Organic Chemistry
Crude oil, hydrocarbons, polymers, and alcohols.
In this topic
Crude Oil and Hydrocarbons
Crude oil is a finite resource formed from ancient marine organisms over millions of years. It is a mixture of hydrocarbons — compounds of hydrogen and carbon only.
Fractional distillation separates crude oil into fractions based on boiling point. The oil is heated and enters a fractionating column. Fractions with lower boiling points rise higher up the column.
Alkanes are saturated hydrocarbons with the general formula CₙH₂ₙ₊₂. - Methane: CH₄ - Ethane: C₂H₆ - Propane: C₃H₈ - Butane: C₄H₁₀
As chain length increases: boiling point increases, viscosity increases, flammability decreases.
Key Points
- Crude oil is a non-renewable fossil fuel
- Short-chain hydrocarbons are more useful as fuels
- Cracking breaks long-chain hydrocarbons into shorter, more useful ones
- Cracking also produces alkenes (unsaturated — used to make polymers)
Example Questions
4Explain the process of fractional distillation of crude oil.
Crude oil is heated until most of it vaporises. The vapour enters the fractionating column, which is hot at the bottom and cool at the top. Different fractions condense at different heights depending on their boiling points. Fractions with high boiling points (long chains) condense near the bottom. Fractions with low boiling points (short chains) rise higher before condensing.
[4 marks]
3What is cracking and why is it carried out?
Cracking is the process of breaking down long-chain hydrocarbons into shorter, more useful molecules. It is carried out because there is more demand for short-chain hydrocarbons (like petrol) than the amount produced by fractional distillation. It also produces alkenes, which are used to make polymers.
[3 marks]