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