Colligative properties of a solution.

1. Colligative properties of a solution. Lector Varikova T.O.

2. The properties of solutions that depend on the number of solute particles in the solution, but not on the nature of the solute are known as colligative properties. These properties include: vapor pressure, freezing point depression, boiling point elevation, and osmotic pressure.

3. In a solution of two liquid components, the vapor pressure of each component is less than the vapor pressure of the component in its pure state at the same temperature. In a solution of a solid solute of negligible vapor pressure, called a nonvolatile solute, the vapor pressure of the solvent at the same temperature.

4. The lowering of the vapor pressure of a liquid solvent in a solution can be explained in terms of solute – solvent interaction. Solute molecules attract solvent molecules, decreasing the number of solvent molecules that can escape from the liquid to the gaseous state in a given time period. • A solution in which the solute – solvent interaction does not differ from solute – solute or solvent – solvent interaction is called an ideal solution.

5. For ideal solution, the vapor pressure of a given component in a solution is directly proportional to the mole fraction of that component. This statment is known as Raoult’s law, after the French chemist François Raoult (1830 – 1901). This law can be expressed mathematically as follows: • where Pi is the vapor pressure of component i in solution at a given temperature, xi the mole fraction of component i, and P0i the vapor pressure of pure component i at the same temperature as the solution.

6. If a nonvolatile solute is dissolved in a liquid, the vapor pressure of the solution is due to the vapor pressure of the solvent alone. The vapor pressure of the solvent in such a solution is lower than that of the pure solvent.

7. Boilingpointelevationandfreezingpointdepression.

8. When a nonvolatile solute dissolves in a liquid, the vapor pressure of the solution is lower than that of the solvent. As a result, the boiling point of the solution is higher than the boiling point of the solvent, and the freezing point of the solution is lower than the freezing point of the solvent. • The boiling point elevation, ΔTb, of an ideal solution is proportional to the molal concentration of the solution, and can be expressed by the equation • where Kb is the boiling point constant. • The boiling point constant equals the boiling point elevation observed for a one molal solution. This means that the boiling point constant, Kb, for a 1 molal solution equals ΔTb.

9. Freezing point depression is directly proportional to the number of solute particles added to given mass of the solvent. The freezing point depression for a solution can be expressed by equation In this equation, Kfis the freezing point constant, or the molal freezing point depression. Kf is the freezing point depression observed when 1 mol of a nonvolatile and nonionizing solute dissolves in 1 kg of solvent. Kf is a characteristic of the solvent but does not depend on the nature of solute.

10. Osmosis and Osmotic Pressure. • Particles of both the solutes and the solvent experience thermal motion to an equal degree in solutions. This causes the mutual diffusion of the solvent and solute particles, which levels out the concentration throughout the volume of a solution. For example, when a solution has two regions with different concentrations c1 > c2, the solute diffuses from region 1 (with the higher concentration) to region 2, while the solvent diffuses from region 2 to region 1. • If these regions are separated from each other by a membrane permeable only for solvent particles, diffusion will be unilateral. But in this case too it levels out the concentration throughout the entire volume of the system.

11. A membrane capable of retaining solutes but permeable for a solvent is said to be semipermeable. Cellophane, parchment, intestine walls, the urinary bladder, etc. have such properties with respect to aqueous solutions. • The phenomenon of the spontaneous transfer of a solvent through a semipermable membrane is called osmosis. • By creating a pressure in the more concentrated solution, the osmotic flow of a solvent through a semipermeable membrane can be prevented. • The pressure that must be created in a solution to stop osmosis from the pure solvent into the solution is called the osmotic pressure of the solution. • When studying osmotic phenomena, Van’t-Hoff in 1887 found that the osmotic pressure does not depend on the nature of the solvent. In very dilute solutions of nonelectrolytes the osmotic pressure is proportional to the molarity of a solution and the temperature:

12. If two solutions have the same osmotic pressure, they are isotonic (“iso-” means the same). If one of the two solutions has a higher osmotic pressure than another, the one of higher osmotic pressure is called hypertonic (“hyper-” means more), and the one having the lesser osmotic pressure is hypotonic (“hypo-”means less)