GCSE · Chemistry · Edexcel · Spec 1CH0

Fractional distillation of crude oil

Petrol, jet fuel and the surface of your road all come out of one liquid. Here's the trick.

Inside the fractionating column

Ride the vapour up the column

Heated crude oil vapour enters near the bottom. The column is hottest at the bottom and gets cooler as you go up. Start at zone 7 and climb: each fraction turns back into a liquid at the height where the column is cool enough for it. (Zones are not drawn to scale.)

↑ Top of the column · coolest

↓ Bottom of the column · hottest

Zone 1 of 7

Petroleum gases

The very top, where the column is coolest. These are the smallest molecules of all, with boiling points so low that even up here they never cool enough to condense. So they leave the top of the column still as gases. They are the most flammable fraction.

Smallest molecules
Use: liquefied petroleum gas (LPG) for heating and cooking at home

Climb the column and every step up gives you smaller molecules, lower boiling points, and fractions that flow more easily and catch fire more easily. Big, thick and hard to light at the bottom; small, runny and flammable at the top.

Exam line: Link height to boiling point: the lower down a fraction is collected, the higher its boiling point and the bigger its molecules.

Size and boiling point

Where do these four boil?

Ethane, propane, butane and pentane are hydrocarbons, listed here from the smallest molecule to the largest. Drag each one to where you think it boils, in °C, then check.

Eth — EthaneProp — PropaneBut — ButanePent — Pentane

Why size matters

Reason it through

Why do bigger hydrocarbon molecules condense lower down the column?

Link 1 of 4

First link · your turn

Molecules are attracted to the molecules next to them. These attractions between molecules are called intermolecular forces. Which has more of them: a big hydrocarbon molecule or a small one?

Locked — reveal the link above first

What the column really does

Sorting, or making?

A refinery starts with crude oil. After fractional distillation it has petroleum gases, petrol, naphtha, kerosene, diesel oil, heavy fuel oil and bitumen.

Which of these is closest to what you think happened?

The heat breaks the big molecules in crude oil into smaller ones, so the column makes new substances such as petrol.Crude oil is one substance, and the column splits it into the parts it is made of.Each fraction that comes out, like petrol, is one pure substance with one boiling point.Crude oil was already a mixture of many hydrocarbons. The column sorts them into groups with similar boiling points, and nothing new is made.

How sure are you?

I'm sureI think soI'm guessing

The heat breaks the big molecules in crude oil into smaller ones, so the column makes new substances such as petrol.

This is a common misconception

It feels that way, because petrol goes in as crude oil and comes out as something different. But the column only separates. The hydrocarbons in petrol were in the crude oil all along, mixed in with everything else. Boiling and condensing pull whole molecules apart from each other and let them gather again; the molecules themselves are not broken, and no new substance is made.

Crude oil is one substance, and the column splits it into the parts it is made of.

This is a common misconception

Crude oil is not one substance. It is a mixture of lots of different hydrocarbons (molecules made of hydrogen and carbon atoms only), most of them alkanes, plus some impurities. That is exactly why it can be separated: its different hydrocarbons have different boiling points. It is also finite: a non-renewable resource that will not be replaced once it has been used.

Each fraction that comes out, like petrol, is one pure substance with one boiling point.

This is a common misconception

Easy to assume, because each fraction has a single name. But a fraction is a group of hydrocarbons with similar boiling points, collected together. It is still a mixture, so it does not boil at one temperature: it boils over a range of temperatures.

Crude oil was already a mixture of many hydrocarbons. The column sorts them into groups with similar boiling points, and nothing new is made.

This matches the evidence

Exactly right. Crude oil is a mixture, fractional distillation separates it by boiling point, and each fraction is itself a smaller mixture of hydrocarbons with similar boiling points. Sorting, not making.

Crude oil is a mixture. The column sorts its hydrocarbons by boiling point without changing them, and every fraction it collects is still a mixture that boils over a range of temperatures.

Your turn to explain it

Describe the whole separation

Describe how crude oil is separated into fractions by fractional distillation. [5 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

The crude oil is heated until it vaporises (turns into a vapour). (1 mark)
The vapour enters low down in the fractionating column and rises up it. (1 mark)
The column is hottest at the bottom and gets cooler towards the top. (1 mark)
Different fractions condense to liquids at different heights, depending on their boiling points. (1 mark)
The smallest molecules do not condense and leave the top as gases; the largest are collected from the bottom. (1 mark)

Model answer

The crude oil is heated until it turns into a vapour. The vapour enters the fractionating column near the bottom and rises up it. The column is hottest at the bottom and cooler towards the top, so as the vapour rises it cools. Each fraction condenses back into a liquid at the height where the temperature has fallen below its boiling point, so fractions with different boiling points are collected at different heights. The smallest molecules have boiling points so low that they never condense; they leave the top as gases. The largest molecules are collected from the bottom.

Heat it, vaporise it, let it rise and cool, collect each fraction where it condenses. Temperature and boiling point do all the sorting.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

How one tall, hot column sorts crude oil into petrol, diesel, bitumen and more, just by boiling point.

What you need to know

Crude oil is a finite (non-renewable) mixture of different hydrocarbons and some impurities. Hydrocarbons contain hydrogen and carbon atoms only, and most of those in crude oil are alkanes.
Fractional distillation separates crude oil into fractions (groups of hydrocarbons of similar size) by their boiling points. It does not make new substances.
Crude oil is heated until it vaporises. The vapour enters low down in a column that is hottest at the bottom and cooler going up, and it condenses at different heights depending on boiling point.
Top to bottom, the fractions are: gases, petrol, naphtha, kerosene, diesel oil, heavy fuel oil, bitumen. The smallest leave the top as gases; the largest are collected from the bottom.
Larger molecules have more intermolecular forces, which need more energy to overcome, so they have higher boiling points. Larger molecules are also less flammable and more viscous.

The big picture

Crude oil is a mixture of many different hydrocarbons, all jumbled together. Fractional distillation separates it using the one thing that differs between those hydrocarbons: their boiling points. The crude oil is heated until it vaporises, and the vapour rises up a column that is hottest at the bottom and cooler towards the top, so each fraction condenses at a different height. Big molecules condense low down and small ones near the top, and nothing new is made along the way.

Key points

1Crude oil is a mixture of hydrocarbons, and fractional distillation sorts them by boiling point.

2The column is hottest at the bottom. Each fraction condenses where the column cools below its boiling point.

3Bigger molecule, more intermolecular forces, more energy needed: a higher boiling point, so it is collected lower down.

4Going up the column: smaller molecules, lower boiling points, more flammable, less viscous.

5Every fraction is still a mixture, so it boils over a range of temperatures. Nothing new is made.

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Worked example

Problem

A hydrocarbon vapour rises up a fractionating column. Its molecules are larger than those in kerosene but smaller than those in heavy fuel oil. In which fraction will it be collected, and how will its boiling point, flammability and viscosity compare with kerosene's?

⚠ Watch out

Saying the biggest molecules end up at the bottom because they are heavy and sink. The column does not sort by weight. Big molecules have high boiling points, so their vapour condenses low down, where the column is still hot enough to cool them below that boiling point.

🧠

Memory hook

Top to bottom: Gary's Pet Newt Keeps Doing Handstands Backwards — Gases, Petrol, Naphtha, Kerosene, Diesel oil, Heavy fuel oil, Bitumen. Small and runny at the cool top; big and sticky at the hot bottom.

✓

Check yourself

Explain why the gases fraction comes out of the column as a gas, when every other fraction is collected as a liquid.

Flashcards

(15)

What is crude oil?

A finite (non-renewable) mixture of lots of different hydrocarbons, plus some impurities.

What is a hydrocarbon?

A molecule made of hydrogen and carbon atoms only.

Which type of hydrocarbon makes up most of crude oil?

Alkanes.

What is a fraction?

A group of hydrocarbon molecules of similar size, with similar boiling points.

Which part of a fractionating column is hottest?

The bottom. The temperature falls as you go up the column.

What makes a rising vapour turn back into a liquid at a particular height?

It has cooled below its boiling point there, so it condenses.

Where are the largest hydrocarbon molecules collected?

From the bottom of the column. Some of them never vaporised at all.

Name the fractions in order, from the top of the column to the bottom.

Gases, petrol, naphtha, kerosene, diesel oil, heavy fuel oil, bitumen.

Why do larger hydrocarbon molecules have higher boiling points?

They have more intermolecular forces, which need more energy to overcome.

Moving UP the column, what happens to flammability and viscosity?

Flammability increases and viscosity decreases: the fractions catch fire more easily and flow more easily.

What does 'viscous' mean?

Thick and slow to flow. A highly viscous liquid does not flow easily.

Why does a fraction boil over a range of temperatures, not at one?

It is a mixture of hydrocarbons whose boiling points are similar but not identical.

Does fractional distillation make new substances?

No. It only separates hydrocarbons that were already in the crude oil.

Which fraction fuels aircraft, and which fuels large ships?

Kerosene fuels aircraft; heavy fuel oil fuels large ships (and some power stations).

What are bitumen and naphtha used for?

Bitumen: road surfaces and roofing. Naphtha: making chemicals.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Ride the vapour up the column

Where do these four boil?

Sorting, or making?

Describe the whole separation

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