Transcript Lecture#1 - WordPress.com
TOPICS:
MODULE 2 : PHASE RULE
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?
• • • • • •
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
SOLID TO LIQUID =
LIQUID TO SOLID =
LIQUID TO VAPOUR =
VAPOUR TO LIQUID =
VAPOUR TO SOLID =
SOLID TO VAPOUR =
CHANGE OF PHASE
• • • • • • SOLID TO LIQUID = MELTING/FUSION
LIQUID TO SOLID = FREEZING
LIQUID TO VAPOUR = VAPORISATION
VAPOUR TO LIQUID = CONDENSATION
VAPOUR TO SOLID = DEPOSITION
SOLID TO VAPOUR = 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 THE GIBB’S PHASE RULE, A RELATIONSHIP GOVERNING ALL HETEROGENEOUS EQUILLIBRIA, WITH THE APPLICATION OF PHASE RULE IT IS POSSIBLE TO PREDICT QUALITATIVELY BY MEANS OF A DIAGRAM PRESSURE, TEMPERATURE AND CONCENTRATION ON A THE EFFECT OF CHANGING 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. CHCl is a …… phase system.
3 and water 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
independent chemical constituents least number of
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:
expressed by one chemical individual rhombic sulphur, monoclinic sulphur liquid sulphur and sulphur vapour and the composition of all these phases is 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 O 2 and N 2 .
Hence a mixture of O 2 and N 2 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 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.
. The composition of both
• • • DECOMPOSITION OF CALCIUM CARBONATE • CaCO 3 (s)= CaO(s) + CO 2 (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 NH 4 Cl: NH 4 Cl(s) = NH 3 (g) + HCl (g)
Ammonium chloride when heated in a closed vessel dissociates into ammonia and HCl gas. The system consists of two phases solid NH 4 Cl and gaseous mixture containing NH 3 and HCl
However the constituents of the mixture are in the same proportion in which they are combined in solid NH 4 Cl.
both the phases therefore be expressed in terms of the same chemical individual NH 4 The composition of the Cl.
Thus the dissociation of NH 4 Cl 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
• • • • 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 N 2 , H 2 AND O 2 .
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 NH 4 Cl DECOMPOSITION OF CALCIUM CARBONATE
END OF LECTURE#1
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