Positive deviation from Raoult’s law:
Most of the liquid pairs showing non-ideal behaviour exhibit positive deviations.
A-A interactions are weaker than Either A-A or B-B
Cause of Positive Deviation:
(A) Difference in extent of association in two liquids (H2O & CH3OH)
(B) Association in one of the liquids through H-bonding (C2H5OH & C6H6) (C) Greater difference in length of hydrocarbon part of members of same homologous series (n-butane & n-heptane)
(D) Difference in polarity of liquids: General Examples are when one is polar & Other is non-polar (CCl4 & CHCl3)
(E) Greater Difference in molar mass of non-polar molecules (CCl4 & C6H6)
Negative deviations from Raoult’s law:
Condition:A-A interactions are Stronger than Both A-A & B-B
Cause of Negative Deviation:
(A) An acidic & a basic liquid Due to STRONG intermolecular hydrogen Bonding between the proton of the acid & lone pair of the donor atom of the basic liquid (C6H5OH & C6H5NH2)
(B) Haloalkanes (like chloroform) with an oxygen or nitrogen or fluorine containing liquid (like ketones, esters, ethers, amines etc) due to formation of Hydrogen – bonding between these BUT excluding ALCOHOLS which are highly associated and would show positive deviations.
(C) Aqueous solutions of strong volatile acids like sulfuric acid, nitric acid etc., which give non-volatile ions with water
Mixtures of ethanol and acetone have vapor pressures that are higher than predicted by Raoult’s law, while mixtures of acetone and water have lower vapor pressures than predicted by Raoult’s law.
Why is this?