Physical Significance and Units of Vander Waal Constants a and b

Physical Significance of Vander Waal Constants a & b

We know that Vander Waal equation or Real Gas equation for n mole is

Where: P = Pressure, R = Universal gas constant, T = Absolute Temperature, a & b = Vander Waal Constant, V = Molar Volume and n = Number of moles

Physical Significance of Vander Waal Constants a

Vander Waal constant 'a' represents the magnitude of intermolecular forces of attraction which is independent of temperature and pressure. A zero value of 'a' indicates the least intermolecular force of attraction and no liquification.

Physical Significance of Vander Waal Constants b

Vander Waals constant 'b' represents the effective size of the molecules.( or average volume excluded from v by a particle).

Unit of Vander Waal Constants

Unit of 'a'

The constant 'a' is obtained from pressure correction term of Vander Waal equation for real gases. The factor a/V² has been added to 'P', hence, the dimension of a/V² will be the dimension of 'P'.

It is simply pressure and therefore, the unit of 'a'can be deduced as-
P = n²a/V²
or, a = PV²/n²
or, a = atm. x liter²/mol²
or, unit of 'a' = atm. liter² mol⁻²

Unit of 'b'

The constant 'b' comes from the volume correction term of Vander Waal equation for real gases. The factor 'b' has been subtracted from 'V', hence, the dimension of 'b' will be the dimension of 'V'.

It is simply volume and therefore, the unit of 'b' can be deduced as-
V = nb
or, b = V/n
or, b = liter/mol
or, unit of 'b' = liter mol⁻¹