Limitations of Van der waal’s equation





Limitations of  Van der waal’s equation

(i) The value of ‘b’ is not constant but varies with pressure and temperature.

(ii) The value of V_c is not equal to 3b, but actually it is equal to, in some case; and in other cases 2b.

(iii)  The value of \dfrac{RT_c}{P_cV_c} is not equal to \dfrac{8}{3} but it is ususlly more than 3 for most of the gases.

 

Inversion Temperature:

It is the temperature above which a gas warms up in adiabatic expansion of a gas from high pressure to low pressure. It is denoted by T_i and is related with van der Waals constants as:

T_i + \dfrac{2a}{bR}

 

Boyle’s Temperature:

The temperature at which a real gas obeys Boyle’s law is known as Boyle’s temperature (T_B). It is related with van der Waals constants as:

T_B = \dfrac{a}{bR}

 

Units of van der Waals constants

 a = L^2 atm mol^{-2} \\[3mm] = Nm^4 mol^{-2} \\[3mm] dyne cm^{4} mol^{-2} \\[3mm] b = L mol^{-1} \\[3mm] = m^3 mol^{-1} \\[3mm] = cm^3 mol^{-1}

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