Ideal and Non-ideal solutions

Ideal and Non-ideal solutions

Ideal solutions

An ideal solution may be defined as the solution which obeys Raoult’s law under all conditions of temperatures and concentrations. Such solutions are formed by mixing the two components which are identical in molecular size and have almost identical intermolecular forces. 

The ideal solutions have the following characteristics :

  1. Heat change on mixing is zero (ΔHmixing = 0)
  2. Volume change on mixing is zero (ΔVmixing = 0)
  3. Obeys Raoult’s law
Raoult's law graphical representations (Vapour pressure of Ideal solutions)

Examples of ideal solution :

  • Benzene + Toluene
  • n-Hexane + n-Heptane
  • Ethyl chloride + Ethyl bromide
  • Chlorobenzene + Bromobenzene

Non-Ideal solutions

The solutions which do not obey Raoult’s law under all conditions of temperatures and concentrations are called non-ideal solutions.

The non-ideal solutions have the following characteristics :

  1. Heat change on mixing is not equal to zero (ΔHmixing ≠ 0)
  2. Volume change on mixing is not equal to zero (ΔVmixing ≠ 0)
  3. Do not obey Raoult’s law

Types of non-ideal solution :

  1. Non-ideal solution showing positive deviations from Raoult’s law.
  2. Non-ideal solution showing negative deviations from Raoult’s law.
Non-ideal solution showing positive deviation from Raoult’s law

When the partial pressure of components is found to be more than expected on the basis of Raoult’s law. As a result, total vapour pressure of the solution is greater than expected.

The intermolecular interactions between components A – B are of less magnitude as in pure components A – A and B – B.

Characteristics :

  1. Heat change on mixing is always positive (ΔHmixing = positive)
  2. Volume change on mixing is also always positive (ΔVmixing = positive)
  3. Partial pressure is found to be more than expected
  4. Behave as minimum boiling azeotropes
Non ideal solutions showing positive deviation from the raoults law Ideal and Non-ideal solutions

Examples :

  • Acetone + Carbon disulphide
  • Acetone + Ethyl alcohol
  • Acetone + Benzene 
  • Methyl alcohol + Water
Non-ideal solution showing negative deviation from Raoult’s law

When the partial pressure of components is found to be less than expected on the basis of Raoult’s law. As a result, total vapour pressure of the solution is lower than expected.

The intermolecular interactions between components A – B are of greater magnitude as in pure components A – A and B – B.

Characteristics :

  1. Heat change on mixing is always negative (ΔHmixing = negative)
  2. Volume change on mixing is also always negative (ΔVmixing = negative)
  3. Partial pressure is found to be less than expected
  4. Behave as maximum boiling azeotropes.
Non ideal solutions showing negative deviation from the raoults law Ideal and Non-ideal solutions

Examples :

  • Chloroform + Acetone  
  • Acetone + Aniline
  • Chloroform + Benzene  
  • HCl + Water
  • Chloroform + Diethyl ether

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