Fossil fuels and crude oil fractions

What you will learn

By the end of this lesson you can:

• name natural gas (mainly methane) and crude oil as non-renewable sources of energy;
• explain how fractional distillation separates crude oil into fractions, and why those fractions compete for use as fuels or as a source of chemicals.

You need two ideas from earlier topics. Distillation separates liquids that have different boiling points. A hydrocarbon is a compound of carbon and hydrogen only.

Fossil fuels and combustion

A fuel releases useful energy when it burns. Natural gas and crude oil formed over millions of years from the remains of living things. We use them far faster than they form, so they are non-renewable: once used, they are not replaced within human timescales.

Natural gas is mainly methane, CH₄. When a hydrocarbon burns in plenty of oxygen, its carbon becomes carbon dioxide and its hydrogen becomes water:

Keep this in mind for the rest of the lesson: complete combustion turns the fuel’s carbon into carbon dioxide. Incomplete combustion can also form carbon monoxide or soot.

Separate crude oil by boiling range

Crude oil is a mixture of many different hydrocarbons, not a single compound. Its molecules vary in size and structure, giving them different boiling points. Fractional distillation uses those differences to separate the mixture:

1. Crude oil is heated until most of it turns to vapour.
2. The vapour enters the bottom of a tall fractionating column. The column is hottest at the bottom and coolest at the top.
3. As the vapour rises it cools. Hydrocarbons condense in regions cool enough for them to become liquids. Higher-boiling fractions condense lower down; lower-boiling fractions are collected higher up.
4. The lowest-boiling hydrocarbons stay as gases and leave near the top. The heaviest material remains liquid at the temperatures used and leaves the bottom as residue, which is used to make bitumen.

Each product is a fraction: a mixture of hydrocarbons with similar boiling points. A fraction therefore boils over a range of temperatures, not at one temperature. No molecule is broken up: fractional distillation is a physical separation.

These are general trends across crude-oil fractions. A boiling range does not tell you an exact molecule size: each fraction contains a mixture.

You do not need to memorise every row. In a question, use the property or use it gives you and link it to molecule size.

Competing uses: fuel or chemicals

Most fractions can be used in two ways. They can be burned as fuels, or used as a source of chemicals (a feedstock) for making plastics, solvents and medicines. These uses compete. Crude oil is non-renewable, so every tonne of a fraction that is burned is a tonne that can no longer be made into chemicals.

Demand is not matched to supply either. Crude oil contains more large molecules than people need, and fewer small ones. Refineries break some large molecules into smaller ones by cracking, which the reactions lesson in this topic explains.

Renewable fuels

Bioethanol is a different source: crops can regrow, and their recent carbon uptake matters when comparing fuels. Learn how in Biofuels and the carbon dioxide balance.

Use boiling ranges to explain collection

Worked example 1

Order two fractions in the column

Problem

Crude-oil fraction A has a boiling range of 200–300 °C; fraction B has a range of 70–170 °C. Which is collected higher in the fractionating column? Explain.

Show full solution

1. Match boiling range to the temperature gradient

   Method

   Compare where each vapour can condense as it rises.

   Reason

   The column is hottest at the bottom and cooler higher up.

   Working

   Answer

   A condenses in a hotter, lower region. B remains vapour through that region and reaches a cooler part higher up.

2. State the order

   Working

   Answer

   B is collected above A. Each fraction is a mixture, so use its boiling range rather than assigning it one pure-substance boiling point. The molecules remain intact during separation.

Try placing fractions in the column

Guided practice 2

Place three fractions in the column

About 5 min

Problem

Three fractions from crude oil have these boiling ranges: fraction P, 250–350 °C; fraction Q, 40–75 °C; fraction R, 150–240 °C. Decide where each is collected relative to the others, which is expected to be most viscous and which suits a car fuel that must vaporise easily.

Use the boiling ranges

Fraction collected highest in the column

PCorrectIncorrectRCorrectIncorrectQCorrectIncorrect

Fraction collected between the other two

PCorrectIncorrectQCorrectIncorrectRCorrectIncorrect

Fraction collected lowest of these three

RCorrectIncorrectQCorrectIncorrectPCorrectIncorrect

Most viscous fraction

PCorrectIncorrectRCorrectIncorrectQCorrectIncorrect

Car fuel that must vaporise easily

QCorrectIncorrectPCorrectIncorrectRCorrectIncorrect

Check answer

Hints

Hint 1

column

The column is coolest at the top. Lower-boiling hydrocarbons remain as vapour as they rise past the warmer collection points.

Hint 2

size

Across crude-oil fractions, a higher boiling range generally goes with larger molecules and higher liquid viscosity.

Show solution step by step

1. Order the fractions

   Method

   Order by boiling range.

   Reason

   Vapour condenses where the column is cooler than its boiling point.

   Working

   Answer

   Top to bottom: Q (40–75 °C), R (150–240 °C), P (250–350 °C).

2. Link to properties and use

   Working

   Answer

   Using the general trends, P is expected to contain larger molecules and be the most viscous. Q has the lowest boiling range, so it vaporises most easily and suits the stated car-fuel requirement.

Quick check

Each fraction is a mixture, which is why it has a boiling range rather than one boiling point.

Correct a distillation explanation

Check your understanding 1 Explain what distillation changes

Correct the claim: “Fractional distillation breaks large hydrocarbon molecules into smaller ones.”

Work on paper or in the optional notes. Compare the physical changes, intact molecules and distinct role of cracking.

Optional working or comparison notes

Show answer

Fractional distillation separates the existing molecules by boiling range; it does not break their chemical bonds. Cracking is the chemical process that breaks large molecules into smaller ones.

Decide between fuel and chemical uses

Check your understanding 2 Burn it or make plastics from it?

A refinery can sell its naphtha to a power station to burn, or to a factory that makes plastics from it. Explain why these two uses compete, and why burning more naphtha now matters for the future.

Work on paper or in the optional notes. Compare what combustion does to the molecules, the one-use trade-off and the non-renewable future supply.

Optional working or comparison notes

Show answer

Naphtha comes from crude oil, which is non-renewable. Each tonne can be used only once: if it is burned for energy it is turned into carbon dioxide and water and can no longer be made into plastics. Burning more now leaves less crude oil for both fuels and chemicals in future, and it adds carbon dioxide to the air.

Practise on your own

Check your understanding 3 Name and explain the fossil-fuel sources

Name the two non-renewable energy sources taught here and the main compound in natural gas. Explain why these sources are non-renewable.

Work on paper or in the optional notes. Compare both sources, natural gas’s main compound and formation versus use timescales.

Optional working or comparison notes

Show answer

Natural gas and crude oil are the sources; natural gas is mainly methane, CH₄. They form over millions of years and are used much faster than they form, so they are not replaced within human timescales.

Check your understanding 4 Explain why kerosene is collected higher

Kerosene generally has a lower boiling range than diesel. Explain why it is collected higher in the fractionating column.

Work on paper or in the optional notes. Compare the temperature gradient, where each vapour condenses and the unchanged molecules.

Optional working or comparison notes

Show answer

The column is cooler higher up. Kerosene remains vapour past the hotter region where diesel condenses, then condenses in a cooler region above it. Fractional distillation separates existing molecules rather than breaking them.

Check your understanding 5 Balance carbon and hydrogen before oxygen

For complete combustion, use one propane molecule in C₃H₈ + ? O₂ → ? CO₂ + ? H₂O. Enter the three missing coefficients. Keep every chemical formula unchanged.

Coefficient of CO₂

Coefficient of H₂O

Coefficient of O₂

Check answer

Show answer

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. Balance C first, then H, then count the ten product oxygen atoms. Now construct the full equation and both-side atom check in the next activity.

Check your understanding 6 Construct and check the complete combustion equation

Write a balanced equation for propane, C₃H₈, burning in plenty of oxygen, and check the atom totals.

Work on paper or in the optional notes. Compare the full equation and the totals of 3 carbon, 8 hydrogen and 10 oxygen atoms on each side.

Optional working or comparison notes

Show answer

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. Three carbons require three carbon dioxide molecules; eight hydrogens require four water molecules. These contain ten oxygen atoms, supplied by five oxygen molecules. Each side has 3 C, 8 H and 10 O.