Freezing Point Depression and vapor pressure

[Master J]

Why is the vapour pressure lowered when non volatile solute is added to a solvent?

What is the physical reason?

Also, why do binary solutions freeze over a range of temperatures, not at one specific temperature?

Thanks guys, as usual, for sharing the knowledge!

 

[Ygggdrasil]

Entropy. The boiling point of a liquid is set roughly by the competition between entropy and enthalpy (potential energy). The liquid has a lower potential energy than the gas phase, but the gas phase has the higher entropy. Adding a solute to the solvent raises the entropy of the solution, which decreases the gain in entropy from going from liquid to gas. Because the gain in entropy is lesser, the competition between entropy and enthalpy is shifted more in favor of the liquid so that the boiling point increases and hence the vapor pressure decreases.

Binary solutions freeze over a range of temperatures because of the same reason. Solids are crystaline and therefore are generally only composed of a single chemical species. Because they are so ordered and are not mixtures, their entropy is very low compared to the entropy of a liquid. However, because solids have a lower enthalpy than liquids, again, a competition between entropy and enthalpy sets the freezing point of a solid. If the liquid is a mixture, it will have a higher entropy than a pure liquid and hence, the entropy loss due to freezing will be greater. This greater loss of entropy leads to a lower freezing point.

Since freezing will create a solid consisting of only (or mostly) solvent molecules, freezing will increase the concentration of solute molecules in the solution further lowering the freezing temperature. Thus, as freezing progresses, the freezing point of the solution decreases, leading to the phenomenon that binary solutions freeze over a range of temperatures.