Question

What does cause deviation from ideal behaviour?

Answer 1

Deviations from ideal solution behaviour occur when the intermolecular interactions in a solution are not equal to those in the pure components. These deviations result in non-ideal solutions that do not obey Raoult’s Law.

1. Difference in intermolecular forces: If A-B interactions ≠ A-A or B-B interactions, the solution behaves non-ideally. This inequality leads to either stronger or weaker attractions than in the pure components.
2. Positive deviation from Raoult’s law: Occurs when A-B interactions are weaker than A-A or B-B. Molecules escape more easily into vapour phase → higher vapour pressure. Endothermic mixing (ΔH_mix > 0), volume expansion. Example: Ethanol + Acetone.
3. Negative deviation from Raoult’s law: Occurs when A-B interactions are stronger than A-A or B-B. Fewer molecules escape → lower vapour pressure. Exothermic mixing (ΔH_mix < 0), volume contraction. Example: Chloroform + Acetone
4. Hydrogen bonding or dipole interactions: If solute and solvent form new hydrogen bonds or strong dipole-dipole attractions, behaviour deviates.