4.2 Crude OilEdexcel IGCSE Chemistry: Revision notes
Section 1
What is crude oil and how is it separated into fractions?
Crude oil is a mixture of many different hydrocarbons, mostly alkanes of varying chain length. Because it is a mixture, its components can be separated by fractional distillation, which relies on the different boiling points of the hydrocarbons:
- Crude oil is heated until it vaporises and enters a fractionating column, which is hotter at the bottom and cooler at the top
- Hydrocarbon vapours rise through the column and cool as they go up
- Each fraction condenses back to a liquid at the height where the column temperature matches its boiling point, and is drawn off
- Fractions with the longest chains (highest boiling points) condense near the bottom; fractions with the shortest chains condense near the top
Section 2
What are the main fractions, and how do their properties differ?
The main fractions, from shortest/lightest chains to longest/heaviest chains, are:
| Fraction | Approx. chain length | Main use |
|---|---|---|
| Refinery gases | shortest | bottled gas, fuel |
| Gasoline (petrol) | short | fuel for cars |
| Kerosene | medium | fuel for aircraft |
| Diesel | medium-long | fuel for lorries, trains |
| Fuel oil | long | fuel for ships, power stations |
| Bitumen | longest | surfacing roads, roofing |
As chain length increases down this list: colour darkens, boiling point increases, and viscosity (how thick/runny the liquid is) increases. Shorter-chain fractions are pale, runny liquids or gases with low boiling points; longer-chain fractions are dark, thick, syrupy liquids or solids with high boiling points.
Section 3
What happens when hydrocarbon fuels burn?
A fuel is a substance that releases heat energy when it is burned.
- Complete combustion (plenty of oxygen): hydrocarbon + oxygen → carbon dioxide + water
- Incomplete combustion (limited oxygen): produces carbon monoxide and/or carbon (soot) instead of, or as well as, carbon dioxide, plus water
Carbon monoxide is poisonous because it binds to haemoglobin in red blood cells more strongly than oxygen does, reducing the blood's capacity to carry oxygen around the body.
In a car engine, the high temperatures reached during combustion are enough to make unreactive nitrogen and oxygen from the air react together, forming oxides of nitrogen.
Section 4
How do impurities in fuels and combustion contribute to acid rain?
Some hydrocarbon fuels contain sulfur impurities. When burned, this sulfur reacts with oxygen to form sulfur dioxide. Both sulfur dioxide and the oxides of nitrogen formed in engines dissolve in atmospheric moisture to form acidic solutions, which fall as acid rain. Acid rain damages plants, aquatic life, and buildings made of limestone.
Section 5
What is cracking, and why is it needed?
Cracking is the process of breaking long-chain alkanes into shorter-chain alkanes and alkenes. It is carried out by passing long-chain hydrocarbon vapour over a hot catalyst (silica or alumina) at 600–700°C, which breaks C-C bonds.
long-chain alkane → shorter-chain alkane + alkene
Cracking is necessary because fractional distillation of crude oil produces far more of the long-chain fractions (like fuel oil and bitumen) than there is demand for, and not enough of the short-chain fractions (like gasoline) that are in high demand. Cracking converts the oversupplied long-chain fractions into more useful shorter-chain products, and also produces alkenes, which are needed as raw materials for making plastics.
If asked to explain why cracking is carried out, always mention both reasons: matching supply to demand for shorter fractions, and producing alkenes as feedstock for plastics.
Must Know
- Crude oil is a mixture of hydrocarbons, separated by fractional distillation based on boiling point
- Down the fraction list (gases to bitumen): colour darkens, boiling point rises, viscosity increases
- Complete combustion gives CO2 + water; incomplete combustion can give CO and/or soot
- Carbon monoxide is poisonous because it reduces the blood's oxygen-carrying capacity
- Sulfur impurities burn to form sulfur dioxide; high engine temperatures form oxides of nitrogen; both cause acid rain
- Cracking uses a hot catalyst (silica/alumina, 600-700°C) to turn long-chain alkanes into shorter alkanes and alkenes, balancing supply/demand and supplying alkenes for plastics
That's the notes covered.
Carry on to the next subtopic.