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Question
What is meant by positive and negative deviations from Raoult's law and how is the sign of ΔmixH related to positive and negative deviations from Raoult's law?
Solution
According to Raoult’s law, the partial vapour pressure of each volatile component in any solution is directly proportional to its mole fraction. The solutions which obey Raoult’s law over the entire range of concentration are known as ideal solutions. If the solution does not obey Raoult’s law over the entire range of concentration, then it is called a non-ideal solution. The vapour pressure of such a solution is either higher or lower than that predicted by Raoult’s law. If the vapour pressure is higher, then the solution is said to exhibit a positive deviation, and if it is lower, then the solution is said to exhibit a negative deviation from Raoult’s law.
Vapour pressure of a two-component solution shows positive deviation from Raoult’s law
Vapour pressure of a two-component solution shows negative deviation from Raoult’s law
In the case of an ideal solution, the enthalpy of mixing the pure components to form the solution is zero.
ΔmixH = 0
In the case of solutions showing positive deviations, absorption of heat takes place.
∴ ΔmixH = Positive
In the case of solutions showing negative deviations, the evolution of heat takes place.
∴ ΔmixH = Negative
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