Question

How to find whether a given non ideal solution have positive or negative deviation?

Answer 1

The deviations are caused due the difference in attractive forces between the molecules.Let us consider we are mixing 2 chemicals – A & B,let the attraction between A-A molecules in pure A solution & B-B molecules in pure B solution is less then the attraction between A-B in the mixture.This causes decrease in volume and ‘-‘ve deviation. ‘+’ve deviantion is caused if A-B attractive force is weak. The difference in attractive force is due to various factors like Hydrogen bonding,dipole-dipole attraction etc..

positive deviation from Raoult’s law, A-B interactions are weaker than those between A-A or B-B, i.e., in this case the intermolecular attractive forces between the solute-solvent molecules are weaker than those between the solute-solute and solvent-solvent molecules
EX Mixtures of ethanol and acetone behave in this manner. In pure ethanol, molecules are hydrogen bonded. On adding acetone, its molecules get in between the host molecules and break some of the hydrogen bonds between them. Due to weakening of interactions, the solution shows positive deviation from Raoult’s law
negative deviations from Raoult’s law, the intermolecular attractive forces between A-A and B-B are weaker than those between A-B and leads to decrease in vapour pressure resulting in negative deviations. An example of this type is a mixture of phenol and aniline. In this case the intermolecular hydrogen bonding between phenolic proton and lone pair on nitrogen atom of aniline is stronger than the respective intermolecular hydrogen bonding between similar molecules.