Crude Oil Notes
Oxford AQA IGCSE Chemistry: Revision notes
Key facts
- Crude oil is a mixture of many compounds, mostly hydrocarbons.
- A hydrocarbon contains hydrogen and carbon atoms only.
- Most are alkanes: saturated, with general formula .
- Fractional distillation separates crude oil into fractions by boiling point.
- Longer chains have higher boiling point and viscosity, and are less flammable.
What crude oil is
Crude oil is a mixture of a very large number of compounds, most of them hydrocarbons.
Crude oil forms over millions of years from the remains of ancient organisms. It is an important source of fuels and raw materials for the petrochemical industry. Most of its compounds are hydrocarbons.
Crude oil
- Hydrocarbonhydrogen and carbon atoms only
What elements does a hydrocarbon contain?
Alkanes
Most are saturated hydrocarbons called alkanes, with formula .
Most are alkanes. In saturated hydrocarbons all the carbon-carbon bonds are single covalent bonds. Alkanes can be shown as molecular, displayed or structural formulae.
- Alkane formula
| Methane | Ethane | Propane | |
|---|---|---|---|
| Carbon atoms | 1 | 2 | 3 |
| Hydrogen atoms | 4 | 6 | 8 |
| Formula |
Methane
- Carbon atoms:
- 1
- Hydrogen atoms:
- 4
- Formula:
Ethane
- Carbon atoms:
- 2
- Hydrogen atoms:
- 6
- Formula:
Propane
- Carbon atoms:
- 3
- Hydrogen atoms:
- 8
- Formula:
Which formula is an alkane?
Fractional distillation
Fractional distillation separates crude oil into fractions that condense at different boiling points.
Crude oil is not very useful as a mixture. Fractional distillation separates it into simpler, more useful mixtures called fractions. Each fraction has hydrocarbons with similar numbers of carbon atoms and similar boiling points.
- 1
Heat
crude oil is heated so that it evaporates
- 2
Vapours rise
up a column that is hotter at the bottom and cooler at the top
- 3
Condense
different hydrocarbons condense at different heights, by boiling point
- 4
Collect
each fraction is drawn off at its own level
In fractional distillation, what decides where a hydrocarbon is collected in the column?
Trends with chain length
Longer chains have higher boiling points and viscosity, and are less flammable.
Short-chain fractions such as petrol have low viscosity (runny) and high flammability. Long-chain fractions such as bitumen are thick and less flammable.
| Short chains (petrol) | Long chains (bitumen) | |
|---|---|---|
| Boiling point | Lower | Higher |
| Viscosity | Lower (runny) | Higher (thick) |
| Flammability | Higher | Lower |
Short chains (petrol)
- Boiling point:
- Lower
- Viscosity:
- Lower (runny)
- Flammability:
- Higher
Long chains (bitumen)
- Boiling point:
- Higher
- Viscosity:
- Higher (thick)
- Flammability:
- Lower
Which fraction is the most viscous: petrol or bitumen?
Try an exam question
Describe how crude oil is separated into fractions by fractional distillation.
[4 marks]
- [1]The crude oil is heated so that it evaporates.
- [1]The vapours rise up a column that is hotter at the bottom and cooler at the top.
- [1]Hydrocarbons condense at different heights depending on their boiling points.
- [1]Fractions with shorter chains (lower boiling points) are collected nearer the top.
That's the notes covered.
Carry on to the next subtopic.