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Non-Ideal Solutions

Non­ideal solutions are the solutions in which solute solvent interactions are different from solute solute and solvent solvent interactions. These solutions do not obey Raoult’s law for all concentrations and

  1. ΔH (mix) ≠ 0
  2. ΔV (mix) ≠ 0

Types of non-ideal solutions

(a) Non-ideal solutions showing positive deviations:  Positive deviation occurs when total vapour pressure for any mole fraction is more than what is expected according to Raoult’s law. This happens when the new interactions are weaker than the interaction in the pure component
(A – B < A – A or B – B interactions).

It forms minimum boiling azeotropes, for example, C2H5OH + cyclohexane. The Bonding present in pure C2H5OH is cut off on adding cyclohexane. For such solution, ΔV and ΔH are positive.

Examples:

  1. Acetone + carbon disulphide,
  2. Acetone + benzene
  3. Carbon tetrachloride + chloroform or Toluene
  4. Methyl alcohol + water
  5. Acetone + C2H5OH

Non-ideal solutions showing negative deviations:  Negative deviation occurs when total vapour pressure for any mole fraction is less than what is expected according to Raoult’s law. This happens when the new interactions are stronger than the interaction in the pure component
(A – B > A – A or B – B interactions).

It forms maximum boiling azeotrope, for example, CHCl3+ CH3COCH3.For such solutions, ΔV and ΔH are negative.

Examples:

  1. Chloroform + benzene or diethyl ether
  2. Acetone + aniline
  3. Nitric acid (HNO3) + water
  4. Acetic acid + pyridine

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