GCSE Chemistry · AQA 8462 · Topic 4.7.1
Carbon compounds as fuels and feedstock
Key points
- Crude oil is a finite resource formed over millions of years from the remains of plankton buried in mud. It is mostly hydrocarbons (compounds of hydrogen and carbon only).
- Most are alkanes: methane, ethane, propane and butane, general formula . They are saturated (only single C–C bonds).
- Fractional distillation separates crude oil into fractions of similar boiling point. Fractions are used as fuels (petrol, diesel, kerosene, LPG) and as feedstock for the petrochemical industry.
- As chain length increases: boiling point and viscosity increase, while flammability decreases.
- Complete combustion oxidises hydrocarbons to carbon dioxide and water, releasing energy.
- Cracking (catalytic, using hot catalyst, or steam cracking) breaks long-chain alkanes into shorter alkanes and alkenes, which are in higher demand and are used to make polymers.
- Alkenes are more reactive and turn bromine water from orange to colourless.
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Crude oil is the source of most of the fuels we burn and the raw materials for plastics. You need to know how it is separated, how the properties of alkanes change with chain length, and why long molecules are cracked into shorter ones.
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