L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 23: Phase Equilibrium III
Chpt: 23 p.1
Three Component Systems
(A)
The Distribution Law
When a solute is added to a pair of immiscible liquids. it may dissolve in both liquids. In this case, the
solute will distribute itself between the two solvents. A dynamic equilibrium is set up when rate of flow
of solute in both directions (from solvent 1 to 2 and vice versa) is equal. This often occurs when the
ratio of the concentration of solute in the two solvents reaches a constant value at a particular
temperature.
TheDistribution Law or Partition Law states that a solute distributes itself between immiscible liquids
in a constant ratio of concentration irrespective of the amount of solute added.
The partition coefficient of a solute between the two immiscible liquids can be expressed as
Partition coefficient (k) =
Note
<1>
concentration of solute in 1
concentration of solute in 2
The partition coefficient
different temperatures.
k is a constant only at a fixed temperature. It may vary under
<2>
The Distribution law hold only for dilute solutions, and when association and dissociation of the
solute does not take place in both solvents.
(B)
Measurement of Distribution (Partition) Coefficient and its Approximation to a Constant Value
Example :
The following is an experimental accoünt to investigate the partition of butane-1,4-dioic
acid between ether (ethoxyethane) and water.
Procedure :
<1>
<2>
<3>
<4>
Experiment
no
1
2
3
4
5
About 25 cm3 of water is delivered into a separating funnel.
Suitable weight of the acid (0.5 to 1.5g) is then added.
About 25 cm3 of ethoxyethane is finally added.
The mixture is shaken well until all the acid has dissolved in both layers, and is
left for equilibrium.
<5> The lower aqueous layer is run into a beaker and 10 cm3 was pipetted and then
titrated with 0.5 M NaOH solution using phenolphthalein as indicator,
<6> The boundary layer is discarded.
<7> 10 cm3 of the ethereal solution is pipetted and small amount of water is then
added. The mixture is titrated with 0.1M NaOH.
<8> Repeat the Procedures 1 to 7 for different masses.
Equilibrium
Equilibrium concentration
concentration
of acid in water layer (M)
of acid in ether layer (M)
0.023
0.152
0.028
0.182
0.036
0.242
0.044
0.300
0.052
0.358
Partition coeffcient of acid
between water and ether
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 23: Phase Equilibrium III
(a)
(b)
(c)
(d)
Chpt: 23 p.2
Why is the initial measurement of the volumes of distilled water and ethoxyethane not
necessary to be highly accurate?
Why should the boundary layer be discarded?
Why is a small volume of water added in the titration of the ethereal solution and why good
shaking is important after each addition of alkali? Calculate the average value of the partition coefficient from the above results. How would you
decide whether any set of result should be discarded or not in the calculation of the
overall average value.
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 23: Phase Equilibrium III
Chpt: 23 p.3
Example:
Iodine can distribute itself between water and tetrachloromethane 0.9656g of iodine is added to a
mixture of 50 cm3 of water and 50 cm3 of tetrachloromethane. The mixture is shaken well and allowed
to stand. 25 cm3 of the aqueous layer require 4.40 cm3 of 0.010 M thiosuiphate for complete reaction.
Calculate the partition coefficient of iodine between water and tetrachloromethane.
ANSWER:
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 23: Phase Equilibrium III
(C)
Chpt: 23 p.4
Application of the Distribution Law
Solvent Extraction
The most common laboratory example of solvent extraction ether extraction. This is used to separate
the products of an organic synthesis from water. The aqueous solution is shaken up with ethoxyethane
(ether) in a separating funnel and allowed to separate. The product is thus removed from inorganic
impurities which are soluble in water. The solution may be subjected to several separations. Finally the
ethoxyethane is evaporated off, leaving the organic product.
Note :
Repeated extractions using small portions of solvent are more efficient than using a single but large
volume of solvent.
Example
A solution of 4g of a substance y in 25.0 cm3 of aqueous solution is in equilibrium at room temperature
with ethereal solution of y containing 80 g of y in 100 cm Calculate the mass of y that could be
extracted by shaking 100 cm3 of an aqueous liquid containing 5.0 g of y with
(a)
50.0 cm of ether
(b)
25.0 cm3 of ether twice at room temperature.
ANSWER
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 23: Phase Equilibrium III
(D)
Chpt: 23 p.5
Chromatography
The theory
1.
Chromatography is a technique used to separate the components of a mixture. There are two
types of chromatography. Both depend on the partition of the components of the mixture
between a stationary phase and a mobile phase.
2.
In adsorption chromatography the stationary phase is a solid. The solid adsorbs a portion of
each component in the mixture.
3.
In partition chromatography the stationary phase is a liquid. The components are partitioned
between the liquid and the mobile phase.
In both types of chromatography, portions of each component remain dissolved in the mobile phase.
The mobile phase flows continuously over the stationary phase as it does so separate the components
on the stationary phase.
4.
Both chromatographic methods thus consist essentially of two steps:
<1> Distribution of the components between the two phases.
<2> Separating of the components on or in the stationary phase by a continuous flow of the
mobile phase.
The distribution of a component between the two phases is given by the partition coefficient D.
D=
Concentration of component in the moving phase
Concentration of component in the stationary phase
A component of a mixture with a high value for D remains largely dissolved in the mobile
phase and thus passes over the stationary phase rapidly.
A component with a small value for D remains largely adsorbed on the stationary phase. As the
moving phase passes over it, this component moves slowly along the stationary phase.
5.
Chromatography is particularly useful in organic synthesis in separating and recovering the
components of a mixture. It is used in both quantitative and qualitative analysis to identify
separated components of a mixture and also to determine the purity of a substance.
II
Paper Chromatography
This technique is a form of partition chromatography since the stationary phase is a liquid. The
stationary phase is water adsorbed on paper. The mobile phase is usually organic liquid.
1.
A solution of the mixture of several components to be separated is applied to a strip of
chromatography paper. This is hung in a glass tank so that the end dips into solvent in the
bottom of the tank. The solvent may be water, ethanol , glacial ethanoic acid, butanol, etc. or a
mixture of any of these.
2.
As the solvent rises through the paper, it meets the sample and the component band spread out.
The separation is stopped when the solvent has traveled nearly to the top of the paper The
distance traveled by the solvent front is measured.
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 23: Phase Equilibrium III
Chpt: 23 p.6
Then for each component, the Rf value, which is related to its partition coefficient, is given by
DisTance traveled by the component
Rf =
DisTance traveled by the solvent front
Example:
The following shows a developed paper with five spots
The Rf value can be used to assist. identifying the components of a mixture. The separated components
can be obtained by cutting the paper into strips and dissolving out each compound.
3.
There is a different design of chromatography tank, with a trough at the top of the tank to hold
the solvent. The paper hangs down from the trough, and the solvent descends through the paper.
The two techniques are naturally described as ascending and descending paper chromatography.
ascending paper chromatography : the solvent moves upwards
descending paper chromatography : the solvent moves downwards
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Chapter 23: Phase Equilibrium III
Chpt: 23 p.7
Paper Chromatography
This technique provides an easy way to separate the components of a mixture.

A drop of mixture is placed in one corner of a square of absorbent paper.

One edge of the paper is immersed in a solvent. (a)

The solvent migrates up the sheet by capillary attraction.

As it does so, the substances in the drop are carried along at different rates. (b)

Each compound migrates at a rate that reflects
o
the size of its molecule and
o
its solubility in the solvent.

After a second run at right angles to the first (often using a different solvent), the various
substances will be spread out at distinct spots across the sheet, forming a chromatogram. (c)

The identity of each spot can be determined by comparing its position with the position
occupied by known substances under the same conditions.

In many cases, a fragment of the paper can be cut away from the sheet and chemical analysis
run on the tiny amount of substance in it.