Fuels, Crude Oil and Alkanes

Crude oil (petroleum) is a mixture of hydrocarbons, separated by fractional distillation: it's heated and vapours rise up a column that is hotter at the bottom; each fraction condenses at its boiling point. Short-chain fractions (refinery gas, petrol) are volatile, runny and flammable; long-chain fractions (fuel oil, bitumen) are viscous with high boiling points. Alkanes are saturated and unreactive except for combustion and substitution with halogens in UV light.

Cambridge 5070 Paper 1Cambridge 5070 Paper 2Cambridge 0620 Papers 2, 4Cambridge 0620 Papers 1, 3Edexcel 4CH1 Paper 1CEdexcel 4CH1 Paper 2C≈ 40 minRevision 1 · updated 2026-10-30

Section 1 of 14

Learning objectives

Describe fractional distillation and the uses of fractions.
Describe trends in properties of fractions.
Write equations for complete and incomplete combustion.
Describe substitution of alkanes with chlorine.

Pause and think

Crude oil is useless straight from the ground but worth a fortune after refining. What does refining do?

Section 2 of 14

Core idea

Start hereBlack gold

A barrel of crude oil is a thick, smelly mixture of thousands of hydrocarbons. Refining sorts them by size, turning it into petrol, jet fuel, plastics and the tarmac under your feet.

The big idea

Fractional distillation separates crude oil by boiling point, which depends on chain length. Alkanes are saturated hydrocarbons that mostly just burn.

1. Fractional distillation

The fractionating column

Drawing diagram…

Hot at the bottom, cool at the top.

2. Combustion

Worked exampleButane in a camping stove

Write equations for complete and incomplete combustion of butane, $\mathrm{C_4H_{10}}$ .

Goal: Two balanced equations.
Strategy: Balance C, then H, then O.

$\mathrm{2C_4H_{10}+13O_2\rightarrow8CO_2+10H_2O}$
Complete: O on right $=16+10=26$ , so $13\,\mathrm{O_2}$ for 2 butane.
$\mathrm{2C_4H_{10}+9O_2\rightarrow8CO+10H_2O}$
Incomplete: CO instead; O $=8+10=18$ .

AnswerAs above.

✓ CheckIncomplete needs less oxygen ✓.

Key takeawaySubstitution In UV light,

\mathrm{CH_4+Cl_2\rightarrow CH_3Cl+HCl}

. Further substitution gives

\mathrm{CH_2Cl_2}

\mathrm{CHCl_3}

and

\mathrm{CCl_4}

Pause and think

Why does a yellow, smoky flame indicate incomplete combustion?

Summary

Separate by boiling point.
Long chains: high bp, viscous.
Complete: CO₂ + H₂O; incomplete: CO/C.

Hydrocarbon: Compound of hydrogen and carbon only.
Fraction: Mixture of hydrocarbons with similar boiling points.
Viscosity: How thick and slow-flowing a liquid is.

HistoryDrake's well. In 1859 Edwin Drake drilled the first commercial oil well in Pennsylvania. Oil was mainly wanted for kerosene lamps; petrol was thrown away as useless.

Fun factAncient sunlight. Crude oil formed from microscopic sea creatures buried millions of years ago. Burning it releases energy that came from sunlight captured by plankton long before the dinosaurs.

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