Kinetic Molecular Theory
- All of the gas laws and the ideal gas equation describe how gases behave but not why they behave the way they do on the molecular level
- For example:
- Why does a gas expand when heated at constant pressure?
- Why does the pressure increase when a gas is compressed at a constant temperature?
- In the nineteenth century, several physicists, notably James Clerk Maxwell and Ludwig Boltzmann, found that the physical properties of gases can be explained in terms of the motion of individual molecules
- This observation resulted in a number of basic assumptions about gas behavior that have since been known as the kinetic molecular theory of gases
The Basic Assumptions of Kinetic Molecular Theory
- Kinetic molecular theory is based on five basic assumptions:
- Gases are composed of molecules whose size is negligible compared with the average distance between them
- These molecules are considered to be “points” which means they possess mass but have negligible volumes
- It also explains why gases are easily compressed
- Gas molecules move randomly in straight lines in all directions and at various speed
- Hence, the properties of a gas which depend on the motion of the molecule, such as pressure, will be the same in all directions
- The forces of attraction or repulsion between two molecules in a gas are very weak or negligible, except when they collide
- Basically, gas molecules will continue moving in a straight line with undiminished speed until they collide with another gas molecule or with the walls of the container
- When molecules collide with one another, the collisions are elastic
- Elastic collisions are collisions in which the total kinetic energy is conserved
- Hence, gas molecules will forever move with the same average kinetic energy per molecule unless the kinetic energy is removed from them, for example as heat
- The average kinetic energy of a molecule is proportional to the absolute temperature
- This means the higher the temperature, the greater the average kinetic energy of the molecules
For more information about the particulate model and its graphical representations, see Solids, Liquids and Gases and Graphical Representations of the Gas Laws