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?
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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