Properties of hydrocarbons (AQA GCSE Chemistry Combined Science): Revision Notes
Properties of hydrocarbons
How molecular size affects properties
The molecular size of hydrocarbon molecules changes their properties. This is really important because it affects how we use different parts of crude oil.
Understanding how molecular size affects properties is fundamental to chemistry because it explains why different hydrocarbon fractions are useful for completely different purposes in industry and everyday life.
Smaller molecules (like methane and ethane) have different properties to larger molecules (like heavy fuel oil).
Key property trends
As hydrocarbon molecules get bigger, three main properties change:
Boiling point
- Small molecules = low boiling points (very cold temperatures)
- Large molecules = high boiling points (hot temperatures)
Temperature Comparison Example: Methane boils at -162°C, but butane boils at 0°C. This shows how even a small increase in molecular size can dramatically change the boiling point.
Viscosity (thickness)
- Small molecules = runny and thin (low viscosity)
- Large molecules = thick and sticky (high viscosity)
Flammability (how easily they catch fire)
- Small molecules = catch fire easily (highly flammable)
- Large molecules = harder to set on fire (less flammable)
These three properties - boiling point, viscosity, and flammability - are directly linked to molecular size and determine how useful each hydrocarbon fraction will be for different applications.
Why these properties matter
These different properties make hydrocarbon fractions useful for different jobs:
- Petrol (small molecules) is runny and flammable - perfect for car engines
- Heavy fuel oil (large molecules) is thick and less flammable - used in big industrial boilers and factories
The petroleum industry takes advantage of these different properties to create products perfectly suited for specific purposes. This is why we can get everything from lighter fuel to road tar from the same crude oil.
Complete combustion of hydrocarbons
When hydrocarbons burn with plenty of oxygen, they undergo complete combustion.
Complete combustion only happens when there is plenty of oxygen available. Without enough oxygen, incomplete combustion occurs, which produces different (and often harmful) products.
What happens during complete combustion:
- Carbon atoms join with oxygen to make carbon dioxide (CO₂)
- Hydrogen atoms join with oxygen to make water (H₂O)
- Lots of heat energy is released
Worked Example: Complete Combustion of Propane
Word equation: Propane + Oxygen → Carbon dioxide + Water
Chemical equation:
Notice how the carbon atoms (3) become 3 CO₂ molecules, and the hydrogen atoms (8) become 4 H₂O molecules.
Fractional distillation
Crude oil can be separated into different fractions because each fraction has different boiling points. The fractions with lower boiling points come off first at the top of the tower.
Fractional distillation works because of the boiling point differences we learned about earlier. The tower is hottest at the bottom and coolest at the top, so smaller molecules (with lower boiling points) rise higher before they condense back into liquid.
Remember!
Key Points to Remember:
- Small molecules: Low boiling point, runny (low viscosity), highly flammable
- Large molecules: High boiling point, thick (high viscosity), less flammable
- Complete combustion always produces carbon dioxide and water
- Different sized molecules are useful for different purposes because of their properties
- The size of the molecule determines its properties and uses