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Unit-Vi Phase Rule UNIT-VI PHASE RULE The phase rule is an important generalization dealing with the equilibrium behavior of a heterogeneous system. This heterogeneous equilibria was first discovered by an American physicist JW Gibbs ( 1874) and continued . The change in the chemical composition or physical state or both may takesplace in chemical equilibrium. These reactions are reversible. TERMINOLOGY Systems are of 2 types. (1) Homogenous (2) Heterogonous. Homogenous:- A system, which exhibit identical physical and chemical properties throughout is called as Homogenous . Eg:- CuSO4--exhibits identical physical and chemical properties even under ultra microscope. Heterogonous:- A system, which exhibit different physical properties is called as Heterogonous. Eg:- (1) Ice-H2O –vapor system--3 portions, physically distinct and mechanically separable from one another. (2) Milk---appears in uniform white colour, but it is a colloidal in presence of fat particles, seen in ultra microscope. Phase:- It is defined as an homogenous part of heterogonous system, physically distinct and mechanically separable portion of system, which is separated from other such parts of the system by definite boundaries. Eg:- H2O consists of 3 phases at freezing point. Ice (s) ↔Water (L)↔ Water vapours (g) At below 00C water exhibit solid state---ice; as the temperature raise it melts to give water (L) then water vapours, reversible reaction. Thus forming a system of 2 or 3 phases; that are homogenous; physically distinct and mechanically separable. It should be noted that each phase in a heterogonous system is homogenous in itself . There are following phases. One phase :- A gaseous mixture, gases are completely miscible in all proportion ---- So form one phase . Eg:- N2 and H2 forms one phase only . Homogenous system consisting of only one phase . If two liquids are miscible (alcohol and H2O) they will form only one phase. The solution of substance in a solvent consists of one phase. Eg: Glucose in H2O. Two phases:- If 2 liquids are immiscible, they will form 2 phases . Eg:-benzene and H2O. Three phases :- Two solid phases of Fe and FeO and one gaseous phase consisting of H2O(g) and H2(g). Heterogonous system consists of 2 or more phases . Eg:- Ice(S) ↔ Water(L) ↔ Watervapour (g). Component:- The smallest number of independent variable constituents, necessary to describe the composition of each phase. Eg:- (1) H2O system Ice (S) ↔ Water (L) ↔ Watervapour (g). The chemical composition of the 3 phases is H2O. Hence it is one component system. (2) The sulphur system consists of 4—phases rhombic, monoclinic, liquid and vapour. The Chemical composition of all phases is S. Hence it is one component system. Two component system:- A system of saturated solution of Nacl consists of solid salt ↔ salt solution ↔ H2O vapour All these phases can be expressed in terms of NaCl and H2O. Hence it is a 2 component system. (2) Thermal decomposition of CaCO3 (s) to give CaO (s) CO2 (g) CaCO3 (s) ↔ CaO (s) + CO2 (g) This 3 phase system can be expressed in terms of at least two of the independently variable constituents ( CaCO3 and CaO / CaO and CO2). Three component system :- Fe(s) + H2O(g) ↔ FeO(s) + H2(g) This 3 phase can be expressed in 3 component. Number of component of a system :- It is defined as the number of chemical constituents of the system minus the number of equations relating to these constituents in an equilibrium state. Eg:- Dissociation of KClO3 2KClO3 (s) ↔ 2KCl(s) + 3O2(g) 2 3 [Kcl] [o2 ] 3 Keq: 2 =[0 2 ] [Kcl03 ] So number of equations relating the concentrations of constituents =1 Hence the number of components = 3-1=2 i.e. 2 component system . Eg:- (2) Dissociation of NH4cl in heated vessel. NH4Cl (s) ↔ NH3 (g) + HCl (g) [NH ] [Hcl] Keg = 3 [NH 4 cl] The active mass of NH4Cl is constant. Keg = [NH3] [HCl] =2 3 – 2 = 1 So, it is a single component system. IV )Degree of freedom or variance :-(F) A system in equilibrium is affected by the factors such as pressure, temperature and composition of the phases. Variance is defined as the number of independent intensive variables such as temperature, pressure and concentration, which must be fixed in order to define the system completely. (The mass dependent properties like weight, volume etc --- extensive) A system having one, two, 3 or 0 degrees of freedom are usually called univariant (F=1), bivariant (F=2), Trivariant(F=3) and invariant(F=0) respectively. Eg:- (1) A pure gas---component =1, P =1 (T and P are fixed ) F = C - P + 2 = 1 – 1 + 2 = 2 A pure gas has 20 s of freedom . (2) A gaseous system consists of 2 gases :- C = 2, P = 1, F = 2 – 1 + 2 = 3 So, this system has 3 degrees of freedom. A gaseous system consists of 3 gases. C = 3, P = 1 F = 3 – 1 + 2 = 4(Tetra variant). Saturated NaCl solution in contact with solid and vapour C = 1, P = 2 F = 1 – 2 + 2 = 1 Ice – H2O - vapour system ( invariant ). C = 1 , P = 3 F = 1 – 3 + 2 = 0 Phase Rule:- It may be stated as equilibrium is dependent only on temperature, pressure and concentration of the phases, is not influenced by gravity/electrical/ magnetic forces etc, then the number of degrees of freedom(F) of the system is related to the number of components( C )and of phases (P) by 2. F = C – P + 2 F = degree of freedom C=components of the system P =number of phases of the system. Phase diagram:- A diagram of substance, which illustrates the conditions of equilibrium between various phases of a substance is called as phase diagram / equilibrium diagram. When system goes from one phase to another phase without any change of chemical composition is called as phase transition. Eg:- Melting (Solid ↔ Liquid), freezing (Liquid↔ Solid) Boiling (L ↔ g) and Condensation (g ↔ L) These different phases of system may be represented as phase diagram as a function of the temperature and pressure. Generally temperature is along the horizontal axis and pressure is along the vertical axis. Phase diagram exhibit following contents. (1) Lines (2) Areas (3) Triple point. Lines /Curves :- There 3 lines separating the areas / regions. Three curves shows the conditions of equilibrium any 2 of the 3 phases. Melting / fusion curve represents equilibrium between liquid ↔ solid. Vaporization curve represents equilibrium between liquid ↔ vapour . Sublimation curve represents equilibrium between vapour ↔ solid. These curves are termed as phase boundaries ;since along these curves the 2 phases are in equilibrium. Areas / Regions:- The diagram divided in to 3 regions or areas which are labeled as solid, liquid and vapour. In the diagram these are represented as A O B, A O C, B O C. Each of the 3 areas shows the conditions of temp and pressure under which the respective phase can exist. Triple point:- The three boundary lines intersect at a common point called Triple point .At which all the 3 phases (L, S and v) can co exist in equilibrium. At this point both temp and pressure on the diagram are fixed. If temperature or pressure is change the equilibrium will be disturbed. One component system (Water system ):- The equilibrium conditions in a one–component system may be conveniently represented by pressure – temperature (P-T) diagrams. This diagram includes Areas:- Represent bivariant system because the temperature and pressure should be fixed to the define system . A line :- Represent a monovariant system, the equilibrium conditions, at any point on the line depends on the conditions of temperature or pressure . A point :- Represent an invariant system, where all the 3 phases are in contact with each other; because it is completely defined by itself and any information regarding pressure or temperature is unnecessary. Eg:- Water system . Water is example for one component and 3 phases.It exhibits following phases. Ice (s) ↔ water (l) ↔ water vapour (g) In all these phases can be represented by one chemical entity H2O. The number of phases which can exist in equilibrium any time depends on the conditions of temperature and pressure. The phase diagram consists of The curves OA, OB and OC The areas AOC, AOB, BOC The triple point O. The curves OA, OB, OC:- These 3 curves meet at the point O and divide the diagram into 3 regions/areas. The curve OA:- It is a vapour pressure curve of liquid water at different temperature. Along this curve OA, the 2 phases water and water vapour co-exit in equilibrium. Above this curve the liquid water alone exist and below it only water vapour exist. This curve terminates at A, critical point (218 atm and 3740c) beyond which the liquid and vapour phases merge into each other resulting in a homogenous phase. When vapour pressure is equal to 1 atm, the corresponding temperature is 1000c (i.e. B.P of H2O). (B) Curve OB:- Is the sublimation curve of ice; this curve separates the ice region from vapour region. This curve represents the equilibrium between ice and vapour. At the lower limit, the curve OB terminates at absolute zero (-2730c) where no vapour exists. (C )Curve OC :- This is melting point curve /fusion curve of ice. This curve divides the solid ice region from the liquid water region .Where ice and water exists in equilibrium.This curve slopes to the left indicates that the melting point of ice is lowered by raise of pressure. Since ice melts with decrease in volume by Le-chatelier‘s principle( M.P is decreased by an increase of pressure. Along the curves OA, OB, OC there are 2 phases (L ↔ G/ G ↔ L/ L↔ S) in equilibrium and one component. According to phase rule F =C – P + 2 = 1 – 2 + 2 = 1 So, the system is univariant.
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