0.17 Phase equilibrium in solutions  (Page 5/9)

This is a nicely intuitive model, but it is somewhat inaccurate. In solution, solute molecules are surrounded by solvent molecules, a process called “solvation.” This is actually why a nonvolatile solute dissolves. This solvation process ties up solvent molecules, reducing their ability to escape from the solution, and it does so in proportion to the fraction of solute molecules in the solution. The effect is exactly the same as described in the simpler model described above, but it is worth keeping in mind that solute molecules are always solvated by solvent molecules. It is an important part of the process of forming a solution.

Let’s conclude by applying our dynamic equilibrium argument to the elevation of the boiling point by the addition of a solute. The rate of condensation is determined by the pressure of the vapor, and for normal boiling to occur, that pressure must equal 1 atm. If the rate of evaporation is lowered by the presence of the solute, then that lowering must be overcome for the rate of evaporation to match the rate of condensation at 1 atm. The straightforward way to do this is to raise the temperature, thereby increasing the fraction of solvent molecules in the liquid with sufficient kinetic energy to escape the liquid. By elevating the temperature, we can restore the dynamic equilibrium, and this is why the boiling point must be elevated by the presence of solute molecules in the liquid phase.

Observation 2: vapor pressures of solutions with two volatile components

So far, we have considered only solutions with a single volatile component, the solvent. In most cases, this means dissolving a solid into a liquid, but it can also mean dissolving a liquid with a very low vapor pressure into another liquid. There are other types of solutions, of course. For example, we can mix together two volatile liquids, dissolving each in the other to form a solution. Note that not all liquids are “miscible,” meaning that it is not always possible to form a liquid-liquid solution. For example, we have all observed that oil and water separate when mixed together and thus do not form a solution. In general, two liquids will tend to dissolve in each other when the intermolecular forces between the two types of molecules are similar. A good example is mixing water with ethanol. In each substance, the molecules attract each other with hydrogen bonding, so therefore the water and ethanol molecules will also attract each other with hydrogen bonding when combined in a solution.

A very good example of such mixing is for benzene and toluene, whose molecules are quite similar as shown in Figure 3:

Because of these similarities, benzene and toluene can be mixed in any proportion. Both of these liquids have significant vapor pressures. At 25 ºC, the vapor pressure of benzene is 95.1 torr and the vapor pressure of toluene is 28.7 torr. What would we observe the vapor pressure to be if we were to take a solution of benzene and toluene? The experimental data are shown in Figure 4 at 25 ºC.