1. Basic Concepts on Phase Rule
Rayat Shikshan Sanstha's
S.M. JOSHI COLLEGE, HADAPSAR, PUNE-28
Basic Concepts on Phase Rule
Dr. Gajanan Wagh
(Associate and Head of Department)
DEPARTMENT OF CHEMISTRY

2. TERMS & Examples Solid Liquid (Melting)
Homogenous Chemical Equilibrium
N2(g) + 3H2(g) NH3(g)
Heterogonous Chemical Equilibrium
CaCO3(s) CaO(s) + CO2(g)
Heterogonous Physical Equilibrium
Solid Liquid (Melting)
Liquid Vapour (Vaporisation)
Solid Vapour (Sublimation)

3. TERMS & Examples (Contd….)
PHASE , P : Physically distinct & mechanically separable
COMPONENTS , C : Minimum number of chemical species by means of which the composition each phase can be described
C = Number of chemical compounds – No of chemical and or physical
( material or charge balance ) relations existing with them
(i) WATER SYTEM: ice water vapor
P = 1, 2 or 3 ; C = 1 ; F = ?
(ii) SYSTEM OF (PCl5 + PCl3 + Cl2 )
PCl PCl3 + Cl2 ; P= ; C = 3 – 1 = 2 ; F = ?

4. TERMS & Examples (Contd…..)
(iii) Pb & Ag System
C= 2 ; P = 1, 2 or 3 ; F = 2 , 1 , 0
NB : There are no (chem or phys) relation between
Pb & Ag
% Pb = % Ag is only a mathematical
relation
DEGREES OF FREEDOM , F (Variables) : Minimum
number of independent variables that must be fixed in order to understand the system fully.
The values of P & F depend on the condition

5. PHASE RULE - Derivation
F = C-P+2 (Gibbs Phase Rule)
Degrees of freedom (F)
= Total No of variables – Total No of
equations connecting them.
Total No of variables:
Conc.variables
C-1 for each phase
P(C-1) for all P phases + 2 ( For P & T)

6. PHASE RULE – Derivation (Contd…)
Total No of equations
P-1 (For each component) ;
C( P-1) For all the ‘ C ’ components
Hence , F = {P(C-1)+2} – C( P-1)
= C-P + 2

7. PHASE CHANGES Clapeyron equation dp/dT = ∆H/(T∆V)
dG=VdP-SdT = 0 (For equilibrium)
Clapeyron equation
dp/dT = ∆H/(T∆V)
Solid Liquid (Fusion or Melting)
Clausius-Clapeyron equations (Vapors)
d(lnp)/dT = ∆H/(RT 2)
Liquid Vapour (Vaporisation)
Solid Vapour (Sublimation)

8. SYSTEMS TO BE STUDIED PHASE DIAGRAM OF One Component Systems
Water system
Sulphur system
Two Component System
Simple eutectic system - Lead-Silver system
Compounds with congruent melting point.
Ferric chloride-water system
Compounds with incongruent melting point(SS).
Sodium sulphate-water system
Three component system.

9. Figure – Phase diagram for water at moderate pressures
One component systems
Figure – Phase diagram for water at moderate pressures

10. One component systems
The previous figure (phase diagram for water at moderate pressures), the curve OA is known as the vapor pressure curve; its upper limit is at the critical temperature, 374ºC for water, and its lower end terminates at ºC, called the triple point. Along the vapor pressure curve, vapor and liquid coexist in equilibrium.
Curve OC is the sublimation curve, and here vapor and solid exist together in equilibrium.
Curve OB is the melting point curve at which liquid and solid are in equilibrium. The negative slope of OB shows that the freezing point of water decreases with increasing external pressure.

11. If t1 is held constant, no matter how much the pressure is raised, the system remains a gas.
At t2 below the critical temperature and above the triple point, water vapor is converted into liquid water by an increase in pressure.
At temperature below the triple point e.g. t3, an increase in pressure on water in the vapor state converts the vapor first to ice and then at higher pressure into liquid water.
4/20/2019

12. One component systems
As mentioned in the table (phase rule with single component systems) and the figure (phase diagram for water at moderate pressures), in any one of the three regions in which pure solid, liquid, or vapor exists, F=1-1+2=2, so two variables (e.g. temperature and pressure| are needed to describe the system. Along any of the three lines, where two phases exist, F=1-2+2=1, hence, only one condition (either temperature or pressure) is needed to define the system. Finally, at the triple point where the three phases exist, F=0.
The triple point for air-free water is at ºC and 4.58mm Hg.
4/20/2019

13. One component systems
At the triple point, all three phases are in equilibrium, that is, the only equilibrium is at this pressure at this temperature of ºC. The triple point for air-free water is at ºC and 4.58mm Hg. Whereas the freezing point (the point at which liquid water is saturated with air is in equilibrium with ice at a total pressure of 1 atm) is 0 ºC.
In increasing the pressure from 4.58mm Hg to 1 atm, the freezing point is lowered by °C. The freezing point is then lowered an additional °C by the presence of dissolved air in water at 1 atm.
4/20/2019

14. Water system

15. Water System-Discussion
Curves-Sublimation, Vaporization, Fusion
Meta stable equilibrium curve
Ice Water curve has –ve slope
M.P of ice decreases with increase in p
Areas :F= 2 ; Lines :F= 1
Triple point:F = 0,0.0075ºC,4.58 mm Hg
Critical state 374ºC , 218 atm

16. SULPHUR SYSTEM

17. Simple eutectic system-General

18. Simple Eutectic(Lead-Silver System)

19. Compounds formation with congruent melting point.

20. Examples of Three component Systems
One pair of partially miscible liquid pair
CH3COOH,CHCl3 & H2O
Two pairs of partially miscible liquid pair
Three pairs of partially miscible liquid pair

21. Three component system (Acetic acid, Choloroform &Water)
CHCl3-H2O-partially miscible pair

22. THREE COMPONENT SYSTEM Two simple salts & water

23. THANK YOU