Question

What is meant by positive and negative deviations from Raoult's law and how is the sign of ${\Delta }_{mix}H$ related to positive and negative deviations from Raoult's law?

Answer 1

Positive deviation from Raoult's law occurs when the total vapour pressure of the solution is more than corresponding vapour pressure in case of ideal solution.

$P=P_A+P_B>P_A^o{X}_A+P_B^o{X}_B$

Negative deviation from Raoult's law occurs when the total vapour pressure of the solution is less than corresponding vapour pressure in case of the ideal solution.

$P=P_A+P_B<P_A^o{X}_A+P_B^o{X}_B$

For positive deviation from Raoult's law, ${\Delta }_{mix}H$ has a positive sign.

For negative deviation from Raoult's law, ${\Delta }_{mix}H$ has a negative sign.