Non-ideal solutions exhibit either positive or negative deviations from Raoult’s law. What are these deviations and why are they caused? Explain with one example for each type.

Ans:

Positive deviation means A—B interactions are weaker than A—A and B—B interactions while, opposite is true in case of negative deviation.

For non-ideal solutions, vapour pressure is either higher or lower than that predicted by Raoult’s law. If it is higher, the solution exhibits positive deviation and if it is lower, it exhibits negative deviation from Raoult’s law.

In case of positive deviation, A—B interactions are weaker than A—A and B—B interactions. Due to this, vapour pressure increases which results in positive deviation. e.g. Ethanol+Acetone and $CS_2$+Acetone show positive deviation.

For positive deviation, $ \Delta H $ (mixing) = Positive

In case of negative deviation, A—B interactions are stronger than A—A and B—B interactions. Due to this, vapour pressure decreases which results in negative deviation. e.g. Phenol + Aniline and Chloroform+ Acetone show negative deviation.

For negative deviation, $ \Delta H $ (mixing) = Negative