07 Phase equilibrium

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L.D.D Advanced Building Physics - Phase equilibrium  Homogenous system: simple system whose properties do not vary from point to point.  Heterogeneous system: system composed of various homogeneous subsystems, i.e. ststem in which there are a certain number of parts, each physically homogeneous within itself, but different from each other for some thermodynamic properties, although not being separated by material constraints; there parts are called Phases. 1 - Gibbs' Phase rule Consider a system composed of c components and f phases, not separated by walls, so that they can exchange among them any type of extensive variable. The intensive variables (T, p, µ1, .. µc) must be equal in all phases. - Conditions of equilibrium for isolated composed system T1 = T2 p1 = p2 µj1 = µj2 More generally, if we have more subsystems, we will have: The number of degrees of freedom is defined as the number of intensive variables that we can fix arbitrarily and independently one from the other without impairing the existence of an equilibrium condition for the system (f = const.)  Gibbs' rule: in a system consisting of c components, the number of intensive parameters susceptible of arbitrary variations without impairing the existence of equilibrium among f phases is: δ = c + 2 - f L.D.D 2 - Equilibrium in a two phase monocomponent system Phase rule --> two phase monocomponent system: one degree of freedom. Consider the Gibbs' potential: G = U - TS + pV for a monocomponent system, it is: G = U - TS + pV = (TS - pV + µN) - TS + pV = µN (Euler equation: U = TS - pV + µN)  The molar Gibbs potential for one-component system is: g = G/N = µ If we talk about molar terms: dg = vdp - sdT (relationship between intensive parameters) 3 - First order phase transition Consider a three-dimensional…