Unit – III C – 02

Non-ideal Solutions:

  • When a solution does not obey Raoult’s law over the entire range of concentration, then it is called non-ideal solution.
  • The vapour pressure of such a solution is either higher or lower than that predicted by Raoult’s law. If it is higher, the solution exhibits positive deviation and if it is lower, it exhibits negative deviation from Raoult’s law.

Positive Deviation:

  • The cause for these deviations lie in the nature of interactions at the molecular level. In case of positive deviation from Raoult’s law, A-B interactions are weaker than those between A-A or B-B, i.e.
  • In this case the intermolecular attractive forces between the solute-solvent molecules are weaker than those between the solute-solute and solvent-solvent molecules. This means that in such solutions, molecules of A (or B) will find it easier to escape than in pure state. This will increase the vapour pressure and result in positive deviation.
  • Mixtures of ethanol and acetone behave in this manner. In pure ethanol, molecules are hydrogen bonded. On adding acetone, its molecules get in between the host molecules and break some of the hydrogen bonds between them. Due to weakening of interactions, the solution shows positive deviation from Raoult’s law.


Positive Deviation

  • In a solution formed by adding carbon disulphide to acetone, the dipolar interactions between solute-solvent molecules are weaker than the respective interactions among the solute-solute and solvent-solvent molecules. This solution also shows positive deviation.
  • For solution showing positive deviation from Rault’s law ΔmixH > 0 and ΔmixV > 0,

    Examples of solutions showing positive deviation from Rault’s law

  • acetone + ethanol
  • acetone + CS2
  • water + methanol
  • water + ethanol
  • CCl4 + toluene
  • CCl4 + CHCl3
  • acetone + benzene
  • CCl4+ CH3OH
  • cycloheane + methanol


Negative Deviation:

  • In case of negative deviations from Raoult’s law, the intermolecular attractive forces between A-A and B-B are weaker than those between A-B and leads to decrease in vapour pressure resulting in negative deviations.
  • An example of this type is a mixture of phenol and aniline. In this case the intermolecular hydrogen bonding between phenolic proton and lone pair on nitrogen atom of aniline is stronger than the respective intermolecular hydrogen bonding between similar molecules. Similarly, a mixture of chloroform and acetone forms a solution with negative deviation from Raoult’s law. This is because chloroform molecule is able to form hydrogen bond with acetone molecule as shown.

Hydrogen Bonding

  • This decreases the escaping tendency of molecules for each component and consequently the vapour pressure decreases resulting in negative deviation from Raoult’s law.
  • This decreases the escaping tendency of molecules for each component and consequently the vapour pressure decreases resulting in negative deviation from Raoult’s law.

Negative Deviations

  • For solution showing positive deviation from Rault’s law ΔmixH < 0 and ΔmixV < 0,


Examples of solutions showing negative deviation from Rault’s law

  • acetone + aniline
  • acetone + chloroform
  • methanol + acetic acid
  • water + nitric acid
  • chloroform + diethyl ether
  • water + HCl
  • acetic acid + pyridine
  • chloroform + benzene

Azeotropes:

  • Azeotropes are binary mixtures having the same composition in liquid and vapour phase and boil at a constant temperature.
  • Some liquids on mixing, form azeotropes. In such cases, it is not possible to separate the components by fractional distillation. There are two types of azeotropes called minimum boiling azeotrope and maximum boiling azeotrope.
  • The solutions which show a large positive deviation from Raoult’s law form minimum boiling azeotrope at a specific composition.
    For example, ethanol-water mixture (obtained by fermentation of sugars) on fractional distillation gives a solution containing approximately 95% by volume of ethanol. Once this composition, known as azeotrope composition, has been achieved, the liquid and vapour have the same composition, and no further separation occurs.
  • The solutions that show large negative deviation from Raoult’s law form maximum boiling azeotrope at a specific composition. Nitric acid and water is an example of this class of azeotrope. This azeotrope has the approximate composition, 68% nitric acid and 32% water by mass, with a boiling point of 393.5 K.

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