Phase Transitions and Equilibria of Pure Substances

A system of two or more phases of a single substance, in the absence of internal constraints, is in an equiUbrium state when each phase has the same temperature, the same pressure, and the same chemical potential. This chapter describes the derivation and consequences of this simple principle, the general appearance of phase diagrams of single-substance systems, and quantitative aspects of the equilibrium phase transitions of these systems. 

If the state of an isolated system is an equiUbrium state, this state does not change over time (Sec. 2.4.4). We expect an isolated system that is not in an equilibrium state to undergo a spontaneous, irreversible process and eventually to reach an equilibrium state. Just how rapidly this process occurs is a matter of kinetics, not thermodynamics. During this irreversible adiabatic process, tbe entropy increases until it reaches a maximum in the equilibrium state. 

A general procedure will now be introduced for finding conditions for equilibrium with given constraints. The procedure is applied to phase equilibria of single-substance, multiphase systems in the next section, to transfer equilibria in multicomponent, multiphase systems in Sec. 9.2.7, and to reaction equilibria in Sec. 11.7.3. 

Write an expression for the total differential of the internal energy U consistent with any constraints and with the number of independent variables of tbe system. 

Impose conditions of isolation for the system, including dU = 0, thereby reducing tbe number of independent variables. 

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CHAPTER 8 PHASE TRANSITIONS AND EQUILIBRIA OF PURE SUBSTANCES 8.1 PHASE Equilibria... 

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