Solutions 3 Flashcards
what are ideal solutions
The solutions which obey Raoult’s law over the entire range of
concentration are known as ideal solutions.
what are the 3 important properties of ideal soluions
i) They obey Raoult’s Law at all concentrations
ii)dmixH=0
dmixV=0
The enthalpy of mixing of the pure
components to form the solution is zero and the volume of mixing is
also zero.It means that no heat is absorbed or evolved when the components
are mixed. Also, the volume of solution would be equal to the sum of
volumes of the two components.
iii). In pure components, the intermolecular attractive interactions will
be of types A-A and B-B, whereas in the binary solutions in addition
to these two interactions, A-B type of interactions will also be present.
If the intermolecular attractive forces between the A-A and B-B are
nearly equal to those between A-B, this leads to the formation of ideal
solution.
examples of ideal solutions
A perfectly ideal solution is rare but some solutions are nearly
ideal in behaviour. Solution of n-hexane and n-heptane, bromoethane
and chloroethane, benzene and toluene, etc. fall into this category.
what are 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.
what is positive deviation from raoults law
If the vapour pressure of a solution is higher than that predited by raoult’s law then its said to exhibit positive deviation.
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.
dmixH>0
dmixV>0
examples of solutions whcih show positive deviation
i)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
ii)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.
what is negative deviation from raoults law
Those solutions in which the vapour pressure is lower than that predicted by raoult’s law is said to exhibit 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.
dmixH<0
dmixV<0
examples of negative deviation from raoult’s law
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.( carbonyl oxygen with acidic hydrogen of chcl3)
This decreases the escaping tendency of molecules for each
component and consequently the vapour pressure decreases resulting
in negative deviation from Raoult’s law
define azeotropes
Some liquids on mixing, form azeotropes which are binary mixtures
having the same composition in liquid and vapour phase and boil at
a constant temperature. 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.
minimum boiling azeotrope example
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.
maximum boiling azeotrope
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.
