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

First we have to define the positive deviation from Raoult's law. We have to give the difference in the vapour pressure in the solution showing deviation from the ideal solution conditions ( apparent increase in the vapour pressure ). Now, apart from that for the second part we have to mention the outcomes of the positive deviation, from which we can see and observe the answer for the ${\Delta _{mix}}H$, as ${\Delta _{mix}}H > 0$ .

Complete step by step answer:
Positive deviation from Rauolt's law is the condition when the observed vapour pressure is more than the expected vapour pressure.
Its occurrence is generally when A-B attractions where (one is component A and the other one is component B ), are weaker than the average power of the attraction in the pure component of the mixture. For example, a mixture of ethanol and acetone show a positive deviation from Rault's law.
Now for the case of the positive deviation there are some outcomes that all the solutions should follow :
For the case of the ${\Delta _{mix}}H$, the change would always be positive. Hence the equation would be ${\Delta _{mix}}H > 0$ . This is because the increased pressure creates more movement in the solution, hereby dissipating much more heat energy.
For the case of the ${\Delta _{mix}}V$, the change would always be negative. Hence the equation would be ${\Delta _{mix}}V > 0$ . This is because the increased pressure hereby increases the volume too due to their direct proportionality.
Hereby the relative volume would be ${P_s} > {P_1} + {P_2}$ .
By the above outcomes we can clearly suggest that for the given solution the sign of the ${\Delta _{mix}}H$ would be positive, as ${\Delta _{mix}}H > 0$ .

Note:
In the field of chemistry ( perhaps physical chemistry ), Raoult's law is a law with some implications in thermodynamics. The law states that the partial pressure of each component of an ideal mixture of liquids ( or preferably solution ) will be equal to the vapour pressure of the pure component multiplied by its mole fraction in the mixture. This law was given by the French chemist François-Marie Raoult in 1887.