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QUESTION 2.14

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


Solution

Positive deviation 'When the vapour pressure of a solution is higher than the predicted value by Raoult's law, it is called positive deviation'. In such cases intermolecular interactions between solute and solvent particles (A and B) are weaker than those between solute-solute (A - A) and solvent-solvent (B - B). Hence, the molecules of (A or B) will escape more easily from the surface of solution than in their pure state. Therefore, the vapour pressure of the solution will be higher.
Characteristics of a solution showing positive deviation
(i) pA>pAxA;pB>pBxB
(ii) ΔHmix>0; i.e., +ve
(iii) ΔVmix>0. i.e., +ve
Examples of solutions showing positive deviation
(i) Ethyl alcohol and water
(ii) Acetone and carbon disulphide
(iii) Carbon tetrachloride and benzene
(iv) Acetone and benzene

Negative deviation 'When the vapour pressure of a solution is lower n the predicted value Raoult's law, it is called negative deviation.' In ase of negative deviation the intermolecular attractive forces between A - A and B - B are weaker than those between A - B. It leads to decrease in vapour pressure resulting in negative deviation.
Characteristics of a solution showing negative deviation
1. pA<pAxA:pB<pBxB
2. ΔVmix<0; i.e., - ve; because weak A - A and B - B bonds are broken and strong A - B bonds is formed. Heat is consequently released.
3. ΔVmix<0; i.e.,-ve
Examples of solutions showing negative deviation
(i) HNO3 and water
(ii) Chloroform and acetone.
(iii) Acetic add and pyridine
(iv) Hydrochloric acid and water.

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