What are the 5 Assumptions of Kinetic Theory?

Matter consists of molecules which are groups of atoms. In solids, the molecules are packed tightly i.e. the interatomic spacing is least in solid. In liquids, the molecules are more free to move about and the interatomic spacing higher. Hence, liquids tend to flow. In gases, the interatomic distance is highest. In gases molecules have maximum freedom to move around. Hence, gas molecules are in constant random motion.

The Kinetic Theory of gas is based on the assumption that the molecules in gas move freely and are in random motion with the molecular force of interaction being negligible and hence, it can be neglected. Kinetic Theory of gas explains properties of gases at a molecular level. In this theory, physical properties of the gas are explained by considering the average over a large number of particles (molecules), and the averages are related to the macroscopic properties of the gas.

Maxwell Boltzmann developed the Kinetic Theory of gases.

5 Assumptions in kinetic theory

• Matter consists of a large number of identical molecules. Hence in Kinetic Theory study of behaviour of a single molecule can be generalised to the system consisting of a large number of such molecules.

Gas does not have definite volume. It occupies the shape of a container. Thus in a given volume gas consists of a large number of identical molecules, to the order of Avogadro number. Hence the assumption in kinetic theory that matter consists of identical molecules applies to the system of gas.

• Molecules in the matter are in constant random motion. They travel in a straight path over a length without losing its energy.

The intermolecular distance between the molecules of gas is more. Thus, gas molecules are in continuous random motion. And they move in a straight path over a greater distance without colliding. As a result Kinetic theory of gas assumes the gas molecule to be moving at constant speed before it undergoes collision. 

• The force of interaction between the molecules is negligible over a large separation between the molecules.

In gas the intermolecular distance is more. Hence for the gas the intermolecular force of attraction or repulsion for a molecule is negligible. Kinetic theory of gas assumes that in gas the interaction force between the molecules can be neglected except when the molecule collides.

• The molecules interact only during collision. The collision is considered to be an elastic collision.

As the gas molecule approaches another molecule it can collide with the other molecule. Also molecules in gas may collide with the walls of the container. Kinetic theory of gas assumes that molecules in gas experience interaction force only during collision. And the collision is assumed to be elastic.

•  The molecules are regarded as particles. And the energy of the molecule is related to temperature.

In Kinetic Theory of gas the collision of molecules is considered to be elastic. The molecule is assumed to be a hard spherical particle which undergoes an elastic collision.

Kinetic Theory of gas relates the kinetic energy of a molecule of gas to the temperature of the gas.

Conclusion  

Kinetic Theory is a molecular theory of matter. Kinetic theory is based on certain assumptions. Based on the assumptions, Kinetic Molecular theory can very well explain macroscopic properties of matter.

Gas does not have definite volume. In gases, the intermolecular distance is highest. In gases molecules have maximum freedom to move around. Hence, gas molecules are in constant random motion. Kinetic Theory when applied to a system of ideal gas it is called Kinetic Theory of gas. Based on the 5 assumptions in Kinetic theory, Kinetic Theory of gas explains the behaviour and physical properties of gas at molecular level.

Kinetic theory of gas assumes that gas consists of a large number of identical molecules. These molecules are assumed to be in continuous random motion. Also Kinetic theory of gas assumes that in gas the interaction force between the molecules can be neglected except when the molecule collides. Kinetic theory of gas assumes the collision of the molecule to be elastic.

Hence Kinetic theory of gas based on the assumptions can very well explain the behaviour of gas at molecular level.