7.1.3Organic Chemistry
Properties of Hydrocarbons — AQA GCSE Combined Science Trilogy 8464 - Chemistry Revision Notes & Topical Questions
5 min read 8 parts Lesson 4 of 5
Part 1 of 6Chain length and physical properties
1Longer molecule
- Longer molecule More carbon atoms in the chain means a larger molecule with a greater surface for contact with its neighbours.
2Stronger intermolecular forces
- Stronger intermolecular forces The attractions between neighbouring molecules increase with size.
3More energy to separate
- More energy to separate More heat is needed to overcome these forces, so the boiling point is higher.
4Other effects
- Other effects Viscosity rises and flammability falls as the chain gets longer.
Part 2 of 6Short versus long chains
| Property | Short-chain hydrocarbon | Long-chain hydrocarbon |
|---|---|---|
| Boiling point | Low | High |
| Viscosity | Runny | Thick and sticky |
| Flammability | Ignites very easily | Difficult to ignite |
| State at room temperature | Gas or liquid | Viscous liquid or solid |
| Flame when burning | Clean and blue or pale yellow | Smoky and yellow |
Part 3 of 6Complete combustion
Plenty of oxygen
- Plenty of oxygen Hydrocarbons burn completely when oxygen is in good supply, releasing a great deal of energy.
- The products are carbon dioxide and water.
Example: propane
- Example: propane C3H8 + 5O2 → 3CO2 + 4H2O
- Both carbon and hydrogen are oxidised, which means the fuel gains oxygen.
Energy transfer
- Energy transfer The reaction is exothermic,
- because more energy is released making the new bonds in the products than is taken in breaking the bonds in the fuel and oxygen.
Part 4 of 6Incomplete combustion
Limited oxygen supply
Products
- Products Water is still formed, but carbon monoxide and/or carbon (soot) form instead of all the carbon becoming carbon dioxide.
Example
- Example 2CH4 + 3O2 → 2CO + 4H2O
- With even less oxygen, carbon itself forms: CH4 + O2 → C + 2H2O.
Carbon monoxide
- Carbon monoxide A colourless, odourless toxic gas that stops red blood cells carrying oxygen.
Soot
- Soot Carbon particles blacken surfaces and can cause breathing problems.
Part 5 of 6Spotting the flame
1
Burn the fuel in a plentiful air supply: a clean, blue flame shows complete combustion.
2
Restrict the air with the collar closed on a Bunsen burner: the flame turns yellow, luminous and sooty.
3
Hold a cold surface in the flame; a black deposit of carbon confirms incomplete combustion.
Part 6 of 6Balancing a combustion equation
1
Write the unbalanced equation: C4H10 + O2 → CO2 + H2O.
2
Balance the carbon first: four carbon atoms in butane give 4CO2.
3
Balance the hydrogen next: ten hydrogen atoms give 5H2O.
4
Count the oxygen on the right: 8 + 5 = 13 atoms, so 6.5O2 are needed.
5
Multiply every term by two to remove the fraction: 2C4H10 + 13O2 → 8CO2 + 10H2O.
Key terms
| Term | Meaning |
|---|---|
| Intermolecular forces | Attractions between separate molecules. |
| Viscosity | The resistance of a liquid to flowing. |
| Flammability | How easily a substance catches fire. |
| Complete combustion | Burning with enough oxygen to form only carbon dioxide and water. |
| Incomplete combustion | Burning with too little oxygen, forming carbon monoxide or soot. |
| Oxidation | A reaction in which a substance gains oxygen. |
Avoid these mistakes
1
Saying that covalent bonds within molecules break when a hydrocarbon boils; only intermolecular forces are overcome.
2
Forgetting to balance combustion equations for hydrogen and oxygen atoms.
3
Stating that carbon monoxide is produced in complete combustion.
4
Saying that long chains are more flammable because they contain more carbon.
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