Everything on the GCSE Chemistry Crude Oil, Fractions & Combustion poster is written out below, section by section. Use it to search the sheet, copy parts into your own notes, or check a fact quickly.
Hydrocarbons & crude oil
- A hydrocarbon contains only hydrogen and carbon.
- Most are alkanes (CₙH₂ₙ₊₂).
- Crude oil is a finite mixture formed over millions of years from ancient marine biomass.
- Also a feedstock for the petrochemical industry, producing plastics, solvents, and lubricants.
Fractional distillation
Crude oil is vaporised and fed into a column that is hot at the bottom (~350°C) and cool at the top (~40°C). Fractions condense at their boiling points and are tapped off at different heights.
| Fraction | Use |
|---|---|
| Refinery gases | Bottled fuel |
| Petrol | Cars |
| Kerosene | Jets |
| Diesel | Lorries |
| Fuel oil | Ships |
| Bitumen | Roads |
As chain length increases: boiling point increases, viscosity increases, flammability decreases. Short chains rise to the top; long chains remain at the bottom.
Cracking
Long-chain alkanes are broken into shorter alkanes + alkenes using high temperature and a catalyst (e.g. silica or alumina).
- Driven by supply vs demand: too much heavy fraction, not enough petrol.
- Alkenes are used as monomers for addition polymers.
Example: cracking turns heavy fuel oil into more petrol.
Combustion
Complete combustion (plenty of O₂)
Hydrocarbon + O₂ → CO₂ + H₂O (clean flame)
Incomplete combustion (not enough O₂)
Hydrocarbon + O₂ → CO + C + H₂O (smoky, yellow flame) - produces soot.
Pollutants
- CO is colourless, odourless, and toxic - binds haemoglobin and blocks oxygen transport.
- SO₂ (from sulfur impurities) dissolves in rain to make acid rain.
- NOₓ (from hot engines) dissolves in rain to make acid rain; also causes respiratory problems.
- Particulates (soot) cause respiratory problems and contribute to global dimming.