Colligative Properties

Colligative properties The colligative properties are shown in dilute solutions with non-volatile solutes and depend only on the number of solute particles, regardless of their chemical nature. In order to make calculations with these properties, molality must be used. Molality is the concentration of a solution expressed by the number of solute moles dissolved in 1000 g of solvent. Here are the colligative properties: Lowering the vapour pressure In a solution the solvent vapour pressure is reduced by the presence of a solute and this reduction is proportional to the solute concentration expressed as a molar fraction (Raoult's law): ΔΡ = X * PO where ∆P is the lowering of the vapour pressure, X is the molar fraction of the solute and PO is the vapour pressure of the pure solvent. This behaviour can be explained if we consider that in a solution only a part of the particles is constituted by solvent, while the remaining part is constituted by solute particles, whose vapour pressure is negligible. Solvent molecules are less likely to evaporate than pure solvent. Raising and lowering the boiling point As a consequence of the change in the vapour pressure of the solution we have a rise in the boiling temperature of the solution and a fall in the solidification temperature of the solution. The rise in boiling point is proportional to the molality of the solution: ATeb = Keb*m where ∆Teb is the ebullioscopic elevation, Keb is the ebullioscopic constant and m is the molal concentration of the solution. As the vapour pressure drops, the solidification temperature of a solution decreases compared to pure solvent. This phenomenon is called cryoscopic lowering which is proportional to the molality of the solution. ∆Tc = Kc*m where ∆Tc is the cryoscopic lowering, Kc is the molal cryoscopic constant and m is the molal concentration of the solution. Osmosis In order to describe osmosis we immediately propose to carry out an experiment. We place, in the centre of a "U" shaped tube, a semi-permeable membrane, that is a membrane that allows only the passage of a solvent even in the presence of solutes, and we place on one side of the membrane a very diluted solution (water with little salt, called solution D) while on the other side a more concentrated solution (sea water, called solution C). In this experiment it is important that the solvents are equal on both sides of the tube and that the levels of the liquids, on both sides of the "U", are equal at the beginning of the experiment. After some time, we will observe that the level of solution D will drop in favour of solution C. This phenomenon sees, the spontaneous passage of solvent from the "diluted" side to the "more concentrated" side generating, therefore, a pressure on the semipermeable membrane due to the movement of solvent itself. The passage of solvent will reach equilibrium when the increase in level of solution C and the dilution of solution C itself, will generate a pressure of equal intensity and direction and in the opposite direction to that which generates the solvent moving from D to C. I have both a dilution of solution C, which decreases the movement of the solvent itself and therefore the osmotic pressure, and an increase in the level of solution C increases the hydrostatic pressure of the column of solution C. The latter increase generates a pressure that acts in the same direction and in the opposite direction to the movement of the solvent itself, leading, in an equilibrium position, to counteract the osmotic pressure.