Section 10.5 Kinetic Theory of Ideal Gas
Kinetic theory of matter is developed to explain the nature of heat and the motion of particles associated with heat energy. According to this theory, every substance is composed of large number of very tiny particles called molecules. These molecules are always in random motion with all possible velocities which increases with the increase of temperature. On this basis, in solids, molecules are very closely packed and attract each other with a large cohesive force. They cannot move freely but can vibrate about their mean positions. This explains the definite size and shape of solids. In liquids, the molecules are farther apart than in solids and attract each other with a comparatively smaller cohesive force. This force does not allow them to escape the liquids however, they can move freely anywhere within the liquids. This explains why a liquid has a definite size (volume) but no shape of its own. In gases, the molecules are very far away from each other than in solids and liquids and have negligibly small force of attraction between them. Hence they can move freely anywhere within the available space. This explains why a gas has no definite shape and size.
To understand the more general behavior of gases kinetic theory of ideal gases is postulated as
- All gases are made up of molecules. Molecules are identical, rigid, and perfectly elastic moving in random directions with all possible velocities.
- The molecules are very far away from each other but they collide with each other and with walls of the container.
- All the collision are elastic, but time spent during each collision is negligibly small compared to the time spent between the collisions.
- Molecules do not interact with each other (no intermolecular force of attraction) except during a collision.
- The average kinetic energy of the molecules of any gas depends only on the temperature.
The distance between the collision is called the free path.
Ideal gas is a hypothetical gas of negligible molecular size and have no interactions among their molecules. The above postulates are the correct description of ideal gas. In practice, a very dilute gas with extremely large volume of container space can be considered as an ideal gas. Remember the real gas molecules has definite size and may interact with each other. Hence this postulates are just a very simplified model to understand gas laws and need modification for real gas behaviors.
