5.7: Kinetic Molecular Theory and the Gas Laws
The test of the kinetic molecular theory (KMT) and its postulates is its ability to explain and describe the behavior of a gas. The various gas laws (Boyle’s, Charles’s, Gay-Lussac’s, Avogadro’s, and Dalton’s laws) can be derived from the assumptions of the KMT, which have led chemists to believe that the assumptions of the theory accurately represent the properties of gas molecules.
The Kinetic Molecular Theory Explains the Behavior of Gases
Recalling that gas pressure is exerted by rapidly moving gas molecules and depends directly on the number of molecules hitting a unit area of the wall per unit of time, the KMT conceptually explains the behavior of a gas as follows:
- Gay-Lussac’s law: If the temperature is increased, the average speed and kinetic energy of the gas molecules increase. If the volume is held constant, the increased speed of the gas molecules results in more frequent and more forceful collisions with the walls of the container, therefore increasing the pressure. This is also termed as Amontons’s law.
- Charles’s law: If the temperature of a gas is increased, constant pressure may be maintained only if the volume occupied by the gas increases. This will result in greater average distances traveled by the molecules to reach the container walls, as well as increased wall surface area. These conditions will decrease both the frequency of molecule-wall collisions and the number of collisions per unit area, the combined effects of which balance the effect of increased collision forces due to the greater kinetic energy at the higher temperature.
- Boyle’s law: If the gas volume is decreased, the container wall area decreases, and the molecule-wall collision frequency increases, both of which increase the pressure exerted by the gas.
- Avogadro’s law: At constant pressure and temperature, the frequency and force of molecule-wall collisions are constant. Under such conditions, increasing the number of gaseous molecules will require a proportional increase in the container volume in order to yield a decrease in the number of collisions per unit area to compensate for the increased frequency of collisions.
- Dalton’s law: Because of the large distances between them, the molecules of one gas in a mixture bombard the container walls with the same frequency, whether other gases are present or not, and the total pressure of a gas mixture equals the sum of the (partial) pressures of the individual gases.
This text is adapted from Openstax, Chemistry 2e, Section 9.5: The Kinetic-Molecular Theory.