Question

What is negative deviation and positive deviation?

Answer 1

Positive deviations :In these type of deviations,the partial vapour pressure of each component A and B of solution is higher than the vapour pressure calculated from Raoult's law,For eg : Water and ethanol,chloroform and water.In the case of positive deviation A-B interactions are weaker than those between A-A or B-B.This means that in such solutions molecules of A (or B) will find it easier to escape than in pure state.This will increase the vapour pressure and results in the positive deviation from Raoult's lawΔHmin=+veΔHmin=+veNegative deviations :In this type of deviations,the partial vapour pressure of solutions is less than that of ideal solution of same composition.Boiling point of such a solution is relatively higher than the boiling points of both the components.This type of behavior is known as negative deviation.Here the intermolecular attractive forces between A-A and B-B are weaker than those between A-B and leads to decrease in vapour pressure.In case of solutions showing negative deviations,a slight decrease in volume and evolution of heat takes place on mixingΔHmix=−veΔHmix=−veΔVmix=−ve

Answer 2

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positive deviation :- when the value of total vapour pressure of the same composition containing solution is greater than the value of total vapour pressure of same composition containing ideal solution is called positive deviation.

$\small{\underline{\sf{\green{example-}}}}$ Acetone + carbondisulphide. ethyl alcohal + water etc.

Negative deviation:- when the value of total vapour pressure of same composition containing ideal solution is greater than the value of total vapour pressure of same composition containing any solution is called negative deviation.

$\small{\underline{\sf{\green{example-}}}}$ chloroform + benzene, HNO3 + H2O etc.