Question

(i) What is meant by positive deviations from Raoult's Law ? Give an example.

(ii) What is the sign of ${\Delta }_{mix}H$ for positive derivation?

Answer 1

Positive deviation from Rauolt's law means that the observed vapour pressure is greater than expected and It occur when A-B attractions are weaker than the average of the attraction in the pure component of the mixture. For example, a mixture of ethnol and ecetone show a positive deviation from Rault's law.
In case of solutions showing positive deviations, absorption of heat takes place, i.e., ${\Delta }_{mix}H$ has a positive (+) sign.