Why ccl4 and Chcl3 , acetone and ethyl alcohol,ethanol and water shows positive deviations according to RAOULT'S LAW?

If intermolecular attractive forces between solute – solvent (A – B) are weaker than that of solute – solute (A – A) and / or solvent – solvent (B – B), then molecules of solute or solvent find easier to escape than in pure state. Because of this the vapour pressure increases and shows positive deviation than ideal vapour pressure in pure state.

Example –

Mixture of ethanol and acetone shows positive deviations from Raoult’s Law.

Explanation:

Molecules of ethanol in pure state are hydrogen bonded. But when acetone is added to the ethanol, molecules of acetone get in between the molecules of ethanol and break some of the hydrogen bond, which weaken the intermolecular attractive forces resulting in increase in vapour pressure of mixture than ethanol in pure state.

Because of this mixture of ethanol and acetone shows positive deviation in vapour pressure from Raoult’s Law.

At the same way Mixture of CCl₄ and CHCl₃ ,ethanol and water shows positive deviations.

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