4.2.1Organic Chemistry: Part 1
Crude Oil and Fractional Distillation — Edexcel International GCSE Science Double Award (Modular) - Chemistry Unit 1 Revision Notes & Topical Questions
5 min read 7 parts Lesson 5 of 12
Part 1 of 5What crude oil is
A mixture of hydrocarbons
- A mixture of hydrocarbons Crude oil, or petroleum, is a thick, dark liquid found in porous rock under the ground and the sea bed.
- A hydrocarbon is a compound containing hydrogen and carbon atoms only.
- The word 'only' is needed for the mark.
How it formed
- How it formed It formed over millions of years from the remains of ancient plants and animals, subjected to high temperature and pressure.
A finite resource
- A finite resource We use crude oil far faster than it forms, so it will eventually run out.
Mostly alkanes
- Mostly alkanes Most of the molecules are alkanes, which have only single bonds between carbon atoms.
- Their chains vary greatly in length.
Part 2 of 5How fractional distillation works
1
The crude oil is heated until it vaporises and the vapour enters the fractionating column.
2
The column is hot at the bottom and cooler towards the top, so there is a temperature gradient.
3
As vapours rise, they cool. Each hydrocarbon condenses where the column temperature drops below its boiling point.
4
Large molecules with high boiling points condense near the bottom. Small molecules with low boiling points rise to the top.
5
Each fraction, a group of hydrocarbons with similar chain lengths and boiling points, is drawn off as a liquid at its own height. The smallest molecules leave the top as gases.
Part 3 of 5Trends in the fractions
1Longer chain
- Longer chain More carbon and hydrogen atoms in each molecule.
2Stronger attractions
- Stronger attractions The forces between molecules become stronger as the molecules get larger.
3Higher boiling point
- Higher boiling point More energy is needed to separate the molecules, so the boiling point rises.
4Thicker and darker
- Thicker and darker The liquid is more viscous (flows less easily), less volatile (does not evaporate easily), and darker in colour.
Part 4 of 5The main fractions and their uses
| Fraction | Approx. carbon atoms | Use |
|---|---|---|
| Refinery gases | 1 to 4 | Domestic heating and cooking |
| Gasoline (petrol) | 4 to 12 | Fuel for cars |
| Kerosene (paraffin) | 12 to 16 | Jet fuel |
| Diesel (gas oil) | 14 to 18 | Fuel for diesel engines |
| Fuel oil | 19 to 25 | Fuel for ships and power stations |
| Bitumen | More than 70 | Surfacing roads and roofs |
Part 5 of 5Why oil is so valuable
Products from crude oil
Fuels
- Fuels Almost all transport fuels come from crude oil, from petrol for cars to kerosene for aircraft.
Raw materials
- Raw materials Polymers, lubricants, solvents, detergents and adhesives are made from fractions of crude oil.
Carbon's bonding
- Carbon's bonding Carbon forms strong covalent bonds with itself, which gives a huge variety of molecules and hence a huge range of products.
Key terms
| Term | Meaning |
|---|---|
| Hydrocarbon | A compound made of hydrogen and carbon atoms only. |
| Fraction | A group of hydrocarbons with similar boiling points collected together in distillation. |
| Alkane | A hydrocarbon with only single covalent bonds between its carbon atoms. |
| Viscosity | How easily a liquid flows; high viscosity means it is thick and flows slowly. |
| Volatility | How easily a liquid turns into a gas. |
Avoid these mistakes
1
Defining a hydrocarbon as a compound that 'contains' carbon and hydrogen without saying 'only'.
2
Thinking the column is hottest at the top. It is hottest at the bottom.
3
Saying that the molecules themselves break when they boil. Only the weak forces between molecules are overcome, not the covalent bonds.
4
Mixing up trendsboiling point and viscosity rise with chain length, while volatility falls.
5
Treating fractional distillation as a chemical reaction. It is a physical separation of a mixture.
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