Crude OilOxford AQA IGCSE Chemistry: Revision notes
Section 1
What is crude oil made of?
Crude oil is a mixture of a very large number of different compounds. Most of these compounds are hydrocarbons — molecules made up of hydrogen and carbon atoms only. Crude oil forms over millions of years from the remains of ancient organisms, and is an important source of raw materials for fuels and the petrochemical industry.
Section 2
What are the hydrocarbons in crude oil?
Most of the hydrocarbons in crude oil are saturated hydrocarbons called alkanes, with the general formula CnH2n+2.
- In saturated hydrocarbons, all the carbon-carbon bonds are single covalent bonds
- The first three alkanes are methane (CH4), ethane (C2H6) and propane (C3H8)
- Alkanes can be recognised from their molecular, displayed or structural formulae
Section 3
How is crude oil separated into useful fractions?
Crude oil itself is not very useful as a mixture — it must be separated by fractional distillation into simpler, more useful mixtures called fractions.
- Crude oil is heated so that it evaporates
- The vapours rise up a fractionating column, which is hotter at the bottom and cooler at the top
- Different hydrocarbons condense at different heights, depending on their boiling points, forming separate fractions
- Each fraction contains hydrocarbons with similar numbers of carbon atoms and similar boiling points
Always describe fractional distillation as separating by boiling point/condensation temperature, not simply as 'heating up the oil'.
Section 4
How do the properties of the fractions change with molecular size?
As hydrocarbon chain length (molecular size) increases:
| Property | Trend with increasing chain length |
|---|---|
| Boiling point | Increases |
| Viscosity (thickness/how easily it flows) | Increases |
| Flammability (how easily it ignites) | Decreases |
This is why short-chain fractions (like petrol) are runny, flammable liquids or gases, while long-chain fractions (like bitumen) are thick, less flammable solids/semi-solids.
Think of short-chain hydrocarbons as light, loosely tangled strands that slip past each other easily (low viscosity, easy to ignite), while long chains are like heavy tangled ropes that resist flowing and are harder to set alight.
Must Know
- Crude oil is a mixture of a very large number of compounds, mostly hydrocarbons (hydrogen and carbon only)
- Most hydrocarbons in crude oil are saturated alkanes, general formula CnH2n+2
- Alkanes have only single covalent bonds between carbons; methane, ethane and propane are the first three
- Crude oil is separated by fractional distillation: evaporation followed by condensation at different temperatures
- As chain length increases: boiling point increases, viscosity increases, flammability decreases
That's the notes covered.
Carry on to the next subtopic.