Colligative Properties

Properties that depend upon the concentration of solute particles are called colligative properties. Generally these properties are measured for dilute solutions (at high concentrations there are other interactions that take place).

Vapor pressure, boiling point, and freezing point are changed by particle concentration.

Vapor Pressure Vapor pressure can be described as the pressure exerted by the vapor above a liquid against the opposing atmospheric pressure. When a nonvolatile substance like sugar or salt are added to water they lower the vapor pressure. How do they lower the vapor pressure? – Some of the nonvolatile particles are at the surface and they interfere with the escape of the volatile water molecules – Since the volatile water molecules need a certain amount of kinetic energy to escape, the number gaining enough energy to escape is less because the nonvolatile particles have absorbed some of that energy

Boiling Point Elevation Boiling Point is defined as the temperature at which the vapor pressure equals the opposing standard atmospheric pressure. When a nonvolatile particles are added to water to make a solution, the boiling point of that solution is raised. Chloride ion Sodium ion Water boils at 100°C at 1 atmNaCl solution at > 100°C at 1 atm

The elevation of the boiling point depends on the number of mols of particles present. For each mol of particles in a kg of water the boiling point of the aqueous solution is elevated by 0.52°C. That is called the boiling point elevation constant for water (0.52°C/m). Molecular compounds such as C 6 H 12 O 6 do not break up into ions, so one mol of C 6 H 12 O 6 would raise the boiling point 0.52°C. Ionic Compounds such as NaCl and CaCl 2 break up into ions in aqueous solution. One mol of NaCl actually yields two mols of ions, one mol of CaCl 2 yields three mols of ions. They would elevate the boiling point more than one mol of C 6 H 12 O 6.

The increase in boiling point relative to that of the pure solvent is related to the molality of the solution by the following expression: /\ T b =K b m /\ T b is the change in boiling point K b is the molal boiling point constant, which depends on the solvent m is the molality of the solution

The boiling point elevation is proportional to the concentration of solute particles, regardless of whether the particles are molecules or ions. When NaCl dissolves in water, 2 moles of solute particles are formed for each mole of NaCl that dissolves. As a result, the boiling point elevation of a 1 m aqueous solution of NaCl is approximately twice as large as a 1 m solution of a nonelectrolyte such as sucrose(C 12 H 22 O 11 ).

To properly access the effect of a particular solute on the boiling point (or any other colligative property), it is important to know whether the solute is an electrolyte or nonelectrolyte. In the previous slide, it was mentioned that NaCl would raise the boiling point of a solution twice as much as a nonelectrolyte. This is important to remember when solving freezing point depression and boiling point elevation problems.

When solving freezing point depression and boiling point elevation problems when a strong electrolyte is the solute, you must use a factor in your calculations called the van’t Hoff factor,i. It represents how many ions the electrolyte forms in solution. Examples: CaCl 2 i = 3 H 2 SO 4 i = 3 NaCl i = 2

Electrolyte Review The strong electrolytes are all ionic compounds and strong acids. Most molecular compounds are nonelectrolytes.

Freezing Point Depression When water freezes the hydrogen bonds give water a rigid structure (water expands as it freezes). Adding NaCl or C 6 H 12 O 6 (any solute) to the water interferes with the formation of the hydrogen bonds. The freezing point of an aqueous solution is lowered 1.86°C for each mol of particles per kg of water (1.86°C/m). The solid water has a definite crystalline structure as a result of the hydrogen bonding

The decrease in freezing point is expressed by the following expression: /\ T f =K f m /\ T f is the change in freezing point K f is the molal freezing point constant, which depends on the solvent m is the molality of the solution

Summary Colligative properties depend upon the concentration of the solute. When determining the boiling point elevation or freezing point depression, you must take into consideration if the solute is an electrolyte or nonelectrolyte.

Colligative Formulas Nonelectrolytes Electrolytes /\ T b =K b m /\T b =K b mi /\ T f =K f m /\ T f =K f mi i= van’t Hoff factor(how many ions of solute are formed)

Study Check What is the boiling point elevation of a solution made from 20.1 g of a nonelectrolyte solute and g of water? The molar mass of the solute is 62.0 g, and the K b for water is 0.51 o C/m.

Study Check What is the freezing point depression of water in a solution of 17.1 g of sucrose, C 12 H 22 O 11, in 200. g of water? What is the actual freezing point of the solution? The K f for water is 1.86 o C/m.

Study Check What is the expected change in freezing point of water in a solution of 62.5 g of barium nitrate, Ba(NO 3 ) 2, in 1.00 kg of water? Ba(NO 3 ) 2 is a strong electrolyte.