.
Why do long-chain alkanes condense lower in the column?: Stronger London forces between longer molecules give higher boiling points, so they condense at lower, hotter positions.
What is a fraction?: A mixture of hydrocarbons with similar chain lengths and a similar boiling range.
Which fraction condenses nearest the top of the column?: Refinery gas, the shortest chains.
What happens in cracking?: C–C bonds in long-chain alkanes are broken to form shorter alkanes and alkenes.
What conditions are used in thermal cracking?: High pressure and high temperature.
What does thermal cracking mainly produce?: A high percentage of alkenes.
What conditions are used in catalytic cracking?: Slight pressure, high temperature and a zeolite catalyst.
What does catalytic cracking mainly produce?: Motor fuels and aromatic hydrocarbons.
What is the name of the catalyst in catalytic cracking?: A zeolite.
Give the economic reasons for cracking alkanes.: Demand for short-chain alkanes exceeds supply, long-chain fractions are in surplus, and alkenes are valuable feedstock for polymers.
Balance: C₁₀H₂₂ → C₈H₁₈ + ?: C₂H₄ (ethene).

Exam questions on Fractional distillation and cracking

A refinery separates crude oil in a fractionating column. The fractions collected, in no particular order, are refinery gas, petrol, kerosene, diesel and fuel oil. Each fraction is a mixture of alkanes.
Explain why alkanes are described as saturated hydrocarbons.2 marks
Tetradecane, C₁₄H₃₀, is a long-chain alkane that is cracked in an oil refinery. Cracking breaks C–C bonds in long-chain alkanes to form shorter, more useful molecules.
Write an equation for the cracking of tetradecane to form octane and ethene as the only products.2 marks
An oil refinery has a large surplus of long-chain hydrocarbons from fractional distillation. At the same time, demand for petrol and for alkenes used in the chemical industry is greater than the amounts the distillation alone provides.
Explain the economic reasons for cracking long-chain alkanes.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).