The fact that all gases behave similarly with change in temperature or pressure led to the kinetic molecular theory or model. It can be summarized as follows:

  • • All matter in the gas phase is composed of discrete, point particles called molecules.
  • • In the gaseous state, molecules are relatively far apart.
  • • The molecules of all substances in the gaseous state are in continuous, rapid, straight-line motion. This motion is in three dimensions and is often called translation.
  • • This continuous motion is a measure of the kinetic energy of the system.
  • • In addition to translation, molecules may also rotate and vibrate, depending on the type of external energy source they are subjected to. These are known as internal modes of energy, and are separate from its kinetic energy.
  • • Collisions between molecules are assumed to be perfectly elastic. This means that when molecules collide with one another or with the walls of a container, they rebound without any loss of kinetic energy.
  • • The average kinetic energy of the molecules is directly proportional to its temperature in Kelvins. All gases at the same temperature will have the same kinetic energy. Particles in a gas are assumed to exert no attractive or repulsive forces.

This leads to gases having the following general properties:

  • 1. Gases exert pressure, which can be measured in any number of units, for example, pounds per square inch, atmospheres, pascals, newtons per square meter, and Torrs.
  • 2 Gases are highly compressible (unlike liquids and solids).
  • 3. Gases diffuse easily.
  • 4. Gases expand upon heating, providing the pressure remains constant.
  • 5. The pressure exerted by a gas increases with temperature, providing the volume is held constant.
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