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Ideal and Non-Ideal Solutions
Ideal solutions:- “Ideal solutions are those solutions which obey the Raoult’s law under all the conditions of temperature and concentration”. An ideal solution must satisfy the following conditions: (graph same as shown for Raoult’s Law above)
- There will be no change in volume on mixing i.e. ΔVmix = 0
- There will be no change in enthalpy i.e. ΔHmix = 0
The above tow conditions are satisfied for a solution of solute (A) and solvent (B) if the intermolecular interactions between solute and solvent (A-B interactions) are of the same magnitude as between solute-solute (A-A interaction) and solvent – (B-B interaction). i.e. like interactions (A-A & B-B) are same as unlike in interactions (A-B).
Practically no solution is ideal solution. However the ideal behavior can be approached at high temperature and low concentration, because under these conditions like interactions are similar to unlike interactions. e.g. solutions of hexane and heptanes; chlorobenzene and brombenzene etc.
Non ideal solutions:- “Non ideal solutions are the solutions which do not obey the Raoult’s Law”. For these solutions ΔVmix≠ 0. And ΔHmix≠ 0. Also fro non ideal solutions like interactions are not equal to unlike interactions. the non-Ideal solutions are of two types:-
- Non- ideal solution showing positive deviations:-
In this type of deviation, the partial vapour pressure of each component (A or B) of solution is greater than the vapour pressure as expected according to Raoult’s law.
This type of deviations are shown by the solutions in which the unlike interactions (A-B) are weaker than like interactions (A-A & B-B). For these solutions ΔVmix> 0 (i.e. +ve) and ΔHmix> 0 (i.e. +ve). For example the solutions of water + Ehtyl Alcohol, water + Methyl Alcohol etc.
- Non – Ideal solutions showing negative deviation:-
In this type of deviation, the partial vapour pressure of each component (A or B) of solution id less than that expected by Raoult’s law (as shown in fig. 2). This type of deviations are shown by the solutions in which the unlike interactions (A-B) are stronger than like interactions (A-A & B-B). For these solutions ΔVmix< 0 (i.e. _ve) and ΔHmix< 0 (i.e. -ve). For example the solutions of water + HCl and water + HNO3 etc.