Question #bd44f

1 Answer
May 4, 2017

One way to describe positive deviation is that the average distance between solute and solvent particles is farther away than in a comparable ideal solution.

We can reconcile this with vapor pressure in that if the solute and solvent particles are farther away on average, the solute or solvent particles on the surface of the solution are interacting less strongly (via intermolecular forces), and thus can leave the solution more easily.

This corresponds to higher vapor pressure than expected from Raoult's law, as we anticipated.

In general, mixtures that show positive deviation are ones where either the solute or solvent exchange interactions so that the new interactions are weaker.

An example is dimethyl ether with hexane. Dimethyl ether started off with dipole-dipole interactions, and hexane starts off with dispersion forces. It ends up that they form dispersion forces.

Therefore, the dimethyl ether/dimethyl ether interactions have become weaker, and the solution deviates positively from Raoult's law.