Real vs Ideal gases 3

 

Significance of compressibility factor

The significance of compressibility factor can be further understood from the following derivation:

If the gas shows ideal behaviour,

Substituting this value of nRT/P in eqn. (1), we get

Thus, compressibility factor is defined as the ratio of the actual molar volume of the gas (For Example: experimentally observed value) to the calculated molar volume (considering it as an ideal gas) at the same temperature and pressure.

Causes of Deviation from Ideal Behaviour

As stated above, the real gases obey ideal gas equation (PV = nRT) only if the pressure is low the temperature is high. However, if the pressure is high or the temperature is low, the real gases show marked deviations from ideal behaviour. The reasons for such a behaviour shown by the real gases have been found to be as follows:

The derivation of the gas laws (and hence of the ideal gas equation) is based upon the Kinetic Theory of Gases which in turn is based upon certain assumptions. Thus, there must be something wrong with certain assumptions. A careful study shows that at high pressure or low temperature, two assumptions of Kinetic Theory of Gases are fails:

• When compared to the total volume of the gas, the volume occupied by the gas molecules is negligible.
• The forces of attraction or repulsion between the gas molecules are negligible.

The above two assumptions are true only if the pressure is low or the temperature is high so that the distance between the molecules is large. However, if the pressure is high or the temperature is low, the gas molecules come close together. Hence, under these conditions:

• The forces of attraction or repulsion between the molecules.
• The volume occupied by the gas may be so small that the volume occupied by the molecules may not be negligible.

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