4.2Organic Chemistry
Crude Oil — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions
5 min read 8 parts Lesson 7 of 18 Unit quiz
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.
- which are not chemically bonded to each other,
- so each keeps its own properties.
Part 2 of 6Fractional distillation
Different hydrocarbons have different boiling points, so the mixture can be separated physically by fractional distillation.
Part 3 of 6How properties change with chain length
| Property | Short chains (top) | Long chains (bottom) |
|---|---|---|
| Boiling point | Low | High |
| Viscosity | Runny | Thick, treacly |
| Flammability | Ignite easily | Difficult to ignite |
| Colour | Colourless / pale | Dark |
| Burning | Clean blue/yellow flame | Smoky 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.
- 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.
Part 6 of 6Worked examples
State why crude oil is called a mixture. [1]1 mark
It contains many different hydrocarbons that are not chemically bonded together.
Explain why bitumen is found at the bottom of the fractionating column. [2]2 marks
It has very large molecules with a very high boiling point, so it does not vaporise or condenses first.
Why does viscosity increase with chain length? [2]2 marks
Longer molecules have stronger intermolecular forces and tangle, so they flow less easily.
Name the two products of complete combustion of a hydrocarbon. [1]1 mark
Carbon dioxide and water.
Key terms
| Term | Meaning |
|---|---|
| Crude oil | Fossil fuel mixture of hydrocarbons |
| Fraction | Hydrocarbons with similar boiling points |
| Fractionating column | Tower where fractions condense at different heights |
| Intermolecular forces | Weak attractions between molecules |
| Non-renewable | Cannot be replaced in a human lifetime |
| Bitumen | Residue used in road making |
Avoid these mistakes
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