Physical Chemistry

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Phase rule

One component system

1] Water System
Water exists in three phases in water system ie solid water liquid water and water vapours.
In water system four equilibrium seen
1) Solid water ⇌ Liquid water
2) Solid water ⇌ water vapours
3) Liquid water ⇌ water vapours
4) Solid water ⇌ liquid water water vapours
         Phase diagram one component system

The phase diagram of water system consists of Three stable curves (BO,OA,OC)
Three areas (BOC,AOC,AOB)
Metastable curve (OA')
Triple point (O)

Sublimation curve (BO)
On this curve solid water is in equilibrium with water vapours.
Solid water ⇌ water vapour
By applying phase rule for curve BO
F=C-P+2             (C=1 and P=2)
F=1-2+2
F=1
As the value of degree of freedom is one the system on curve is univariant or Monovariant.

Freezing or melting curve (OC)
On this curve solid water is in equilibrium with liquid water.
solid water ⇌ liquid water
By applying phase rule we have
F=C-P+2            (C=1 and P=2)
F=1-2+2
F=1
As the value of degree of freedom is one the system on curve OC is Monovariant or univariant.

Vapourisation curve or vapour pressure curve (OA)
On this curve liquid water is in equilibrium with water vapours
Liquid water ⇌ Water vapours
By applying phase rule
F=C-P+2            (C=1 and P=2)
F=1-2+2
F=1
As the value of degree of freedom is one the system on curve OA is univariant or Monovariant.
On curve OA the pressure increases with increase in temperature
Curve OA have upper limit at point 'A' this point is known as critical point.
At critical point T=374°C and P=218atm.
Critical Temperature(374°C) :- the temperature on which and above which water exist in only vapour phase known as critical temperature.
Critical Pressure (218atm) :- the pressure on which and above which water exist only in vapour phase known as critical pressure.

Area BOC :- in this area water exist only in solid phase.
Then P=1 and C=1
By applying phase rule
∴ F=C-P+2=1-1+2=2
Hense in area BOC the system is Bivariant.

Area AOC :- in this area water exist only in liquid phase.
By applying phase rule
∴ F=C-P+2=1-1+2=2
Hense the system is bivariant

Area AOB :- in this area water exist only in liquid phase.
By applying phase rule
P=1 and C=1
By applying phase rule
∴ F=C-P+2=1-1+2=2
Hense the system is bivariant

Metastable curve (OA')
Liquid water may be cooled below it's freezing point without solidifying.
Curve OA' represent metastable equilibrium which is extension of curve AO.
The curve represent supercooled water.
Vapour pressure of supercooled water is greater than that of vapour pressure of solid water.

Triple point (O)
At triple point O three phases of water ie solid water, liquid water and water vapour coexist at equilibrium.
Solid water ⇌ liquid water ⇌ Water vapour
Temperature at triple point is 0.0075°C and pressure is 218 atm.
By applying phase rule we get
F=C-P+2              (C=1 and P=3)
F=1-3+2
F=0
Hense the system at triple point is nonvariant or invariant or zerovariant.



 

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