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Gaseous State: Behaviour Of Real Gases

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Gaseous State: Behaviour Of Real Gases - Lesson Summary

Gases that obey the ideal gas equation are called ideal gases.

The ideal gas equation is PV = nRT

In graph of pV Vs p at constant temperature, according to Boyle's law a straight line parallel to the x axis is expected but for a real gas the line obtained is not a straight line.

In the graph between pressure Vs volume at higher pressures, real gas deviates from ideal gas.

Dutch physicist Johannes van der Waals gave an explanation for the deviations and modified the ideal gas equation in order to make it applicable to real gases.

According to Kinetic molecular theory:

  1. There are no forces of attraction between gas molecules.
  2. The actual volumes of the gas molecules are negligible when compared to the total volume of the gas.

If there no forces of attraction, then it is difficult for liquefaction of the gases, but gases can be liquefied by decreasing the temperature and increasing the pressure.

At high pressure, molecules of gases are very close to each other and the intermolecular attractive forces are high. These molecules then do not exert full impact on the walls of the container as they are dragged back by the attractive force of other gas molecules. Hence, the molecules strike the walls of the container with reduced velocity. Thus, the pressure exerted by the real gas is lower than the pressure exerted by the ideal gas.

Hence the pressure in ideal gas equation is corrected as,

                           Pideal  = Preal + an2/v2
                Where  a = Proportionality Constant Characteristic of a Gas
                           n = Number of Moles of the Gas  
                          V = Volume Occupied by the Gas

The actual volume of the gas molecules is negligible when compared to the total volume of the gas, which is valid only at very low pressure.

At high pressure, the molecules are restricted to move, hence the volume occupied by the molecules become significant.

Hence the volume (V) in ideal gas equation is corrected as,

                       V - nb
                V = Ideal Volume
                b = Volume Occupied by Gas Molecules

By substituting these in the ideal gas equation,

               (p + an2/V2)(V - nb) = nRT

This equation is known as the Van der Waal's equation.

Where 'n' = number of moles

'a' and 'b' are called Van der Waal's constants.

The extent to which a real gas deviates from an ideal gas can be measured in terms of the compressibility factor.

At high pressure, all the gases have compressibility >1. So, these are very difficult to compress.

At intermediate pressure, most of the gases have compressibility <1. So these are easily compressible.

Thus it can be concluded that the behavior of a gas becomes more ideal when the pressure is very low. The temperature at which a real gas obeys ideal gas law over an appreciable range of pressure is known as Boyle temperature or Boyle point. Above Boyle temperature, real gases are difficult to compress.

Compressibility factor for a gas is calculated as,

                  Z = pV/nRT

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