Fugacity: It is derived from Latin, expressed as fleetness or escaping ...

Phase Equilibrium in single component system
Consider a thermodynamic equilibrium system consisting of two or more phases of a
single substance. Though the individual phases can exchange mass with each other.
Consider equilibrium between vapor and liquid phases for a single substance at constant
temperature and pressure. Appling the criteria of equilibrium
dG 0
a
dG
b
dG 0
a
dG and
b
dG are chane in free energies of the phases a and b respectively.
We know that
i i dn G SdT VdP dG
For phase a
a
dG
a a a a a a
V dP S dT G dn
For phase b
b
dG
b b b b b b
V dP S dT G dn
At constant temperature and pressure
a
dG
a a
G dn ,
b
dG
b b
G dn
For a whole system to be at equilibrium
a
dn
b
dn 0
or
dn
a
dn
a b a
G G dn 0
b
a b
G G
When two phases are in equilibrium at constant temperature and pressure, Gibbs free
energies must be same in each phase for equilibrium.
a
b
RT ln f C RT ln f C
a
f
f
b
Clapeyron Clausius Equation
Clapeyron Clausius Equation are developed two phases when they are in equilibrium
(a) Solid- liquid
(b) Liquid-Vapor
(c) Solid Vapor