Question

What type of deviation from Raoult's law is shown by mixture of ethanol and acetone? Give reason. What will happen to the boiling point of the solution on mixing ethanol and acetone?

Hint:

Raoult's law deviations arise from differences between A-A, B-B and A-B intermolecular forces. Pure ethanol has strong hydrogen bonding between its molecules.

Answer 1

Concept: Raoult's law deviations arise from differences between A-A, B-B and A-B intermolecular forces. Pure ethanol has strong hydrogen bonding between its molecules. When acetone is added, the acetone molecules come between the ethanol molecules and break these hydrogen bonds. The new ethanol-acetone interactions are weaker than the original ethanol-ethanol hydrogen bonds.
Answer: The mixture of ethanol and acetone shows positive deviation from Raoult's law. This is because acetone molecules break the hydrogen bonds present between ethanol molecules, so the molecules escape into the vapour phase more easily. As a result, the vapour pressure of the solution becomes higher than that expected from Raoult's law. Since vapour pressure increases, the boiling point of the solution decreases on mixing ethanol and acetone.