Concise Physical Chemistry

c09 JWBS043-Rogers September 13, 2010 11:26 Printer Name: Yet to Come

136 THE PHASE RULE

A and B to come to equilibrium with its vapor and then separate and condense the

vapor, we shall have two solutions, one richer in A than the original solution and one

richer in B. Three such equilibrations are represented in Fig. 9.8 as horizontal lines at

different temperaturesT, between the twoTvs.XBcurves. If we start with a solution

of A and B, well to the right of the figure, and allow it to come to equilibrium with

its vapor, the composition of the vapor is given by the leftmost terminus of the top

horizontal. Now separate that vapor from the original solution and condense it. The

new liquid is richer in A than the original solution was.

A second equilibration and separation gives a solution still richer in A according

to the middle horizontal in Fig. 9.8. A third repetition yields a vapor still richer in

A at the lowest temperature. Now the purified concentration is given by the leftmost

terminus of the three equilibration steps diagrammed in Fig. 9.8.

Of course, real distillations are not carried out by such a laborious stepwise process

of equilibration and condensation. Nevertheless, a process very like this, of continual

vaporization equilibration followed by condensation, does take place in the distillation

columns we actually use. Real distilling columns may range in height from a hand’s

breadth for use in the laboratory to columns several stories tall used in the petroleum

industry. By comparing the composition of the input with that of the effluent, the

number of theoretical plates of a “still” can be calculated. Within normal practical

considerations, the more theoretical plates, the better the still.

9.6.2 Type II

Type II phase diagrams describe liquid–liquid systems in which the components are

completely miscible at some temperatures but undergo phase separation at others.

Systems with a composition and temperature above the dome-shaped coexistence

curve in Fig. 9.9 are completely miscible. Those with a temperature and composition

below the curve split into two phases. The transition can be brought about by cooling

a miscible solution so that the temperature drops along a vertical and enters the two-

phase zone or it can be brought about at constant temperature by adding one or the

XB

T, K

A B

FIGURE 9.9 A Type II phase diagram. Solutions having a composition and temperature on

the line split into an A-rich phase and a B-rich phase. The horizontaltie lineis unique at each

temperature.