Non-Ideal Solutions: Positive and Negative Deviations from Raoult's Law

Most real liquid-liquid solutions do not obey Raoult's law exactly across their whole composition range; instead, their actual total vapour pressure deviates either above or below the value Raoult's law would predict, and the sign of that deviation is set by how the strength of A–B intermolecular attractions compares with the strength of A–A and B–B attractions in the two pure liquids.

A positive deviation occurs when A–B interactions are weaker than the average of A–A and B–B interactions. Because molecules of A and B then attract each other less strongly than they attract their own kind, both components escape into the vapour phase more readily than Raoult's law predicts, so the solution's actual vapour pressure is higher than the ideal value, and correspondingly its boiling point is lower than a simple weighted average would suggest. Mixing the two liquids under these conditions requires energy to be absorbed to partially separate the stronger like-molecule attractions, so $\Delta_{mix}H > 0$ (mixing is endothermic), and the total volume typically expands slightly on mixing, $\Delta_{mix}V > 0$. Ethanol and acetone is a standard example: pure ethanol's hydrogen-bonded network is partly disrupted by the acetone, releasing some molecules that would otherwise stay hydrogen-bonded, and raising the vapour pressure above the ideal prediction.

A negative deviation occurs in the opposite case, when A–B interactions are stronger than A–A and B–B interactions — typically because mixing introduces a new, favourable interaction (often hydrogen bonding) that neither pure liquid enjoyed on its own. Molecules are then held in the liquid more tightly than Raoult's law assumes, so fewer escape into the vapour, the actual vapour pressure is lower than the ideal value, and the boiling point is correspondingly higher. Mixing releases energy as these new, stronger attractions form, so $\Delta_{mix}H < 0$ (mixing is exothermic) and $\Delta_{mix}V < 0$. Chloroform and acetone is the standard example: the chloroform C–H hydrogen bonds to the acetone carbonyl oxygen, an attraction absent in either pure liquid. …