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4.2Organic Chemistry

Crude Oil — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions

5 min read 8 parts Lesson 7 of 18 Unit quiz

📚 Edexcel Linear - OL (Chemistry) Hub / Topical Revision
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Lesson summary

Crude Oil: Formation, Fractional Distillation and Properties of Fractions — EDEXCEL LINEAR – OL (CHEMISTRY) notes: Explain how crude oil formed, how it is separated into fractions and how chain length affects the properties of fractions.Crude oil is a fossil fuel formed over mill…

  • non-renewable
  • intermolecular forces
  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4

Part 1 of 6Formation and composition

Crude oil is a fossil fuel formed over millions of years from the remains of ancient marine plankton and animals that were buried under layers of sediment.

Heat and pressure in the absence of air converted the organic matter to oil and gas, trapped under impermeable rock.
Mixture
It is a mixture of hydrocarbons, mostly alkanes of different chain lengths,
  • which are not chemically bonded to each other,
  • so each keeps its own properties.
Non-renewable
Because it takes millions of years to form, crude oil is non-renewable.

Part 2 of 6Fractional distillation

Different hydrocarbons have different boiling points, so the mixture can be separated physically by fractional distillation.

Crude oil is heated until most of it vaporises and fed into a tall column that is hot at the bottom (about 350 °C) and cooler at the top (about 20 °C).
Vapours rise and cool; each fraction condenses at the level where the temperature is below its boiling point and is drawn off.
Fraction
A fraction is a mixture of hydrocarbons with similar boiling points and chain length.
The residue (bitumen) stays at the bottom, and gases leave at the top.

Part 3 of 6How properties change with chain length

PropertyShort chains (top)Long chains (bottom)
Boiling pointLowHigh
ViscosityRunnyThick, treacly
FlammabilityIgnite easilyDifficult to ignite
ColourColourless / paleDark
BurningClean blue/yellow flameSmoky flame

The reason is the strength of the intermolecular forces: longer molecules have more contact and stronger attractions between them, so more energy is needed to separate them and they are more viscous. Only these weak forces are broken in boiling; covalent bonds inside the molecules remain intact.

Usesrefinery gases (LPG) for heating and cooking, petrol for cars, naphtha for chemicals, kerosene for aircraft, diesel for lorries and trains, fuel oil for ships and power stations, lubricating oil and bitumen for roads.

Part 4 of 6Fuels and pollution

Burning hydrocarbons in plenty of air forms carbon dioxide and water; in limited air carbon monoxide and soot form. Sulfur impurities form SO2, causing acid rain, and engines make nitrogen oxides. Carbon dioxide is a greenhouse gas that contributes to global warming. Fuels can be made cleaner by removing sulfur before burning, and by fitting catalytic converters.

Part 5 of 6Worked examples and exam links

A typical question gives a table of fraction names, number of carbons and boiling ranges, and asks you to predict the properties of an extra fraction.

Worked examples and exam links
Use the trend: as the number of carbons increases, boiling point and viscosity go up and flammability goes down.
A fraction with 18 carbon atoms is therefore a diesel-type liquid that is thicker and less flammable than petrol, and boils at higher temperatures.
Another style asks about fuel choice: petrol vaporises easily so is suitable for spark-ignition engines,
  • while heavy fuel oil needs pre-heating to flow,
  • so is used in large slow engines such as ships.

The industrial separation can also be described as a physical process driven by temperature gradient, whereas cracking is chemical because bonds break.

Worked examples and exam links
Exam answers lose marks by saying that the compounds "evaporate and condense at different boiling points" without linking to the column temperature or the size of the molecules.
Quote the full chain: larger molecules, stronger intermolecular forces, higher boiling point, condense lower in the column.

Part 6 of 6Worked examples

  1. State why crude oil is called a mixture. [1]1 mark

  2. Explain why bitumen is found at the bottom of the fractionating column. [2]2 marks

  3. Why does viscosity increase with chain length? [2]2 marks

  4. Name the two products of complete combustion of a hydrocarbon. [1]1 mark

Key terms

TermMeaning
Crude oilFossil fuel mixture of hydrocarbons
FractionHydrocarbons with similar boiling points
Fractionating columnTower where fractions condense at different heights
Intermolecular forcesWeak attractions between molecules
Non-renewableCannot be replaced in a human lifetime
BitumenResidue used in road making

Avoid these mistakes

1
Saying covalent bonds break when crude oil is distilled.
2
Putting bitumen at the top.
3
Calling crude oil a compound.
4
Saying boiling points decrease with chain length.

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