CHEM-GUIDE: Kinetic model of gas and its postulates

In order to explain the observed behaviour of gases, a model was proposed based on the molecular and kinetic concept of gas molecules. This model takes into account the particulate nature of matter and the constant movement of particles.

Postulates of Kinetic Theory

All gases are made up of large number of minute particles called molecules.

Large distances separate the molecules so that the actual volume of the molecules is negligible as compared to the total volume of the gas.

The molecules are in a state of constant rapid motion in all directions, colliding with one another and also with the walls of the container.

The molecular collisions are perfectly elastic with no loss of energy and only redistribution of energy during collision.

There are no attractive or repulsive forces between the molecules.

The pressure exerted by the gas is due to the bombardment of its molecules on the walls of the container per unit area.

The average kinetic energy of the gas molecules is directly proportional to the absolute temperature.

On the basis of these postulates an equation for the pressure of the gas is derived as

This is called the Kinetic gas equation where 'N' is the number of molecules in volume 'V', 'm' is the mass of the molecule and 'u' is the root mean square velocity of the molecules.

On plotting a fraction

of molecules having different speeds against the speeds of the molecules (along x-axis) a curve known as Maxwell's distribution curve is obtained. The important features of which are:

The fraction of molecules with very low or very high speeds is very small.

The fraction of molecules possessing higher and higher speeds goes on increasing till it reaches a peak and then starts decreasing.

The maximum fraction of molecules possess a speed, corresponding to the peak in the curve which is referred to as most probable speed.

Fig: 2.8 - Maxwell-Boltzmann's distribution of speeds

The increase in temperature of the gas results in increase in the molecular motion. Consequently, the value of the most probable speed increases with increase in temperature. It may be noted that as long as the temperature of a gas is constant, the fraction having the speed equal to most probable speed remains the same but the molecules having this speed may not be the same. In fact, the molecules keep on changing their speed as a result of collisions.

Molecular motions may be described in terms of different types of molecular speeds. These are defined and described below.

Most probable speed

The most probable speed of a gas is the speed possessed by the maximum fraction of gas molecules at a given temperature denoted by a.

where, R is the gas constant

T is the absolute temperature and

M is the molecular mass.

Average speed

This is the average of speeds possessed by the molecules in a sample of any gas. This is defined as,

It can be shown that

Root mean square speed

The root mean square speed is expressed by the relationship,

Root mean square speed in any gas is given by the relationship,

The root mean square speed is commonly used and can be calculated from the following relations:

Relation between most probable, average and root mean square speeds

The three molecular speeds are expressed as

Thus for gases, the root mean square speed is directly proportional to , and inversely proportional to . Therefore heavier molecules move slower.

Average kinetic energy of a gas

From the kinetic model of gases,

For 1 mole of the gas, this equation becomes,

For an ideal gas,

Since PV = RT for 1 mole of gas

Therefore

For 'n' moles of gas, the kinetic energy is

The units of E_k depend upon the units of R. Hence the following point holds true:

The assumption of kinetic theory that the average kinetic energy or molecular velocity of any gas is directly proportional to its absolute temperature.

K.E a u²