Question

Why does a solution of ethanol and cyclohexane show positive deviation from Raoult's law?

Answer 1

A mixture of cyclohexane and ethanol shows positive deviation from Roult’s law. In ethanol there is hydrogen bonding while in cyclohexane there is weak van der Waal 's forces of attraction. When they are mixed the hydrogen bonding of ethanol is disturbed as cyclohexane molecules are placed in between ethanol molecules. There is no possible bonding between ethanol and cyclohexane molecules but only  weak van der Waals' forces exist so the molecules in the mixture have higher tendency to escape. This reduce the boiling point of solution.

Whereas,

A mixture of acetone-chloroform shows negative deviation from Roult’s law. The chloroform and acetone are held together by van der waal force of attraction. When they are mixed it give rise to hydrogen bonding between the hydrogen of CHCl3 and oxygen atom of CH3-O-CH3 .

Hence the intermolecular interaction between chloroform and acetone is stronger than the intermolecular interactions of chloroform-chloroform and acetone -acetone molecules.This results in decreasing tendency to go in vapour phase and increases the boiling point of solution.