The solutions which obey Raoult’s law at all compositions of solute in solvent at all temperature are called ideal solution.
Ideal solutions can be obtained by mixing two components with identical molecular size, structure and they should have almost same inter molecular attraction e.g., two liquids A and B form and ideal solution when A – A and B – B molecular attractions will be same and hence A-B molecular attraction will be almost same as A-A and B-B molecular attraction.
An ideal solution should have following characteristics
1. It should obey Raoult’s law i.e., PA = XA P°A and PB = XB P°B
2. ΔHmixing = 0, i.e. no heat should be absorbed or evolved during mixing
3. ΔVmixing = 0, i.e. no expansion or contraction on mixing
Examples of Ideal solutions :
Ethyl chloride and ethyl bromide
n-hexane and n-heptane
CCl4 and SiCl4
The solution which deviate from ideal behaviour are called non ideal solution or real solutions and they do not obey Raoult’s law over entire range of composition. It has been found that on increasing dilution, a non ideal solution tend to be ideal.
For non ideal solutions,
PA ≠ XA P°A , PB ≠ XB P°B i.e, they do not obey Raoult’s law
ΔHmixing ≠ 0
(ii) ΔHmixing ≠ 0
Here we may have two cases
(A) When i) PA > XA P°A
PB > XB P°B
(ii) ΔHmix > 0
(iii) ΔVmix > 0
Such a solution show positive deviation from Raoult’s Law and the observed boiling point of such solutions is found to be less than the calculated value.
e.g. cyclohexane and ethanol.
In ethanol the molecules are held together due to hydrogen bonding, when cyclohexane is added to ethanol the molecules of cyclohexane tend to occupy the space between ethanol molecules due to which some hydrogen bonds break due to which inter molecular attraction between cyclohexane and ethanol is reduced and hence show higher vapour pressure.
(B) (i) PA < XA P°A
(ii) ΔHmix < 0
(iii) ΔVmix < 0
Solutions of above type show negative deviation from Raoult’s law and their observed boiling point is found to be higher than the calculated value .
eg. When acetone and chloroform are mixed together a hydrogen bond is formed between them which increases inter molecular attraction between them and hence decreases the vapour pressure.