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Question:

What is meant by positive & negative deviations from raoult’s law & how is the sign of ΔmixH related to positive & negative deviations from Raoult’s law?

Answer:

Raoult’s law states that at a given temperature, the vapour pressure of a solution containing non volatile solute is directly proportional the mole fraction of the solvent.

Non ideal solutions shows positive & negative deviations from ideal behavoiur.

Non ideal solutions showing  positive deviations from Raoult’s law- Consider a binary solution of two components A & B .If the A-B interaction in the solutions are weaker than A-A & B-B interactions in the two liquids forming the solution,then the escaping tendency of A & B  types of molecules from the solution becomes more than from pure liquids.

As a result ,each component of solution has a partial vapour pressure greater than expected on the basis of Raoult’s law.This is called positive deviations from Raoult’s law,i.e PA> PA °xA  & PB >PB°xB

ΔmixH is positive because energy is required to  break A-A & B-B attractive forces.Hence endothermic process.

Non ideal solutions showing  Negative  deviations from Raoult’s law- in such solutions,the A-B interactions are stronger than A-A & B-B interactions .Due to stronger A-B interactions ,the escaping tendency of A & B types of molecules from the solution becomes less than from pure liquids. Consequently, each component of the solution has a  partial vapour  pressure  less than expected on the basis of Raoults law. This is called negative deviations form Raoults law,i.e PA< PA °xA  & PB B°xB

ΔmixH is negative because energy is released due to increase in attractive forces. Hence exothermic process.


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