Gases X, Y, Z, P and Q have their van der Waals constants ′a′ and ′b′ (in CGS units) as shown below:
XYZPQa66200.0530b0.0250.150.10.020.2
The gas with the highest critical temperature is:
Critical Point (Point of inflection):
At this point on critical isotherm, gas and liquid are at equilibrium.
At point J (the tip of the dome) :
(dPdV)=(d2PdV2)=0
At Higher Temperature
T>Tc
Gas can not be liquefied for any T>Tc
Critical Temperature (Tc) : The maximum temperature at which a gas can be liquefied by the application of pressure alone.
Critical Pressure (Pc) :The minimum pressure required to liquefy a gas at critical temperature (Tc)
Critical Volume (Vc) : The volume occupied by one mole of the gas at critical temperature (Tc), and critical pressure (Pc)
Critical constants
Critical Temperature (Tc), Critical Pressure (Pc) and Critical Volume (Vc) are called critical constants.
Significance of Critical Temperature (Tc)
Tc∝Intermolecular attractions∝Ease of liquefaction
Critical Constants in Terms of van der Waals Constants
Vc=3b
Pc=a27×b2
Tc=8a27Rb
Compressibility Factor and Critical Constants for every gas
Zc=PcVcRTc=27ab(9b)(8a)=38=0.375