6.6 Phase Diagrams of Nonideal Mixtures 283
Anazeotropic mixtureis sometimes called aconstant-boiling mixture, since it distills
without any change in composition. It is impossible to distill from one side of an
azeotrope to the other. For example, ethanol and water at 1.00 atm have an azeotrope
at an ethanol mole fraction equal to 0.90. Any mixture of ethanol and water can be
distilled to this composition, but no further.
Exercise 6.27
At 1.000 atm the boiling temperature of water is 100◦C and that of ethanol is 78.3◦C. The
azeotrope boils at 78.17◦C at this pressure.
a.Sketch the liquid–vapor temperature–composition phase diagram of ethanol and water.
b.By drawing a “staircase” of line segments representing distillation, as in Figure 6.7, show
that a distillation process beginning with a mole fraction of ethanol less than 0.90 cannot give
a distillate with an ethanol mole fraction greater than 0.90.
300
200
100
P
/torr
0
0
Mole fraction of ethanol
Azeotrope
Tie line
area
Tie line
area
Area of
liquid states
Coexisting
states of
two phases
Area of
vapor states
0.2 0.4 0.6 0.8 1
Figure 6.13 Liquid–Vapor Pressure–
Composition Phase Diagram of
Ethanol and Benzene, Showing an
Azeotrope. Drawn from data in J. Tim-
mermans, Physicochemical Constants
of Binary Systems, Vol. 2, Interscience
Publishers, New York, 1959, p. 61.
Area of liquid states
Tie line
areas
Azeotrope
Area of vapor states
65
t 60
/C
8 C
CHCl 3 mole fraction
55
0 0.2 0.4 0.6 0.8 1
Figure 6.14 Liquid–Vapor Tempera-
ture–Composition Phase Diagram of
Acetone and Chloroform.
If a binary mixture has a sufficiently large positive deviation from ideality, the
system at equilibrium can contain two liquid phases. Figure 6.15 shows schematically
the temperature–composition phase diagram of two hypothetical substances, called A
and B. Below the temperature labeledTcthere is a region of tie lines. A tie line in this
region connects points representing the compositions of the two liquids that can be at
equilibrium with each other. The compositions to the right or to the left of this region
are possible equilibrium compositions of a single liquid phase.
The highest point in the tie-line region at temperatureTcis called anupper critical
solution pointor anupper consolute point. It has a number of properties similar to
those of the gas–liquid critical point in Figure 1.5. If a mixture has the same overall
composition as that of the consolute point, it will be a two-phase system at a temperature
below the consolute temperature. If its temperature is gradually raised, the meniscus
between the phases becomes diffuse and disappears, in the same way that the meniscus
Area of vapor states
Tie line areas
(one vapor phase,
one liquid phase)
Area of states
of one liquid
Azeotrope
Consolute
point
Two coexisting
liquid states
Tie line
region –
two liquids
01
Mole fraction
T
Ta
Tc
Figure 6.15 Temperature–Composition Phase Diagram of a System with Liquid–
Liquid Phase Separation (Schematic).