Negative deviation from Raoult’ s Law
o ΔHmix > 0
o ΔVmix > 0
o Total vapor pressure is more than as expected from Raoult’ s law.
o Solution of ethanol and acetone behaves in the same manner. Molecules of pure ethanol are hydrogen bonded. The molecules of acetone get in between the molecules of ethanol on addition of acetone and break some of the hydrogen bonds between them.
Negative deviation from Raoult’ s Law
o
o
The solute-solute and solvent – solvent interactions are weaker than solute-solvent interaction.
Let pA = Partial vapour pressures of component A
PB = Partial vapour pressures of component B
Hmix = Change in enthalpy of mixing.
Vmix = Change in volume of mixing. o o pA < pA o xA
pB < pB o xB
o ΔHmix < 0
o ΔVmix < 0 o Total vapour pressure is less than as expected from Raoult’ s law. o
o
Solution of phenol and aniline. It has strong intermolecular hydrogen bonding between phenolic proton and lone pair of nitrogen atom of aniline than the intermolecular hydrogen bonding between similar molecules.
A solution of chloroform and acetone shows negative deviation from Raoult’ s law due to the formation of hydrogen bond with acetone molecule by chloroform molecule. This decreases the chances of molecules of component to escape leading to the decrease in vapour pressure. And hence the solution exhibits negative deviation from Raoult’ s law