Crude Oil and HydrocarbonsAQA GCSE Chemistry: Revision notes
Section 1
What Is Crude Oil?
Crude oil is a finite resource found in rock, and is a mixture made up mainly of hydrocarbons — molecules containing only hydrogen and carbon atoms.
- Most of the hydrocarbons in crude oil are alkanes, with the general formula CₙH₂ₙ₊₂
- The first four alkanes are methane, ethane, propane and butane
Because crude oil is a mixture, its components can be separated by physical methods (like distillation) without any chemical reaction taking place.
Section 2
How Is Crude Oil Separated Into Useful Fractions?
Crude oil is separated into simpler, more useful mixtures called fractions by fractional distillation. This relies on the different hydrocarbons in crude oil having different boiling points:
- Crude oil is heated until it evaporates
- Vapours rise up a fractionating column, which is hotter at the bottom and cooler at the top
- Hydrocarbons condense back to liquid at different heights depending on their boiling point, and are collected as separate fractions
The fractions can be processed into fuels (petrol, diesel oil, kerosene, heavy fuel oil, liquefied petroleum gases) and feedstock for the petrochemical industry, used to make solvents, lubricants, polymers and detergents.
Section 3
How Do Properties Change Across the Fractions?
As molecular size (chain length) increases across the hydrocarbon fractions of crude oil:
| Property | As molecular size increases |
|---|---|
| Boiling point | Increases |
| Viscosity (thickness/how easily it flows) | Increases |
| Flammability (how easily it ignites) | Decreases |
These property changes influence how the different fractions are used as fuels — smaller, more flammable, less viscous hydrocarbons (like those in petrol) are used where an easily ignited, free-flowing fuel is needed, while larger, more viscous fractions (like heavy fuel oil) are used for less demanding, larger-scale burning.
Don't assume flammability increases with size along with boiling point and viscosity — it's the opposite: larger hydrocarbon molecules are actually harder to ignite.
Section 4
What Happens When Hydrocarbons Burn?
Hydrocarbons undergo complete combustion when there is a good supply of oxygen, producing carbon dioxide and water as the only products:
hydrocarbon + oxygen → carbon dioxide + water
For example, the complete combustion of methane:
CH₄ + 2O₂ → CO₂ + 2H₂O
Balanced equations for combustion must always be checked to ensure the numbers of each type of atom match on both sides.
Complete combustion of propane: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
Must Know
- Crude oil is a finite mixture of hydrocarbons, mostly alkanes (CₙH₂ₙ₊₂); the first four are methane, ethane, propane, butane
- Fractional distillation separates crude oil into fractions using differences in boiling point
- Fractions are processed into fuels (petrol, diesel, kerosene, heavy fuel oil, LPG) and feedstock for the petrochemical industry
- As molecular size increases: boiling point increases, viscosity increases, flammability decreases
- Complete combustion of a hydrocarbon (in plenty of oxygen) produces carbon dioxide and water only
- Always check combustion equations are correctly balanced
That's the notes covered.
Carry on to the next subtopic.