In a solution of benzene and toluene, the vapor pressure of each component obeys
Raoult’s law. Figure 12.7 shows the dependence of the total vapor pressure (PT) in
a benzene-toluene solution on the composition of the solution. Note that we need
only express the composition of the solution in terms of the mole fraction of one
component. For every value of Xbenzene, the mole fraction of toluene, Xtoluene, is given
by (1 2 Xbenzene). The benzene-toluene solution is one of the few examples of an
ideal solution, which is any solution that obeys Raoult’s law. One characteristic of
an ideal solution is that the heat of solution, DHsoln, is zero.
Most solutions do not behave ideally in this respect. Designating two volatile
substances as A and B, we can consider the following two cases:
Case 1: If the intermolecular forces between A and B molecules are weaker than
those between A molecules and between B molecules, then there is a greater tendency
for these molecules to leave the solution than in the case of an ideal solution. Consequently,
the vapor pressure of the solution is greater than the sum of the vapor
pressures as predicted by Raoult’s law for the same concentration. This behavior gives
rise to the positive deviation [Figure 12.8(a)]. In this case, the heat of solution is
positive (that is, mixing is an endothermic process).
Case 2: If A molecules attract B molecules more strongly than they do their own
kind, the vapor pressure of the solution is less than the sum of the vapor pressures as
predicted by Raoult’s law. Here we have a negative deviation [Figure 12.8(b)]. In this
case, the heat of solution is negative (that is, mixing is an exothermic process).