12.9: Ideal Solutions
According to Raoult’s law, the partial vapor pressure of a solvent in a solution is equal or identical to the vapor pressure of the pure solvent multiplied by its mole fraction in the solution. However, Raoult's Law is only valid for ideal solutions. For a solution to be ideal, the solvent-solute interaction must be just as strong as a solvent-solvent or solute-solute interaction. This suggests that both the solute and the solvent would use the same amount of energy to escape to the vapor phase as when they are in their pure states. This is only possible when the different components of the solution are chemically similar, as in the case of benzene and toluene or hexane and heptane.
Since many solutions do not have uniform attractive forces, the vapor pressure of these solutions deviates away from the pressure predicted by Raoult’s law. For instance, when ethanol is dissolved in water, there are strong attractions between the water molecules and the ethanol molecules. These attractive forces tend to slow down the loss of water molecules from the surface of the solution. However, if the solution is sufficiently dilute, the surface will have more water molecules. Some of these surface water molecules may not be surrounded by any ethanol molecules and can still escape to the vapor phase at the same rate as they would in pure water. Such dilute solutions are said to approach ideal behavior.
For non-ideal solutions, deviation from Raoult’s law can either be negative or positive. The negative deviation takes place when the vapor pressure is lower than that expected due to Raoult’s law. A solution of water and hydrochloric acid exhibits negative deviation because the hydrogen bonds between water and hydrochloric acid prevent the surface water molecules from vaporizing as readily.
Alternatively, positive deviation occurs when the attraction between the molecules of each component, either solute-solute or solvent-solvent, is greater than the attraction between the solvent and the solute. In such solutions, both components can easily escape into the vapor phase. An example of a positive deviation is a solution of benzene and methanol as the intermolecular forces between the benzene and methanol are weaker than found in pure methanol.