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Home  >>  CBSE XII  >>  Chemistry  >>  Solutions
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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?

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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 law
$\Delta H_{min}=+ve$
Negative 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 mixing
$\Delta H_{mix}=-ve$
$\Delta V_{mix}=-ve$
answered Aug 5, 2014 by sreemathi.v
 
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