Phase Equilibrium between Mutually Insoluble Solids

Phase Equilibrium between Mutually Insoluble Solids.—In the 

Fi j xvii-5. Phase equilibrium mixture of liquid and phase a, and fiom diagram for system with almost x3 to X it is a mixture of liquid and phase 

As the temperature rises to 773, the melting point of pure material b, the phase p disappears, and at 77, the melting point of pure a, the phase a disappears, leaving only the liquid as the stable phase above this temperature. 

The most important examples of the phase diagrams we have discussed are found in metallurgy. There, in alloys of two metals with each other, 

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The usual situation, true for the first three cases, is that in which the reactant and product solids are mutually insoluble. Langmuir [146] pointed out that such reactions undoubtedly occur at the linear interface between the two solid phases. The rate of reaction will thus be small when either solid phase is practically absent. Moreover, since both forward and reverse rates will depend on the amount of this common solid-solid interface, its extent cancels out at equilibrium, in harmony with the thermodynamic conclusion that for the reactions such as Eqs. VII-24 to VII-27 the equilibrium constant is given simply by the gas pressure and does not involve the amounts of the two solid phases. 

Let us begin an analysis of the process of formation of chemical compounds in heterogeneous systems with the simplest case of growth of a solid layer between elementary substances A and B which form, according to the equilibrium phase diagram of the A-B binary system, only one chemical compound ApBq, p and q being positive numbers (Fig. 1.1). The substances A and B are considered to be solid at reaction temperature 7, and mutually insoluble. 

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