Question

1.What is meant by positive deviations from Raoult’s law? Give an example.
2.What is the sign of \[{\Delta _{mix}}H\] for positive deviation?

Answer 1

Hint: Positive deviation indicates that solution no longer behaves as an ideal solution. In general, we can say that, the resultant pressure of solution A-B is more than that of initial pressure of solution i.e. A-A and B-B. Mainly, this deviation is based only on the interactive forces between the particles.

Complete step-by-step answer:
1.Because of Raoult’s law, when we plot the vapour pressure of an ideal mixture of two liquids against their composition in a graph, we get a straight-line graph. But this law is applicable only for ideal mixtures because in such mixtures, the forces between the particles are exactly the same to those in their pure liquids. We know that this is not true for non-ideal mixtures.
This results in deviation from Raoult’s law which can either be positive or negative. In mixtures that show positive deviation from Raoult’s law, the vapour pressure of the mixture is always higher than what we obtain from an ideal mixture. Due to this, instead of a straight line, we observe a slight curve in the graph.
High vapour pressure means that molecules can break away more easily as compared to pure liquids. This is because the intermolecular forces between molecules of A and B are less as compared to that of pure liquids. We can experimentally observe that when we mix such liquids, less heat is evolved when the new interaction is set up then was absorbed to break the original bonds. Therefore, heat is absorbed when the liquids are mixed and the enthalpy change becomes endothermic.
The best example of this deviation is a mixture of ethanol and water. They produce a highly distorted curve with a maximum vapour pressure containing \[95\% \] of ethanol by weight.

2.The solute-solvent forces of attraction are weaker than solute-solute and solvent-solvent interaction in case of positive deviation i.e., \[A-B < A-A\] or \[B-B\] . this corresponds to the enthalpy of mixing which is positive in this case i.e. \[{\Delta _{mix}}H > 0\] because the heat absorbed to form new molecular interaction is less than the heat liberated on breaking of original molecular interaction.

Note: There is no solution mixture in reality which is completely ideal. The reason behind this is the existence of other types of forces such as dipole forces, ion-dipole forces and Vander Waal forces in every molecule. There exists either positive or negative deviations i.e. non-ideal solutions only.