Physical Chemistry

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Phase rule
Two Component System

I] Silver Lead System
●In this system there are two components.
If we consider single phase then
F=C-P+2
F=2-1+2
F=3
●Therefore three variables are possible ie Temperature pressure and concentration.
●In two component system solid and liquid equilibrium are important and vapour phase is almost negligible.
●Therefore this system studied by taking pressure constant.
●Only Temperature and concentration are considered as variables.
●Such kind of system known as condensed system.
●Generally carried out in absence of gas phase and at atmospheric pressure
Therefore according to phase rule for condensed system Pressure=constant and C=2
F=C-P+1
F=2-P+1
F=3-P
●Where Temperature and concentration are variables.
●Therefore this phase diagram also known as temperature composition diagram.

I) Point A and B is equal to melting point or freezing point of pure Ag and pure Pb respectively.
II) Point C is known as eutectic point
Temperature correspond to point C known as eutectic Temperature  (Te=303°C)
Composition correspond to point C known as eutectic composition (Ce=2.6%Pb)
At point C two solid phases and one liquid phase exist in equilibrium.
According to phase rule
F=C-P+1          (C=2 and P=3)
F=2-3+1
F=0
Degree of freedom is zero hense the system is nonvariant, zerovariant or invariant at eutectic point C.
Curve AC and BC
AC represent freezing curve of Ag and BC represent freezing curve of Pb
On curve AC and BC
Solid ⇌ Liquid
According to phase rule
F=C-P+1        (C=2 and P=2)
F=2-2+1
F=1
The curve AC and BC represent Monovariant or univariant.
Area (ACB) :- only liquid phase present hense C=2 and P=1 then degree of freedom is
F=2-1+1
F=2
The system is Bivariant.
Below curve AC
Represent solid Ag and Liquid phase hense C=2 and P=2 then
F=C-P+1
F=2-2+1
F=1
Hense below curve AC system is univariant.
Below curve BC
Represent Solid Pb and Liquid phase
C=2 and P=2 then
F=C-P+1
F=2-2+1
F=1
Hense below curve BC represent univariant system.
Below point C
Solid Ag ⇌ Solid Pb
C=2 and P=2
By applying phase rule
F=C-P+1
F=2-2+1
F=1
Hense below point C the system is univariant or Monovariant.





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