MODULE 2 : PHASE RULE
TOPICS:
PHASE RULE,
EXPLANANATION OF THE TERMS,
DERIVATION OF PHASE RULE ON THERMODYNAMIC
CONSIDERATION, PHASE DIAGRAM, REGIONS, LINES
AND TRIPLE POINT, METASTABLE EQUILIBRIUM,
POLYMORPHISM: ENANTIOTROPY, MONOTROPY,
DYNAMIC ALLOTROPY, PHASE DIAGRAMS OF ONE
AND TWO COMPONENT SYSTEMS,
Dr. J. K. Sarmah
WHAT IS A PHASE?
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ICE
LIQUID WATER
WATER VAPOUR
ICE AND LIQUID WATER
LIQUID WATER AND WATER VAPOUR
ICE AND WATER VAPOUR
– HOW MANY PHASES IN EACH?
CHANGE OF PHASE
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SOLID TO LIQUID
LIQUID TO SOLID
LIQUID TO VAPOUR
VAPOUR TO LIQUID
VAPOUR TO SOLID
SOLID TO VAPOUR
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CHANGE OF PHASE
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SOLID TO LIQUID
LIQUID TO SOLID
LIQUID TO VAPOUR
VAPOUR TO LIQUID
VAPOUR TO SOLID
SOLID TO VAPOUR
= MELTING/FUSION
= FREEZING
= VAPORISATION
= CONDENSATION
= DEPOSITION
= SUBLIMATION
CHANGE OF PHASE
DEFINITIONS
• Sublimation is the process of transformation
directly from the solid phase to
the gaseous phase without passing through an
intermediate liquid phase.
• Sublimation is an endothermic (REQUIRES
ADDITIONAL ENERGY) phase transition that
occurs at temperatures and pressures below a
substance's triple point in its phase diagram.
SELF STUDY: DEFINITIONS WITH EXAMPLES
PHASE EQUILLIBRIA
• DEALS WITH THE STUDY OF EQUILLIBRIUM
CONDITIONS OF HETEROGENEOUS SYSTEMS.
• WILLIARD GIBBS, AN AMERICAN PHYSICIST IN
1874, FIRST DISCOVERED A RELATIONSHIP
GOVERNING ALL HETEROGENEOUS EQUILLIBRIA,
THE GIBB’S PHASE RULE,
• WITH THE APPLICATION OF PHASE RULE IT IS
POSSIBLE TO PREDICT QUALITATIVELY BY MEANS
OF A DIAGRAM THE EFFECT OF CHANGING
PRESSURE, TEMPERATURE AND CONCENTRATION
ON
A
HETEROGENEOUS
SYSTEM
IN
EQUILLIBRIUM.
PHASE RULE
• IN EVERY HETERGENEOUS SYSTEM AT EQBM THE SUM OF NO. OF
PHASES AND DEGREES OF FREEDOM IS GREATER THAN THE NO.
OF COMPONENTS BY TWO:
F=C-P+2
• F = Degrees of freedom = variance
Number of intensive variables that can be changed
independently without disturbing the number of phases in
equilibrium
• P = Number of Phases
gas, homogeneous liquid phases, homogeneous solid phases
• C = Components
Minimum number of independent constituents
Example 1: methanol and water
---- 2 components
EXPLANATION:
• Phase:
• A phase is defined as any homogeneous and physically
distinct part of a system having all physical and chemical
properties the same throughout the system. A system
may consist of one phase or more than one phase.
• E.g. A system containing only liquid water is one-phase
system
• A system containing liquid water and water vapour (gas)
is a two phase system
• A system containing liquid water, water vapour and solid
ice is a three phase system.
• Pure substances (solid, liquid, or gas) made of one
chemical species only, is considered as one phase, thus,
oxygen, benzene, and ice are all one phase.
EXPLANATION
• Mixture of gases of oxygen and nitrogen which
mix freely is …….. phase system, two completely
miscible liquids, water and ethanol is ……. phase
system, An aqueous solution of solid NaCl or a
sugar solution is …….. phase system.
• A mixture of two immiscible liquids forms two
separate layers on standing e.g. CHCl3 and water
is a …… phase system.
• Calcium carbonate decomposes on heating in a
closed chamber to form solid calcium oxide and
carbon di oxide(gas). It is ……. phase system.
EXPLANATION
• Component:
• The term component is defined as the least number of
independent chemical constituents in terms of which
the composition of every phase can be expressed by
means of a chemical equation. E.g.
• Water system has three phases, ice, water and water
vapour and the composition of all these phases is
expressed in terms of one chemical individual water.
Thus water system has one component only.
• Similarly Sulphur system has four phases: rhombic
sulphur, monoclinic sulphur liquid sulphur and sulphur
vapour and the composition of all these phases is
expressed by one chemical individual sulphur.
Therefore Sulphur system is one component system.
• Thus, all the phases in one component system is
expressed by only one chemical individual.
EXPLANATION
• Similarly, a mixture of gases viz. oxygen and nitrogen
are miscible freely and form one phase only but its
composition can be expressed by two chemical
substances O2 and N2. Hence a mixture of O2 and N2 is
……… component system.
• A solution of NaCl in water is a one phase system and
its composition is expressed by both NaCl and water,
hence it is a ……… component system.
• A saturated solution of NaCl in contact with excess
solid NaCl has two phases. The composition of both
the phases can be expressed in terms of two chemical
individual NaCl and water. Hence a saturated solution
of NaCl in water in contact with excess solid NaCl is a
two component system.
DECOMPOSITION OF CALCIUM CARBONATE
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CaCO3(s)= CaO(s) + CO2(g)
• It has three phases but the composition of the system
can be expressed in terms of two of the three
chemical substances in equilibrium. Hence it is a two
component system
• Dissociation of NH4Cl: NH4Cl(s) = NH3 (g) + HCl (g)
• Ammonium chloride when heated in a closed vessel
dissociates into ammonia and HCl gas. The system
consists of two phases solid NH4Cl and gaseous mixture
containing NH3 and HCl However the constituents of
the mixture are in the same proportion in which they
are combined in solid NH4Cl. The composition of the
both the phases therefore be expressed in terms of the
same chemical individual NH4Cl. Thus the dissociation
of NH4Cl is one component system.
DEGREES OF FREEDOM(F)
• It is defined as the least number of variable factors of a
system which must be specified so that the remaining
variables are fixed automatically and the system is
completely defined. E.g.
• For a pure gas, PV=RT, if P and T values are specified there
can have be only one definite value of V or that the volume
is fixed automatically. Thus it has two degrees of freedom.
THE SYSTEM IS BIVARIANT
• A mixture of two or more gases is completely defined only
when P, T and Composition are specified. If P and T be
specified the third variable i.e. composition may be varied.
Since it is necessary to specify three variables to define the
system completely, it has three degrees of freedom.
THE SYSTEM IS TRIVARIANT
DISCUSSION
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FOR A PURE GAS DEGREES OF FREEDOM, F = 2 ?
FOR A MIXTURE OF GASES, F = 3?
FOR WATER = WATER VAPOUR, F = 1?
FOR ICE = WATER = VAPOUR SYSTEM, F = 0?
DISCUSSION
• For Water = Water Vapour system, F=1, The
system has two variables, P and T. At definite T,
the vapour pressure of water can have only one
fixed value. Thus if one variable is specified , the
other is fixed automatically. Hence this system
has one degree of freedom.
• For ice, water, water vapour system, F=0, In this
system, the three phases coexist at the freezing
point of water. Since the freezing temperature of
water has a definite value, the vapour pressure of
water has also a fixed value. Since both the
variables are already fixed, the system is defined
automatically and there being no need to specify
any variable. Hence this system has no degree of
freedom.
NONVARIANT SYSTEM.
DISCUSSION
• HOW MANY PHASES ARE PRESENT IN EACH
OF THE FOLLOWING SYSTEMS?
• A PIECE OF MOLTEN ICE PLACED IN A BEAKER
COVERED WITH A WATCH GLASS
• MIXTURE OF N2, H2 AND O2.
• WATER = WATER VAPOUR,
• KCl + WATER = KCl HYDRATE
DISCUSSION
• HOW MANY COMPONENTS ARE PRESENT IN
THE FOLLOWING SYSTEM?
• WATER = WATER VAPOUR,
• KCl + WATER = KCl HYDRATE
• FOR ICE = WATER = VAPOUR SYSTEM
• A SOLUTION OF COMMON SALT
• DISSOCIATION OF NH4Cl
• DECOMPOSITION OF CALCIUM CARBONATE
END OF LECTURE#1
• LECTURE #2
• PHASE DIAGRAM
• APPLICATION OF PHASE RULE TO
ONE COMPONENT SYSTEM