8.5
Mixing and
Solution
403
3. Pure water at 32"F is used to dilute a 90 wt% H2SOa solution (aq,32"F). Estimate the
maximum temperature the product solution can achieve and the concentration of sulfuric
acid (wt%) in this solution.
4. Estimate (a) the specific enthalpy of a 30 wt% H2SOa solution (aq,77'F) and (b) the speciflc enthalpy of a 30 wt% solution obtained by mixing pure water at 77"F and pure sulfuric
acid atTJ'F adiabatically. What is the physical significance of the difference between these
two enthalpies?
Using Enthalpy-Concentration Charts for Vapor-Liquid Equilibrium Calculations
Enthalpy-concentration charts are particularly useful for two-component systems in which vapor and liquid phases are in equilibrium. The Gibbs phase rule (Equation 6.2-1) specifies that
such a system has (2 + 2 - 2) : 2 degrees of freedom. If as before we fix the system pressure,
then specifying only one more intensive variable-the system temperature, or the mass or mole
fraction of either component in either phase-flxes the values of all other intensive variables in
both phases. An fr-x diagram for the ammonia-water system at L atm is shown in Figure 8.5-2.
900
:=
800
f
do
700
_a
600
!
E
500
to
to
400
.9
3oo
200
100
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
0.9
x (mass fraction NH3)
Figure 8.5-2 Enthalpy-concentration diagram
for the ammonia-water system at 1 atm. (From
G. G. Brown et al., Unit Operations, @1950, Figure 551. Reprinted by permission of John Wiley
&
Sons.)
The specific enthalpies of aqueous solutions and gaseous mixtures of ammonia and water are
shown on the two curves on this flgure.
Suppose the mass fraction of ammonia in a liquid solution of NH3 and H2O at 1 atm is
specifled to be 0.25. According to the phase rule, the system temperature and the mass fraction
of NH3 in the vapor phase are uniquely determined by these speciflcations. (Veri[y.) A tie line
may therefore be drawn on the enthalpy-concentration chart from x : 0.25 on the liquidphase curve to the corresponding point on the vapor-phase curve, which is at y : g.95. and the