Trouton's Rule and Free Volume Theories

DHv = heat energy required to free the molecules of a liquid from the intermolecular forces without change of temperature. his can be measured by calorimetry, vapour pressure or by electrical energy required to vaporize.
Calculated values for molecular substances show DHv decreases with temperature - e.g.
for H2O @ 0 C = 44.8( kJ/mol) and at 200 C = 34.8 : for diethyl ether its 28.8 @ 0 C and 13.6 @ 160 C. From this arises DHv = f(T) and DHv -> 0 at the critical point ( liquid/equilibrium vapour merge)
dH/dT = DCp
In words- the rate of change of enthalpy of vaporization with respect to temperature at constant pressure equals the rate of change of heat capacity (DC) of the liquid at constant pressure.The pattern of energy level occupation may change, but essentially

DHv = f( Ep free moles- Ep bound moles) assuming ideality

Of course nett attraction in a liquid is multi-molecular but easier mathematically to consider in terms of just two molecules
Consider two polar molecules with dipole = u and distance r (r>>> +/-istance)

u(d-d) = -2/3{( u*4/4pie0*2)kT*1/2

Induction must also play a part ( a = dipole moment induced by unit electric field)

u ind. = -2au*2/4pie0r*6


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Kiwiman

Author:Kiwiman
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