Difference between non ideal solution with positive deviation and non ideal solution with negative deviation
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Non-ideal solutions showing negative deviation from Raoult’s Law
i) Positive Deviation from Raoult’s Law
Positive Deviation from Raoult’s Law occurs when the vapour pressure of the component is greater than what is expected in Raoult’s Law. For Example, consider two components A and B to form non-ideal solutions. Let the vapour pressure, pure vapour pressure and mole fraction of component A be PA , PA0 and xA respectively and that of component B be PB , PB0 and xB respectively. These liquids will show positive deviation when Raoult’s Law when:
PA > PA0 xA and PB > P0B xB, as the total vapour pressure (PA0 xA + P0B xB) is greater than what it should be according to Raoult’s Law.
The solute-solvent forces of attraction is weaker than solute-solute and solvent-solvent interaction that is, A – B < A – A or B – B
The enthalpy of mixing is positive that is, Δmix H > 0 because the heat absorbed to form new molecular interaction is less than the heat released on breaking of original molecular interaction
The volume of mixing is positive that is, Δmix V > 0 as the volume expands on the dissolution of components A and B
Examples of Positive Deviation
Following are examples of solutions showing positive deviation from Raoult’s Law:
Acetone and Carbon disulphide
Acetone and Benzene
Carbon Tetrachloride and Toluene or Chloroform
Methyl Alcohol and Water
Acetone and Ethanol
Ethanol and Water
Negative Deviation from Raoult’s Law
Negative Deviation occurs when the total vapour pressure is less than what it should be according to Raoult’s Law. Considering the same A and B components to form a non-ideal solution, it will show negative deviation from Raoult’s Law only when:
PA < PA0 xA and PB < P0B xB as the total vapour pressure (PA0 xA + P0B xB) is less than what it should be with respect to Raoult’s Law
The solute-solvent interaction is stronger than solute-solute and solvent-solvent interaction that is, A – B > A – A or B – B
The enthalpy of mixing is negative that is, Δmix H < 0 because more heat is released when new molecular interactions are formed
The volume of mixing is negative that is, Δmix V < 0 as the volume decreases on the dissolution of components A and B