Trends in physical properties
- Some properties of hydrocarbons depend on the size of their molecules
- These properties include:
- Viscosity (the ability of the substance to flow)
- Flammability (how easily the substance burns)
- Colour
- Cleanliness of burn
- These properties influence how hydrocarbons are used as fuels
Boiling Point
- The bigger the hydrocarbon, the higher it's boiling point
- This is because the size of the intermolecular forces increases requiring more energy to overcome them
Graph to show the trend in boiling points of hydrocarbons
A graph of the boiling points of the first eight alkanes showing a gradually increasing trend
Examiner Tip
The boiling points of some of the alkanes are below zero meaning they are gases at room temperature.
Viscosity
- Viscosity refers to the ease of flow of a liquid
- The opposite of high viscosity is to say a liquid is runny
- High viscosity liquids are thick and flow less easily
- Viscosity also increases with increasing chain length
- This is also due to the increased intermolecular forces of attraction as molecular size increases
- Longer chain hydrocarbons would not be as useful as fuels for cars as they would be too thick and clog the engine
- Increased viscosity means that longer chained alkanes are useful as lubricants in machinery as they are less likely to burn and function to reduce friction between moving parts
Flammability
- Molecular size again influences the ease of ignition or flammability of hydrocarbons
- Smaller hydrocarbon molecules are more flammable and are easier to ignite than larger molecules
- This makes them very useful as fuels, releasing large amounts of energy when they burn