Explain Phase Rule

The relation between the number of phases, components and the degrees of freedom is known as the phase rule.

One constituent systems: the identification of an area on a P-versus-T with one phase of a component system illustrates the two degrees of freedom that exist, these usually being specified as pressure and temperature.

For a two phase system, the requirement of equality in the molar free energies of the two phases imposes a relation, such as dP/dT = ?S/?V, and thus the pressure and temperature cannot both be arbitrary varied. A two phase component system thus has a single degree of freedom, as shown by the identification of a line on a P-versus-T diagram with two phases in equilibrium.

Finally, for three phases to coexist, the molar energy of the first pair would have to be equal that of the additional phase. One molar restrictive equation then exists, and thus the last degree of freedom is entirely removed. No arbitrary assignment of variable can be made; the system is entirely self determined. The one component P-versus-T diagram feature for three phase is a point.

All this can be assumed by the equation:

= 3 - P [one component]

Multi component systems: rules similar to the above equation can be deduced for systems of more than one component. It is possible, however, to proceed more generally and to obtain the phase rule, which gives the number of degrees of freedom of a system with C components and P phases, this rule was first obtained by J. Willard Gibbs in 1878, but it was published in rather obscure Transactions of the Connecticut Academy and overlooked for 20 years.

Consider the two components to be published in the rather obscure Transaction of the Connecticut academy and overlooked the degrees of freedom of the system can be calculated by first adding the total number of intensive variables required to describe separately each problem and subtracting these variables, whose values are fixed by free energy equilibrium relations between the different phases. To begin, each component is assumed to be present in every phase.

In each phase C - 1 quantity will be define the composition of the phase quantitatively. Thus, if mole fraction are used to measure the concentrations, one needs to be specify the mole fraction of the components, the remaining one being determined because the sum of P (C - 1) such composition variables. In addition the pressure and the temperature if the system is considered phase by phase is denoted by the main composition of phase rule.

The number of degrees of freedom, i.e. of net arbitrary adjustable intensive variables, is therefore:

= P(C - 1) + 2 - (P - 1) = C - P + 2

If a component is not present or is present to a negligible extent in one of the phases of the system, there will be one fewer intensive variable for that phase since the neglible concentration of the species is is of no interest. There will also be one fewer equilibrium relation. The phase rule applies, therefore, to all systems regardless of whether all phases have the same number of components.

The phase rule is an significant generalization. Although it tells us nothing that could not be deduced in any given system, it is a valuable guide for unraveling phase equilibrium in more complex systems.

   Related Questions in Chemistry

  • Q : Crystals of covalent compounds Crystals

    Crystals of the covalent compounds always contain:(i) Atoms as their structural units  (ii) Molecules as structural units  (iii) Ions held altogether by electrostatic forces (iv) High melting pointsAnswer: (i)

  • Q : Question based on maximum vapour

    Provide solution of this question. Which has maximum vapour pressure: (a) HI (b) HBr (c) HCl (d) HF

  • Q : Dipole attractions-London dispersion

    Describe how dipole attractions, London dispersion forces and the hydrogen bonding identical?

  • Q : Meaning of molality of a solution The

    The molality of a solution will be: (i) Number of moles of solute per 1000 ml of solvent (ii) Number of moles of solute per 1000 gm of solvent (iii) Number of moles of solute per 1000 ml of solution (iv) Number of gram equivalents of solute per 1000 m

  • Q : Explain the process of adsorption of

    The extent of adsorption of a gas on a solid adsorbent is affected by the following factors: 1. Nature of the gas Since physical adsorption is non-specific in nature, every gas will get adsorbed on the

  • Q : Analytical chemistry 37% weight of HCl

    37% weight of HCl and density is 1.1g/ml. find molarity of HCl

  • Q : What is depression in freezing point?

    Freezing point of a substance is the temperature at which solid and liquid phases of the substance coexist. It is defined as the temperature at which its solid and liquid phases have the same vapour pressure. The freezing point o

  • Q : Law of vapour pressure Select the right

    Select the right answer of the question. "The relative lowering of the vapour pressure is equal to the mole fraction of the solute." This law is called: (a) Henry's law (b) Raoult's law (c) Ostwald's law (d) Arrhenius's law

  • Q : Moles of chloride ion Select the right

    Select the right answer of the question. A solution of CaCl2 is 0.5 mol litre , then the moles of chloride ion in 500ml will be : (a) 0.25 (b) 0.50 (c) 0.75 (d)1.00

  • Q : The three facts on the evaporation

    Describe briefly the three facts on the evaporation?

©TutorsGlobe All rights reserved 2022-2023.