Ideal Gas Law
Kinetic theory of gases
- The kinetic theory of gases states that molecules in gases are constantly moving
- The theory makes the following assumptions:
- The gas molecules are moving very fast and randomly
- The molecules hardly have any volume
- The gas molecules do not attract or repel each other (no intermolecular forces)
- No kinetic energy is lost when the gas molecules collide with each other (elastic collisions)
- The temperature of the gas is related to the average kinetic energy of the molecules
- Gases that follow the kinetic theory of gases are called ideal gases
- However, in reality, gases do not fit this description exactly but may come very close and are called real gases
Ideal gases
- The volume that an ideal gas occupies depends on:
- When a gas is heated (at constant pressure) the particles gain more kinetic energy and undergo more frequent collisions with the container wall
- To keep the pressure constant, the molecules must get further apart and therefore the volume increases
- The volume is therefore directly proportional to the temperature (at constant pressure)
How the volume of a gas increases upon heating
The volume of a gas increases upon heating to keep a constant pressure (a); volume is directly proportional to the temperature (b)
Limitations of the ideal gas law
- At very high pressures and low temperatures real gases do not obey the kinetic theory as under these conditions:
- Molecules are close to each other
- There are instantaneous dipole- induced dipole or permanent dipole- permanent dipole forces between the molecules
- These attractive forces pull the molecules away from the container wall
- The volume of the molecules is not negligible
- Real gases therefore do not obey the following kinetic theory assumptions at high temperatures and pressures:
- There is zero attraction between molecules (due to attractive forces, the pressure is lower than expected for an ideal gas)
- The volume of the gas molecules can be ignored (volume of the gas is smaller than expected for an ideal gas)