11.2.1Organic Chemistry
Fossil Fuels — Combined Science 0653 - Chemistry Revision Notes & Topical Questions
5 min read 7 parts Lesson 5 of 9 Unit quiz
Part 1 of 5The fossil fuels
What is a fuel?
- What is a fuel?
- A fuel is a substance that releases heat energy when it is burned.
- That energy can be used directly or transferred to generate electricity.
The three main fossil fuels
- The three main fossil fuels Coal, natural gas and petroleum (crude oil).
- Natural gas is mainly methane, CH4, which is a hydrocarbon.
Petroleum
- Petroleum A thick, black, sticky liquid found trapped under porous rock beneath land and sea.
- It is a mixture of many hydrocarbons, and as a mixture it is not very useful until it is separated into fractions.
Part 2 of 5Fractional distillation of petroleum
1
The crude oil is heated until most of it vaporises.
2
The hot vapours pass into the bottom of a fractionating column, which is very hot at the base and gets cooler towards the top.
3
As the vapours rise, each hydrocarbon condenses back to a liquid when it reaches the level where the temperature is just below its boiling point.
4
Hydrocarbons with high boiling points condense low down, while those with low boiling points rise much higher before condensing.
5
Each fraction, a group of hydrocarbons with similar chain lengths, is drawn off at its own level.
Part 3 of 5Uses of the fractions
| Fraction | Use |
|---|---|
| Refinery gas | Heating and cooking |
| Gasoline (petrol) | Fuel for cars |
| Naphtha | Raw material for making chemicals |
| Diesel (gas oil) | Fuel for diesel engines |
| Bitumen | Surfacing roads |
Part 4 of 5Trends in the fractions (extended tier)
| Fraction | Chain length | Boiling point |
|---|---|---|
| Refinery gas | Shortest | Lowest |
| Gasoline | Short | Low |
| Naphtha | Medium | Medium |
| Diesel | Long | High |
| Bitumen | Longest | Highest |
Part 5 of 5Why longer molecules boil at higher temperatures
1Longer chain
- Longer chain The molecule has more atoms and a larger surface.
2Stronger forces
- Stronger forces Attractive forces between the molecules increase with size.
3More energy needed
- More energy needed More heat is needed to pull the molecules apart, so the boiling point is higher.
Key terms
| Term | Meaning |
|---|---|
| Fossil fuel | A fuel formed from the remains of ancient living organisms, such as coal, natural gas and petroleum. |
| Hydrocarbon | A compound containing only hydrogen and carbon. |
| Fraction | A group of hydrocarbons with similar boiling points separated from crude oil. |
| Fractional distillation | Separating a mixture into fractions according to boiling point. |
| Fractionating column | A tall tower with a temperature gradient in which crude oil vapours condense at different heights. |
Avoid these mistakes
1
Saying crude oil is a single compound. It is a mixture of hydrocarbons.
2
Stating that the fractions are separated by chemical reactions. Fractional distillation is a physical process using different boiling points.
3
Placing the largest molecules at the top of the column. They condense at the bottom because they have the highest boiling points.
4
Saying that molecules break up when they boil. Only the forces between molecules are overcome, and no covalent bonds break.
5
Giving a vague use such as 'fuel' for every fraction. Learn a specific use for each one.
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