Essay on ideal solutions, non-ideal solutions and Raoult’s law
Write an essay on ideal and non-ideal solution of two miscible liquids. In your essay, it
should include the intermolecular forces, variation of vapour pressure and boiling point
against composition of the solution.
Scheme of Work
I
Ideal solutions
(5 marks)
(a) Interaction of molecules in solution
- escaping tendency; H ; V in mixing
(b) Examples: benzene & methylbenzene; hexane & heptane ; two different esters
- structures, polarity, chemical properties of the components
(c) VP can be predicted by Raoult's Law
(d) Variation of VP of solution vs composition
- vapour is richer in more volatile component
(e) Variation of boiling point of solution vs composition
II
Non-ideal solutions - positive deviation from ideality
(5 marks)
(a) Interaction of molecules in solution
- escaping tendency; H ; V in mixing
(b) Examples: chloroform & propanone
- polarity, hydrogen bond
(c) VP lower than that predicted by Raoult's Law
(d) Variation of VP of solution vs composition
- minimum point, azotropic mixture
(e) Variation of boiling point of solution vs composition
- maximum point, azotropic mixture
(f)
Fraction distillation
- incomplete separation of components in ideal solution
II
Non-ideal solutions - negative deviation from ideality
(2 marks)
Suggested Essay
What is an ideal solution?
An ideal solution is a mixture of liquids of similar chemical structures and polarities. For
an example, a mixture of hexane and heptane is an ideal solution. Both hexane and
heptane are hydrocarbons and non-polar molecules. When they are mixed together, it is
found experimentally that the boiling points of mixtures varies almost linearly with
composition of constituents.
(3 x ½ M)
temp.
temp.
b.p.
of
pure A
b.p.
of
pure B
0.0
1.0
0.2
0.8
0.4
0.6
0.6
0.4
0.8
0.2
1.0 cB
0.0 cA
(½ M)
The boiling point-composition relationship can be explained in terms of their intermolecular
forces. Since they have similar chemical structures and they are non-polar.
Their
intermolecular forces are van der Waals’ forces, so that
Intermolecular attraction
between hexane & heptane
in the mixture
=
Intermolecular attraction
between heptane & heptane
in the pure liquid
=
Intermolecular attraction
between hexane & hexane
in the pure liquid
(½ M)
Since the intermolecular forces are similar, the escaping tendency of each component from
the solution into vapour is the same as their respective escaping tendency in pure liquids.
Tthe ideal solution obeys Raoult’s law.
(½ M)
Raoult’s law states that the partial pressure of a component in an ideal solution is equal
to the product of its mole fraction and the vapour pressure of the pure liquid at that
temperature.
(½ M)
For an ideal solution of A and B,
Partial pressure of A, PA = XA PAo
Partial pressure of B, PB = XB PBo
where XA and XB = mole fraction of A and B respectively
PAo and PBo = vapour pressure of pure liquids A and B respectively
(½ M)
By Dalton's law of partial pressure, the total pressure over the solution AB is
PAB =Ptotal = PA + PB = XA PAo + XB PBo
(½ M)
VP
20 PBo
pure B
20
Liquid curve
15
15
PAo 10
10
pure A
5
5
0.0
1.0
0.2
0.8
0.4
0.6
0.6
0.4
0.8
0.2
1.0 cB
0.0 cA
(½ M)
During the mixing process, molecules A try to get in between the molecules B.
Energy is
required to separate B and B; and A and A.
However, energy releases as different
molecules A and B come together.
Since the interaction A-A, B-B and A-B are similar,
there is no energy change , H  0 , or no volume change , V  0 , in mixing.
(2 x ½ M)
However, many liquids mixture do not obey Raoult’s law. In other words, they deviated
from Raoult’s law. Deviations from Raoults law can be positive or negative.
Positive Deviation from Raoult’s Law
When cyclohexane and ethanol are mixed together, the vapour pressure is greater
than that predicted by Raoult’s law.
(2 x ½ M)
Partial pressure of cyclohexane (A), PA > XA PAo
Partial pressure of ethanol (B), PB > XB PBo
where XA and XB = mole fraction of cyclohexane and ethanol respectively
PAo and PBo = vapour pressure of pure liquids cyclohexane
and ethanol respectively
In mixing cyclohexane and ethanol, cyclohexane molecules get in between the ethanol
molecules.
Many hydrogen bonds between ethanol molecules are broken up.
Since there is only weak van der Waals' forces between different molecules.
The total
intermolecular forces in the solution are reduced and the boiling point of the solution is
lower than expected.
(3 x ½ M)
Weak van der Waal's forces
between ethanol &
cyclohexane in the mixture
Strong hydrogen-bond
< between ethanol &
ethanol in the pure liquid
and
Strong van der Waals' forces
between cyclohexane &
cyclohexane in the pure liquid
Therefore, the molecules in the mixture have higher escaping tendency. The total vapour
pressure is greater than that predicted by Raoult’s law.
(½ M)
azeotropic
mixture
x
VP
temp.
PBo
pure B
VP for
ideal solution
PA o
pure A
temp.
Vapour curve
b.p.
of
pure A
Liquid curve
VP for
non-ideal solution
of negative deviation
azeotropic
mixture
x
0.0
1.0
0.2
0.8
0.4
0.6
0.6
0.4
0.8
0.2
1.0 cB
0.0 cA
0.0
1.0
0.2
0.8
0.4
0.6
0.6
0.4
0.8
0.2
.
b.p.
of
pure B
1.0 cB
0.0 cA
(2 x ½ M)
Moreover, it requires more energy to break intermolecular hydrogen-bonds between ethanol
molecules.
The mixing process is endothermic (H > 0) and the temperature decreases.
(½ M)
Besides, due to the weaker interaction, the total volume of solution increases (V > 0) in the
mixing process.
(½ M)
Negative Deviation from Raoult’s Law
When trichloromethane, CHCl3, and propanone are mixed together, the vapour pressure is
lower than that predicted by Raoult’s law.
Partial pressure of trichloromethane (A), PA < XA PAo
Partial pressure of propanone (B), PB < XB PBo
where XA and XB = mole fraction of CHCl3 and propanone respectively
PAo and PBo = vapour pressure of pure liquids CHCl3 and propanone
respectively
In pure liquids of trichloromethane and propanone, there are relative weak van der Waals’
forces. After mixing, hydrogen bonds are formed between trichloromethane and propanone
molecules and the boiling point of the solution is higher.
Strong hydrogen-bond
between propanone &
trichloromethane
in the mixture
>
Weak van der Waals' forces
between trichloromethane &
trichloromethane
in the pure liquid
and
Weak van der Waal's forces
between propanone &
propanone
in the pure liquid
Therefore, the molecules in the mixture have lower escaping tendency. The total vapour
pressure is lower than that predicted by Raoult’s law.
azeotropic
mixture
x
VP
VP for
non-ideal solution
of negative deviation
temp.
PBo
pure B
temp.
b.p.
of
pure A
VP for
ideal solution
Vapour curve
.
PA o
pure A
b.p.
of
pure B
Liquid curve
azeotropic
x mixture
0.0
1.0
0.2
0.8
0.4
0.6
0.6
0.4
0.8
0.2
1.0 cB
0.0 cA
0.0
1.0
0.2
0.8
0.4
0.6
0.6
0.4
0.8
0.2
1.0 cB
0.0 cA
Moreover, it releases more energy to form intermolecular hydrogen-bonds between
propanone and trichloromethane molecules.
The mixing process is exothermic (H < 0)
and the temperature increases.
Besides, due to the stronger interaction, the total volume of solution decreases (V < 0) in the
mixing process.