Solutions and Colligative Properties :  Colligative Properties

Colligative property - Any property that depends on the number of solute particles in a solution, but not their identity.

Boiling point elevation - The increase in the boiling point of a solvent that occurs when a non-volatile solute is added to form a solution.

Boiling point - The temperature at which the vapor pressure of a liquid is equal to the pressure on the liquid.

• Normal (Standard) Boiling Point - The temperature at which the vapor pressure of a liquid is equal to standard pressure (1.00 atm = 760 mmHg = 760 torr = 101.325 kPa)

Vapor pressure - The pressure of the gas that collects above a liquid in a closed container.

P = vapor pressure of the pure solvent
Po = vapor pressure of the solvent in a solution

When a solute is dissolved in a solvent, the boiling point of the solution is raised according to the equation:

T = change in temperature
i = the van't Hoff factor, which is the number of particles into which the solute dissociates
m = the molality, which is the moles of solute per kilograms of solvent
Kb = the molal boiling point constant (for water, Kb = 0.515C/m)

The van't Hoff factor is an important factor in the extent of the change in boiling point or freezing point of a solution after solute has been added.  For substances that do not dissociate in water, such as sugar (C12H22O11), has an i value of 1.  Substances that dissociate in water to give two ion particles, such as salt (NaCl), have an i value of 2.  This pattern continues for any number of particles into which a solute can dissociate.
 Jacobus Henricus van't Hoff

CaCl2 which dissociates to give 3 ion particles is often used as road salt instead of a salt such as NaCl which only dissociates to give 2 ion particles.  The change in freezing point temperature is 1.5 times greater when adding CaCl2 versus NaCl.

There are two reasons why the addition of a solute raises the boiling point (i.e. why the vapor pressure of the solvent in a solution is lower than the vapor pressure of a pure solvent):

1. The solute particles occupy space at the surface.
• This consequently slows the rate at which the solvent molecules into he liquid phase can escape into the gas phase

P < Po

• Raoult's Law - The vapor pressure of the solvent above a solution is equal to the product of the mole fraction of the solvent and the vapor pressure of the pure solvent:

2. The solute particles introduce a new set of attractive forces with the solvent molecules.

The vapor pressure of the solvent with the solute is lower than the vapor pressure of the pure solvent.  In order to raise the vapor pressure of the solvent with the solute to atmospheric pressure, the temperature must be raised.

Freezing point depression - The decrease in the freezing point of a solvent that occurs when a solute is added to form a solution.

Freezing Point - The temperature at which the solid and liquid phases of a substance are in equilibrium at atmospheric pressure.

When a solute is dissolved in a solvent, the freezing point of a solution  is lowered according to the equation:

T = change in temperature
i = the van't Hoff factor, which is the number of particles into which the solute dissociates
m = the molality, which is the moles of solute per kilograms of solvent
Kf = the molal freezing point constant (for water, Kf = 1.853 C/m)

In order for a substance in the liquid phase to freeze, the molecules must begin to form a cluster of molecules which eventually grows into a solid.  When a solute is added to a solvent, the solute particles hinder the solvent molecules from being able to cluster by effectively forcing the solvent molecules way from the cluster when they collide.  In order for the solute molecules to reach the cluster and add themselves to the freezing solid, they must be slowed down, have a lower kinetic energy.  This is achieved by lowering the temperature.  Therefore, the freezing point of a solvent with a solute is lower than the freezing point of a pure solvent.

Next:  "Antifreeze"