Group Number: 3
Exercise No. 2
In liquid solutions, particles are close together and the solute molecules or ions disrupt intermolecular forces between the solvent molecules, causing changes in those properties of the solvent that depend in intermolecular attraction. For example, the freezing point of a solution is lower than that of the of the pure solvent and the boiling point is higher. Colligative properties of solution are those that depend on the concentration of solute particles in the solution, regardless of what kinds of particles are present the greater the concentration of any solute, the lower the freezing point and the higher the boiling point of a solution. FREEZING POINT LOWERING
A liquid begins to freeze when temperature is lowered to the substance’s freezing point and the first few molecules cluster together into a crystal lattice to form a tiny quantity of solid. As long as both solid and liquid phases are present at the freezing point, the rate of crystallization equals the rate of melting and there is a dynamic equilibrium. When a solution freezes, a few molecules of solvent cluster together to form pure solid solvent and a dynamic equilibrium is set up between the solution and the solid solvent. In the case of a solution, the molecules in the liquid in contact with the solid solvent are not all solvent molecule. The rate at which molecules move from solution to solid is therefore smaller that in the pure liquid to achieve dynamic equilibrium there must be a corresponding smaller rate of escape of molecules from solid crystal lattice. This slower rate occurs at a lower temperature and so the freezing point of the solution is lower than that of liquid solvent. The change in freezing point ∆Tf is proportional to the concentration of the solute in the same way as the boiling point elevation. ∆Tf = Kf × msolute × ¡solute
Here also, the proportionality constant Kf depends on the solvent and not the kind of solute and isolute represents the number of particles per formula unti of solute. For water, the freezing point constant is -1.86 oC-kg/mole.
Freezing point or melting point is the temperature of transition between solid and liquid. Melting point can be measured more accurately than freezing points. This is becauses so in freezing point measurements, supercooling may occur which would yield a lower than sslkdjs freezing (melting point). CHANGES IN VAPOUR PRESSURE: RAOULT’S LAW
At the surface of an aqueous solution, there are molecules of water as well as ions or molecules from the solute. Water molecules can leave the liquid and enter the gas phase, exerting a vapour pressure. However, there are not as many water molecules at the surface as in pure water, because some of them have been displaced by dissolved ions or molecules/ therefore, not as many water molecules are available to leave the liquid surface, and the vapour pressure is lower than that of pure water at a given temperature. From this analysis, it should make senses that the vapour pressure of the solovent above the solution, Psolvent, solution, that is , to their mole fraction. Thus, since Psolvent α Xsolvent, we can write Psolvent = Xsolvent × K
(where K is a constant). This equation tells you that, if there are only half as many solvent molecules present at the surface of a solution as at the surface of the pure liquid, then the vapour pressure of the solvent above the solution will only be half as great as that of the pure solvent at the same temperature. If we are dealing only with pure solvent, the above equation becomes Posolvent = Xsolvent × K
where Posolvent is the vapour pressure of the pure solvent and Xsolvent is equal to 1. This means that Posolvent = K; that is, the constant K...