Fuels
- Coal: A solid fuel.
- Natural gas: A gaseous fuel.
- Petroleum (crude oil): A liquid fuel.
Methane (CH_4) is the main constituent of natural gas. It is a simple hydrocarbon consisting of one carbon atom and four hydrogen atoms.
A hydrocarbon is defined as a compound that contains only hydrogen and carbon atoms. This 'only' is critical; if any other element (like oxygen or nitrogen) is present, it is not a hydrocarbon.
Petroleum is not a single substance but a complex mixture of many different hydrocarbons. These hydrocarbons vary in the length of their carbon chains.
Note: The word 'only' is essential. Compounds like alcohols or carboxylic acids contain oxygen, so they are not hydrocarbons.
Because petroleum is a mixture, its components must be separated to be useful. This is done by fractional distillation.
How it works:
- Petroleum is heated in a furnace until it vaporises (turns into gas/vapour).
- The hot vapour enters a fractionating column. This column is tall and has a temperature gradient: it is hottest at the bottom and coolest at the top.
- As the vapour rises, it cools down.
- Hydrocarbons with higher boiling points condense (turn back into liquid) lower down in the column where it is still hot enough to keep smaller molecules gaseous but cool enough for larger ones to liquefy.
- Hydrocarbons with lower boiling points rise to the top of the column before they can condense, and are collected there as gases or very volatile liquids.
| Property | Trend (Bottom → Top) |
|---|---|
| Chain length (number of carbon atoms) | Decreases |
| Boiling point | Decreases |
| Volatility (ease of evaporation) | Increases (more volatile) |
| Viscosity (resistance to flow/thickness) | Decreases (less viscous/thinner) |
Each fraction has specific uses based on its physical properties. The table below lists the standard fractions and their primary uses as required by the syllabus.
| Fraction | Primary Uses |
|---|---|
| Refinery gas | Gas used in heating and cooking (e.g., bottled gas). |
| Gasoline / Petrol | Fuel used in cars. |
| Naphtha | Chemical feedstock (used to make other chemicals, e.g., plastics). |
| Kerosene / Paraffin | Jet fuel for aircraft. |
| Diesel oil / Gas oil | Fuel used in diesel engines (e.g., trucks, trains). |
| Fuel oil | Fuel used in ships and home heating systems. |
| Lubricating oil | Lubricants, waxes, and polishes. |
| Bitumen | Making roads (tarmac) and roofing felt. |
The Correct Understanding:
- Volatility refers to how easily a liquid evaporates. Short chains have weak intermolecular forces, so they evaporate easily. Therefore, as chain length decreases (moving up the column), volatility increases.
- Viscosity refers to thickness/resistance to flow. Long chains tangle together, making them thick and sticky. As chain length decreases, molecules slide past each other more easily. Therefore, as chain length decreases, viscosity decreases.
Mnemonic: Volatile liquids are thin and runny; Viscous liquids are thick and slow.
The Correct Understanding: You must specify that no other elements are present. If a compound contains oxygen (like ethanol), it is not a hydrocarbon, even though it has C and H.
Why this matters: Examiners look for the exclusion of other elements to test your precise understanding of organic classification.*
Why examiners accept this: The markscheme requires you to mention the temperature gradient and the condensation process. Simply saying 'it separates by boiling point' is often insufficient for full marks because it doesn't explain the mechanism.
Key phrases to include:
- 'Petroleum is heated/vaporised.'
- 'The column has a temperature gradient (hotter at the bottom, cooler at the top).' OR 'Vapours rise and cool down.'
- 'Fractions condense at different heights/temperatures according to their boiling points.'
Correct usage example:
Q: Describe how petroleum is separated.
A: Petroleum is heated to form vapour which rises up the fractionating column. The column is cooler at the top than at the bottom. Hydrocarbons with higher boiling points condense lower down, while those with lower boiling points rise further up before condensing.
Why examiners accept this: The syllabus explicitly lists three distinct applications for lubricating oil: lubricants, waxes, and polishes. For most other fractions (like bitumen or gasoline), one primary use is sufficient. However, because the syllabus groups these three together for lubricating oil, listing all three ensures you capture the full scope of the mark if the question implies plural uses or tests detailed recall.
Correct usage example:
Q: Name the uses of the lubricating oil fraction.
A: Lubricants, waxes, and polishes. (Listing all three is the safest approach to ensure full marks for this specific fraction).*
- Boiling point decreases. 2. Viscosity decreases. (Or Volatility increases / Chain length decreases).