Question: 100% NCERT covered from Particular Keyword.Explanations are 100% from NCERT | [Difficult level: Easy] From NCERT NEET Derived Question.
Which one of the following binary liquid systems shows
positive deviation from Raoult’s law?
(a) Benzene-toluene
(b) Carbon disulphide-acetone
(c) Phenol-aniline
(d) Chloroform-acetone
Answer: b
Mixtures of ethanol and acetone
Showa positive daviation. 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
.
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.
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.
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.
