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Kinetic Molecular Threory of Gases

The laws what we have studied are (e.g., Boyle's law, Charles' law etc.) concise statements of experimental facts observed in the laboratory. An important aspect of scientific method is conducting careful experiments. It also tells us how the particular system is behaving under different conditions. However, once the experimental facts are established, a scientist is curious to know why the system is behaving in that way.

Postulates of the kinetic molecular theory of gases are given below. These postulates are related to atoms and molecules which cannot be seen, hence, it is said to provide a microscopic model of gases.
  • Gases consist of large number of identical particles (atoms or molecules).These particles are so small and so far apart on the average that the actual volume of the molecules is negligible in comparison to the empty space between them. They are considered as point masses. Great compressibility of gases is explained by this assumption.
  • At ordinary temperature and pressure, there is no force of attraction between the particles of a gas. The fact that gases expand and occupy all the space available to them is the support for this assumption.
  • Always particles of a gas are in constant and random motion. If the particles were at rest and occupied at fixed positions, then a gas would have had a fixed shape which is not observed.
  • Particles of a gas take all possible directions in straight lines. They collide with each other and with the walls of the container during the random motion. As a result of collision of the particles with the walls of the container, pressure is exerted by the gas.
  • Collisions of gas molecules are perfectly elastic. This means that total energy of molecules remains same before and after the collision. There may be exchange of energy between colliding molecules. Their individual energies may change. But the sum of their energies remains constant. The motion of molecules will stop and gases will settle down if there were loss of kinetic energy. This is contrary to actual observation.
  • Different particles in the gas have different speeds and hence different kinetic energies at any particular time. This assumption is reasonable because as the particles collide, we expect their speed to change. The molecular collisions will disrupt this uniformity even if initial speed of all the particles was same. Consequently, the particles must have different speeds, which go on changing constantly. It is possible to show that though the individual speeds are changing, the distribution of speeds remains constant at a particular temperature.
  • It must have a variable kinetic energy, if a molecule has variable speed. We can talk only about average kinetic energy under these circumstances. In kinetic theory, it is assumed that average kinetic energy of the gas molecules is directly proportional to the absolute temperature. It is seen that on heating a gas at constant volume, the pressure increases. On heating the gas, kinetic energy of the particles increases and they strike the walls of the container more frequently thus exerting more pressure.
Kinetic theory of gases allows us to derive theoretically all the gas laws studied in the previous sections. Calculations and predictions based on kinetic theory of gases agree very well with the experimental observations and thus establish the correctness of this model.

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