288 6 The Thermodynamics of Solutions
capacity of the sample material can be determined if no phase transition occurs. If a
phase transition occurs, the temperature of the phase transition and the enthalpy change
of the phase transition can be determined.
Figure 6.20 shows the solid–liquid temperature–composition phase diagram of
p-xylene and bromobenzene at 1 atm. This diagram is similar to that of silver and
copper in Figure 6.18 except that the solids are almost completely insoluble in each
other. The regions of solid solubility are too small to show in the figure. The two solids
that occur in the diagram are nearly pure substances and the tie lines extend to the edge
of the diagram.280290270260250240230220
0 0.2 0.4 0.6 0.8 1T/Kx(p-xylene)EutecticLiquid
Tie line
area“Pure”
bromobenzene“Pure”p-xylene“Pure”
p-xyleneLiquid solution
at equilibrium
with xyleneFigure 6.20 Solid–Liquid Tempera-
ture–Composition Phase Diagram of
p-Xylene and Bromobenzene. From
M. L. McGlashan,Chemical Thermo-
dynamics, Academic Press, New York,
1979, p. 268.Solid–Liquid Phase Diagrams with Compounds
Sometimes two substances form solid-state compounds. Figure 6.21 shows the solid–
liquid temperature–composition phase diagram of aniline (A) and phenol (P), which
exhibit a compound C 6 H 5 NH 2 ·C 6 H 5 OH (abbreviated by AP) in the solid state. Such
a compound has a crystal lattice containing both substances in a stoichiometric ratio.
This crystal lattice is different from the crystal lattice of either A or P and A, and the
three solids, A, P, and AP are almost completely insoluble in each other. The compoundT/KAniline mole fraction0 0.2 0.4 0.6 0.8 1.0Tie line area
solid AP 1 ATie line area
solid P 1 solid APTie line
areaTie line
area
liquid 1
solid ATie line
areaTie line area 2 liquid 1 solid PLiquid 1
AP (solid)Liquid 1
AP (solid)One liquidEutecticEutectic310300290280270260Figure 6.21 Solid–Liquid Temperature–Composition Phase Diagram of Aniline and
Phenol.