Fractional distillation and combustion
Fractional distillation
separates crude oil into . Each fraction contains hydrocarbons with a similar number of carbon atoms, so they have similar .
- Crude oil is heated until most of it turns into vapour.
- The vapour enters a , which is hot at the bottom and cooler at the top.
- The vapours rise. Each hydrocarbon condenses where the temperature drops below its boiling point, and is collected there.
Short-chain hydrocarbons have low boiling points and leave at the top as gases. Long-chain hydrocarbons condense near the bottom.
Fractions from top to bottom: refinery gases, petrol, kerosene, diesel oil, heavy fuel oil, and bitumen.
Key terms
Properties and chain length
| Property | Short chains | Long chains |
|---|---|---|
| Low | High | |
| (how thick) | Low — runny | High — thick |
| (how easily it burns) | High | Low |
The longer the chain, the stronger the intermolecular forces between molecules, so boiling point and viscosity go up.
Short-chain hydrocarbons make better fuels: they ignite easily and burn with a clean flame. The fractions are used as fuels (petrol, diesel, kerosene for jets) and as for making new chemicals, such as solvents, lubricants and polymers.
Key terms
Combustion
Burning hydrocarbon fuels releases energy — combustion is . The carbon and hydrogen are .
happens with plenty of oxygen. The products are :
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
happens when there isn't enough oxygen. The products include (a toxic gas) and/or carbon (soot), as well as water:
2CH₄ + 3O₂ → 2CO + 4H₂O
CH₄ + O₂ → C + 2H₂O
Incomplete combustion releases less energy and gives a smoky yellow flame.