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What is meant by positive and negative deviations from Raoult's law and how is the sign of delta_mixH related to positive and negative deviations from Raoult's law?

2.14    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 and negative deviation:- A non-ideal solution is defined as a solution which does not obey Raoult’s law over the entire range of concentration i.e.,     \Delta _{Mix}H \neq 0 and \Delta _{Mix}V \neq 0. The vapour pressure of these solutions is either higher or lower than that expected by Raoult’s law. If vapour pressure is higher, the solution shows a positive deviation and if it is lower, it shows a negative deviation from Raoult’s law.

Enthalpy relation to positive and negative deviation can be understood from the following example:- 

 Consider a solution made up of two components -  A and B. In the pure state the intermolecular force of attraction between them are A-A and B-B. But when we mix the two, we get a binary solution with molecular interaction A-B. 

If A-B interaction is weak than A-A and B-B then enthalpy of reaction will be positive thus reaction will tend to move in a backward direction. Hence molecules in binary solution will have a higher tendency to escape. Thus vapour pressure increases and shows positive deviation from the ideal behaviour.

Similarly, for negative deviation, A-B interaction is stronger than that of A-A and B-B.

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