Physical Chemistry 2.pdf - OER@AVU - African Virtual University

Interactive exercise: Vapour pressure
African Virtual University
An ideal solution is a solution in which the forces of intermolecular attraction in
the two components (solute and solvent) are very similar to one another and to the
forces of attraction (solvent-solute) in the solution. Ideal solutions are therefore ones
for which the enthalpy of solution is zero. In other words they occur when solute
and solvent are very similar-for example, benzene and toluene; hexane and heptane;
pentanol and hexanol. If the forces of attraction differ in the two components and
the solution, the solution is said to deviate from Raoult’s law. Real solutions follow
a different law known as Henry’s law. William Henry found that the vapour pressure
of a dilute solution is proportional to the mole fraction, x B .
P B = x B K B Henry's law (1.23)
The proportionality constant K B has pressure units and is unique for to each gas.
Henry’s law constant depends on temperature and the solvent/solute combination.
1.9 Colligative Properties
Colligative properties are solution properties dependent only on the amount of solute
present. The particles in solutions may be ions or molecules. Colligative properties
include vapour pressure, boiling point, melting point and osmotic pressure. In the
section that follows we examine the effects of solutes on a liquid on the properties
just mentioned.
Vapour pressure of solutions
(a) (b)
F ig ure 1.8 (a) Liquid-vapour
equilibrium with a relati vely rate of
vap orisat ion (b) Some solvent
molecules have b een replaced with
solute molecules. T he rate at which
solvent molecules are escaping is
lower. Low rate of vaporisation.
Figure 1.8 (a) Liquid-vapour equilibrium with a relatively high rate of vaporisation
(b) Some solvent molecules have been replaced with solute
molecules. The rate at which solvent molecules are escaping is lower.
Low rate of vaporisation.