16.3. Colligative Properties http://www.ck12.org
Vapor Pressure
As we saw when we studiedStates of Matter, the driving force for particles in the liquid phase to escape into the gas
phase depends on both the temperature and identity of the substance. The vapor pressure of a liquid is a measure
of this ability. Specifically,vapor pressureis the pressure exerted by a vapor that is in equilibrium with its solid or
liquid phase. For substances with a stronger drive to enter the gas phase, more vapor particles will be present in the
same amount of space, resulting in a higher pressure. At a given temperature, the vapor pressures of various liquids
depends primarily on the strength of intermolecular attractions between individual particles. Figure16.11 shows
the relative vapor pressures of several different substances.
FIGURE 16.11
Vapor Pressure –Temperature CurveOverall, molecules that can participate in hydrogen bonding, such as acetic acid, tend to have lower vapor pressures
at a given temperature than similarly sized molecules without the ability to hydrogen bond, such as acetone.
Vapor Pressure of Solutions
The vapor pressure of a solution is lower than that of the pure solvent at the same temperature. This decrease in vapor
pressure is one example of acolligative property. Colligative properties are properties of solutions that depend only
on the concentration of dissolved particles and not on their identity.
As we see inFigure16.12, the vapor pressure of the solution is lower than the vapor pressure of the pure solvent.
This phenomenon can be understood by considering the equilibrium between the liquid and the gas phases for a
given solvent. When a pure solvent reaches equilibrium with its vapor, the liquid particles are escaping into the
gas phase at the same rate as the gas particles are condensing into the liquid phase. If we add a non-volatile solute
(one that does not escape into the vapor phase under standard conditions), the liquid becomes a mixture of solute and
solvent particles. The surface will then be composed of solute particles in addition to solvent molecules, which slows
the rate at which the liquid particles can evaporate due to fewer solvent molecules in contact with the liquid/vapor
interface. However, it does not slow down the rate of condensation. As a result, there is a net shift into the liquid
phase. Less solvent is present in its vapor form, so the resulting vapor pressure is lower. This effect is illustrated on
the molecular level inFigure16.13.
This effect is quantified byRaoult’s law, which states that the vapor pressure of a solution is equal to the vapor
pressure of the pure solvent multiplied by its mole fraction. Raoult’s law can be expressed mathematically as
follows:
P =χsolventP°
where P is the vapor pressure of the solution,χsolventis the mole fraction of the solvent, and P° is the vapor pressure