Concise Physical Chemistry

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

PROBLEMS AND EXAMPLES 139

A

B C

FIGURE 9.12 A ternary phase diagram with a tie line. At constantpandT, f= 3 −

1 =2 within the two-dimensional triangular surface, but f= 3 − 2 =1 on the coexistence

curve. In general, the area under the curve is smaller at higher temperatures. Numerous, more

complicated forms are known.

Addition of A may cause a milky suspension of immiscible phases to suddenly

clear. Conversely, addition of C to a clear stirred solution of A and B may cause it to

go milky at some concentration. The transition is sudden and sharp and can be used

as the end point of aphase titration.

PROBLEMS AND EXAMPLES

Example 9.1 The Enthalpy of Vaporization of H 2 O

Handbook values for the vapor pressures of water between 273 and 373 K are

available. Plotpvapvs.− 1 /Tfor several temperatures in the vicinity of 298 K and

use a commercial curve fitting routine to determine the enthalpy change for the

vaporization of water at that temperature.

Solution 9.1 The handbook gives pvapat temperatures from 273 to 373 K. We

have selected six values symmetrically grouped around 298 K and presented them as

1 /TK−^1 in Table 9.1.

TABLE 9.1 Negative Inverse Temperatures

and Vapor Pressures for Water.

− 1 /Tpvap

−3.6600e−3 0.6110

−3.5300e−3 1.2300

−3.4100e−3 2.3400

−3.3000e−3 4.2500

−3.1900e−3 7.3800

−3.1000e−3 12.3400

The curve ofpvapvs.− 1 /Tfor water is shown in Fig. 9.13.