Question

What type of deviation from Raoult's law is shown by a mixture of phenol and aniline? Give reason.

Hint:

Raoult's law says that in an ideal solution each component's vapour pressure is directly proportional to its mole fraction.

Answer 1

Concept:
Raoult's law says that in an ideal solution each component's vapour pressure is directly proportional to its mole fraction. A solution shows a deviation when the new forces between the two different molecules are different from the forces that existed in the pure liquids.

Step 1:
Look at the two molecules. Phenol (\(C_6H_5OH\)) has an O-H group that can donate a hydrogen bond, and aniline (\(C_6H_5NH_2\)) has a nitrogen lone pair that can accept a hydrogen bond.

Step 2:
When phenol and aniline are mixed, the O-H of phenol forms a strong new hydrogen bond with the N of aniline. This new attraction between the two different molecules is stronger than the attractions present in the separate pure liquids.

Step 3:
Because the molecules are now held together more tightly, fewer molecules escape into the vapour. So the actual vapour pressure becomes lower than the value predicted by Raoult's law.

Answer: A mixture of phenol and aniline shows negative deviation from Raoult's law, because the strong hydrogen bonding between phenol and aniline molecules is stronger than the interactions in the pure components, which lowers the vapour pressure of the solution.