Component ' It is defined as minimum number of chemically independent species which are required to express the composition of all the phases present in the system. ' Water system has three phases and the composition of all these phases is expressed in one chemical formula. Hence one component only. ' 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. 'Therefore Sulphur system is one component system.
Degrees of freedom or Variance ' It is defined as smallest number of intensive variables (such as temperature, pressure and conc.) that can be changed independently without disturbing the number of phases in equilibrium 'State of a pure gas may be described by any two of the three variables P, T and density. If any two known third can be calculated. Hence it has two degrees of freedom called bivariant system. 'Univariant in which water and water vapor are in one system 'If all phases of water exist in equilibrium, no condition needs to be specified as the phases can coexist only at particular temp and pressure. 'This system has no degrees of freedom hence called invariant system.
Phase rule , It was first presented by Gibbs in 1875. ' It is very useful to understand the effect of intensive variables, such as temperature, pressure, or concentration, on the equilibrium between phases as well as between chemical constituents. ' It is used to deduce the number of degrees of freedom (F) for a system. Sometimes called: "the variance of the system".
1. 2. 3. 4. 5. 6. Advantages of Phase rule Provides convenient method of classification of equilibrium states of system. Predict the behavior of system with changes in the intensive variables. Indicate that different systems having the same number of degrees of freedom behave in the same manner. Applicable to macroscopic system It takes no account of nature of reactant and products in phase Applicable to physical and chemical equilibria.
1. 2. 3. 4. 5. Limitation of Phase rule Applicable only for the system in equilibrium Applicable to a single equilibrium state Considers only intensive variables Considers only number of phases not quantity of phases It requires that all the phases to be present under the same conditions of temperature and pressure.
1. 2. 3. 4. 5. Phase Diagram It is a convenient graphical representation formed by plotting the values of intensive variables for equilibrium conditions between two phases. It shows the properties such as mp, bp, phase transition point and triple point. The complex city of phase diagram increase with increase in number of component in the system. For a simple substance (one component system) phase diagram is two dimensional plot where P & T are independent variables. The phase diagram of a two component system is a three dimensional plot, where third axis is for composition.
Phase Diagram 6. Three dimensional plot can also converted into two dimensional plot by keeping one variable constant. Isobaric Isothermal 7. When one of the variable kept constant then phase rule equation is reduced to : F=C-P+I 8. This is known as reduced phase rule.
1. 2. Water System Water exist in three possible phases: ice, water, vapors It is a one component system so maximum degrees of freedom is two, when one phase is stable at equilibrium. C-P+2 = 1-1+2=2 3. Phase diagram of water is two dimensional plot where P & T are taken as axes.
Areas of Phase Diagram of Water System Phase diagram is divided into three areas: 1. Area BOC — where ice has stable existence 2. Area COA — where water has stable existence 3. Area BOA — where water vapors has stable existence Phase Rule for this system: F = C-P+2 = 2 Degrees of freedom is two hence bivariant system.
Vaporization Curve or vapor pressure curve (Curve OA) 6. At higher end, curve OA terminates at point A which is critical temperature (374C) and Pressure (218atm.) of water. 7. At this point liquid & vapor phases become indistinguishable & merge into single fluid phase. 8. Under normal conditions terminus point is O where water freezes to form ice.
1. 2. 3. 4. 5. Vaporization Curve or vapor pressure curve (Curve OA') Curve OA' represents the meta-stable equilibrium. Under some special conditions pure water may be cooled down much below the freezing point without forming ice. Thus it is possible to extend vapor pressure curve even below freezing point of water. This equilibrium can be approached by cooling liquid water and not by heating ice. Metastable vapor pressure of super cooled liquid is higher than the vapor pressure of ice.
1. 2. 3. 4. Sublimation Curve (Curve 0B) Represents the condition for equilibrium between ice and vapors. Shows vapor pressure of ice at different temperature In order to describe the system along line 0B either value of T or P need to be specified. Because at any temperature, value of vapor pressure of ice is fixed.
1. 2. 3. 4. Phase Diagram for Two component system In two component system with P=l, the number of degrees of freedom are highest order is three. In two component system phase diagram may be represented by three dimensional diagram of P, composition and T. In this diagram two axes represents two variables while third variable is held constant. Phase Diagram may be constructed as : 1. P-T diagram keeping conc. Constant 2. P- Conc. Diagram T constant 3. T-C diagram keeping P constant.
Solid — Liquid Equilibria or condensed phase rule or reduced phase When P is kept constant, vapour phase of system is not considered. In this case system is said to be condensed and phase rule reduced to: F=C-P+I This is called condensed phase rule or reduced phase rule. T vs. Composition diagrams are shown to represents the solid- Liquid equilibria. Two component system is to be classified depending upon miscibility of two components in molten state and also on basis of solid phases that separates out on cooling.
Ag-Pb System It is a simple eutectic system in which silver and lead are completely soluble in liquid state. They do not react to form a any compound but on solidification they form an easy melting mixture.(eutectic system) Four phases that take part in equilibrium are: 1. solid silver 2. solid lead 3. solution of Lead & silver 4. Vapor Boiling point of Ag & Pb being high, so Gas phase is absent, so pressure has negligible effect. So S-L equilibrium without gas phase is known as condensed system.
Ag-Pb System Areas (Bivariant System) 1. Area above AOB: only molten mixture of Ag and Pb exist in this system 2. Areas enclosed by BOF: only molten mixture of Ag and Pb and solid Pb exist. 3. Areas enclosed by AOE: only molten mixture of Ag and Pb and solid Ag exist.
Curves of Ag-Pb System (Univariant System) 2. Curve BO Since mp of Pb is 327C, continuous addition of Ag lowers mp along BOO Two phases exists in equilibrium along BO solid Pb and solution System is univariant.
Ag-Pb System AO and BO meets at point at O, O is common to both the curves, solid Pb, solid Ag and their liquid solution coexist. At this point degrees of freedom is zero and it is non- variant. Point O is lowest T of 303C at which liquid exist. If liquid cooled below this temperature both Ag & Pb separates in solid form as 2.6% Ag and 97.6 % Pb. In an alloy of Ag and Pb, Ag melts at 303C which is lower than melting point of Ag and Pb called eutectic mixture.
Ag-Pb System Diagram is divided into two regions below the line EF: On the left of the line OD, crystalline Ag and eutectic mixture are stable On the right of the line OD crystalline Pb and eutectic mixture are stable.
Zn-Mg System Zn-Mg System is a two component system and having congruent melting point. In this system zinc and magnesium, which melt at 4190C and 6500C. Both metals enter into chemical combination and form an intermetallic compound MgZn2 and melts at 5900C to give a liquid of the same composition. Hence, 5900C is the congruent melting point of the system. In the reduced form, the system has the following four phases: Solid magnesium, solid zinc, Solid MgZn2 Liquid solution of Zn and Mg.
1. 2. Zn-Mg System On applying the reduced phase rule Therefore, at point D constitutes a non-variant system. Point E (Eutectic point): Point E represents the eutectic point of the system at a temperature of 3450C which is the least melting point of Mg-MgZn2 system. Here, also three phases existing together in equilibrium at point E are solid Mg, solid MgZn2 and liquid MgZn2. The system is non-variant.
3. Zn-Mg System Point C (Eutectic point): This point also represents the eutectic point (3800C) which is the least melting point of Zn-MgZn2 system. At this point, the three phases—solid Zn, solid MgZn2 and liquid MgZn2 exist together in equilibrium. Hence, point C represents a non-variant system.
Areas of Zn-Mg System The area above the curve BCDEA constitutes a single phase system. i.e. liquid melt consisting of a liquid solution of zinc and magnesium. The system is bivariant. Other areas of the Zn-Mg system consists of two phases and they are univariant systems.
Thermal Analysis Thermal analysis is a branch of materials science where the properties of materials are studied as they change with temperature, Several methods are commonly used — these are distinguished from one another by the property which is measured.
Thermal Analysis Depending upon the variables being measured as a function of temperature or 1. 2. 3. 4. 5. time: Thermal Gravimetric Analysis (TG): Weight Loss due to decomposition Differential Thermal Analysis (DTA): Temperature change between a substance and inert reference is calculated as a function of temperature. Differential scanning colorimetry (DSC): Enthalpy required to record zero temperature difference between a substance and reference material. Dilatometry : changes in size or dimension of substance are measured. Evolved gas analysis: volatile product evolved are analysed.
FaQs What do you mean by polymerization? 1. How will you distinguish between addition and 2. condensation polymerization? Write short notes on : a. PVC 3. b. Nylon 4. Write applications of polymers. 5. Explain vulcanization of rubber.