Distribution law

Distribution Law
What is Distribution law?
Immiscible liquids
Explanation
APPLICATION OF DISTRIBUTION LAW
Limitations of Distribution Law
Contrast and Comparison between separation through Separating funnel and Fractional Distillation

1. Assignment No.1

Course Code # CHEM-706
Course Title # Adv. Chromatographic Techniques

Submitted by:
Student name Kashif Ali
Roll no. 029
Section B
Semester 2
nd

Class M Phil Chemistry
[email protected]

Submitted to:
Ma’am Ayesha Bukhari

Department of Chemistry
Minhaj University Lahore

2. Distribution Law
1.1 Introduction
This law was given by Nernst, Hermann Walther in 1891. He was a German chemist known for his work in
thermodynamics, physical chemistry, electrochemistry, and solid state physics. His formulation of the Nernst
heat theorem helped pave the way for the third law of thermodynamics, for which he won the 1920 Nobel Prize
in Chemistry.
1.2 What is Distribution law?
When a solute is taken up with two immiscible liquids, in both of which the solute is soluble, the solute
distributes itself between the two liquids in such a way that the ratio of its concentration in the two liquid phases
is constant at a given temperature provided the molecular state of the distributed solute is same in both phases.
1.3 Immiscible liquids Those liquids which do not dissolve with each other e,g Water and Ether etc.
Note This law is also known as Nernst Distribution law, Nernst Partition law, Distribution law or simply
Partition law.
1.4 Explanation
If a solute X distributes itself between two immiscible solvents A and B at constant temperature and X is in the
same molecular condition in both solvents, then


= KD (1
If C1 denotes the concentration of the solute in solvent A and C2 the concentration in solvent B, Nernst’s
Distribution law can be expressed as


= KD (2
In equation (1 and (2 constant KD (or simply K) is called the Distribution coefficient or Partition coefficient
or Distribution ratio.
This is an equilibrium law. When the distribution of the solute X has reached dynamic equilibrium, the rate (R1)
at which molecules of X pass from solvent A to B is proportional to its concentration (C1) in A. The rate (R2) at
which molecules of X pass from solvent B to A is proportional to its concentration (C2) in B. Also, at
equilibrium, the rate of migration of solute from one solvent to the other is equal as shown in fig 1. Thus we
have,

Fig. 1 Distribution of two solutes A and B in immiscible liquids at equilibrium can be seen according to Distribution law.

At equilibrium, the number of molecules of X passing from solvent A into B is proportional to its concentration
of A and vice versa. Also, the rate of migration of solute molecules from A to B and B to A is equal. So it can
be written as
R1 ∝ C1
Or R1 = k1 × C1 where k1 is a constant

3. Again R2 ∝ C2
Or R2 = k2 × C2 where k2 is a constant
Since at equilibrium R1 = R2
k1 × C1 = k2 × C2





KD (3
Equation (3 is the Nernst’s Distribution law equation. Since k1 and k2 are constants at the same temperature,
the distribution coefficient KD is also constant if temperature is fixed.
1.5 APPLICATION OF DISTRIBUTION LAW
There are numerous applications of distribution law in the laboratory as well as in industry
1) Solvent Extraction-
This is the process used for the separation of organic substances from aqueous solutions. The aqueous solution
is shaken with an immiscible organic solvent such as ether or benzene in a reparatory funnel. The distribution
ratio being in favor of ether, most of the organic substance passes into the ether layer. The ethereal layer is
separated and ether distilled off. Organic substance is left behind.
2) Partition Chromatography
A paste of the mixture is applied at the top of a column of silica soaked in water. Another immiscible solvent
(hexane) is allowed to flow down the column. Each component of the mixture is partitioned between the
stationary liquid phase (water) and the mobile liquid phase (hexane). The various components of the mixture are
extracted by hexane in order of their distribution coefficients.
(3) De silverization of Lead (Parke’s Process)
When molten zinc is added to molten lead containing silver (argentiferous lead), zinc and lead form immiscible
layers and silver is distributed between them. Since the distribution ratio is about 300 in favor of zinc at 800º C,
most of silver passes into the zinc layer. On cooling the zinc layer, an alloy of silver and zinc separates. The Ag-
Zn alloy is distilled in a retort when zinc passes over leaving silver behind. The lead layer still contains un-
extracted silver. This is treated with fresh quantities of molten zinc to recover most of the silver
(4) Confirmatory Test for Bromide and Iodide
The salt solution is treated with chlorine water. Small quantity of bromine or iodine is thus liberated. The
solution is then shaken with chloroform. On standing chloroform forms the lower layer. The free bromine or
iodine being more soluble in chloroform concentrates into the lower layer, making it red for bromine and violet
for iodine.
5) Determination of Association
When a substance is associated (or polymerized) in solvent A and exists as simple molecules in solvent B, the
Distribution law is modified as
n√Ca/Cb = k
When n is the number of molecules which combine to form an associated molecule.
(6) Determination of Dissociation
Suppose a substance X is dissociated in aqueous layer and exists as single molecules in ether. If x is the degree
of dissociation (or ionisation), the distribution law is modified as
C1 /C2)(1-x) = K
Where C1 = concentration of X in benzene and C2 = concentration of X in aqueous layer
The value of x can be determined from conductivity measurements, while C1 and C2 are found experimentally.
Thus the value of K can be calculated. Using this value of K, the value of x for any other concentrations of X
can be determined.
(7) Determination of Solubility
Suppose the solubility of iodine in benzene is to be determined. Iodine is shaken with water and benzene. At
equilibrium concentrations of iodine in benzene (Cb) and water (Cw) are found experimentally and the value of
distribution coefficient calculated.
Cb / Cw = Kd
Sb/ Sw = Kd
Where Sb = solubility in benzene; and Sw = solubility in water.

4. (8)The degree of hydrolysis of substances
The hydrolysis of aniline hydrochloride in water can be followed from the distribution of aniline hydrochloride
between water and benzene. Thus for this reaction, Aniline hydrochloride (Salt) + Water (H2O) ↔ Aniline
(Base) + Hydrochloric acid (Acid)
Thus, Kh = {[Base] x [Acid]} / {[Salt] x [H2O]}
By applying the principle of equilibrium it can be shown that,
Kh = {C12 (1+vK)} / {C2 – C1 (1+vK)}
Where C1 is the concentration of the base in aqueous layer (C1 and C2 are expressed in Mol L-1), C2 the
original concentration of the salt, K the distribution coefficient and v is the volume of benzene in liters added to
1 liter of water.
1.6 Limitations of Distribution Law
 Constant temperature. The temperature is kept constant throughout the experiment.
 Same molecular state. The molecular state of the solute is the same in the two solvents. The law does
not hold if there is association or dissociation of the solute in one of the solvents.
 Equilibrium concentrations. The concentrations of the solute are noted after the equilibrium has been
established.
 Dilute solutions. The concentration of the solute in the two solvents is low. The law does not hold when
the concentrations are high.
 Non-miscibility of solvents. The two solvents are non-miscible or only slightly soluble in each other.
The extent of mutual solubility of the solvents remains unaltered by the addition of solute to them.

2. Contrast and Comparison between separation through
Separating funnel and Fractional Distillation
Separation through Separating funnel Separation through Fractional distillation

Separating funnel
A separating funnel is used to separate immiscible
liquids. Liquids that do not mix with each other are
said to be immiscible. Two immiscible liquids, such
as oil and water, can be separated by
using a separating funnel.
Example
Ether extraction is the common example of
separation through separating funnel.
Principle
The principle behind separating funnel is that
immiscible liquids separate out in layers depending
on their densities.
Parts:
 Stopper
 Body
 Stopcock
 Drain Tip
 Stand





Fractional distillation
Fractional distillation is a process by which components in
a chemical mixture are separated into different parts
(called fractions) according to their different boiling
points.
Example
Fractional distillation of Crude oil is the common example
of separation through fractional distillation.
Principle
The principle of fractional distillation is that different
liquids boil at different temperature. So, When the mixture
of immiscible liquids is heated, the liquid with lower
boiling point boils and turns into vapors.
Parts
 Thermometer
 Condenser
 Collecting flask
 Benson burner/Stove/Heater
 Boiling flask
 Stand

5. Diagram



Applications
1) To separate mixture of oil and water.
2) In the extraction of iron from its ore, the
lighter slag is removed from the top by this
method to leave the molten iron at the bottom
in the furnace.
3) In the extraction of the iron from its ore. The
lighter slag is removed from the top by this
method to leave the molten iron at the bottom
in the furnace.
Advantages
 Good separation is achieved
 Running cost is low
 Easy to handle the apparatus.
 Equally used in the lab as well as industry.
Disadvantages
1) Provide good separation for only small
fraction of mixture.
2) Operation process is time consuming
3) The largest risk when using a separating
funnel is that of pressure build-up.










Diagram

Applications
1) Fractional distillation is used for the purification of
water as well as separating acetone and water.
2) Fractional distillation is used in several industries
like oil refineries and chemical plants mainly for
purification and separation of many organic
compounds.
3) Fractional distillation is also used for the
separation of (liquefied) air. Components like
liquid nitrogen and oxygen as well as concentrated
argon are obtained.
4) Distillation is used in the production of high-
purity silicon from chlorosilanes. The silicon is
widely used in semiconductors.
Advantages
 Better separation of liquids
 Good at purifying liquids containing many
different components.
 Equally used in the lab as well as industry.
Disadvantages
1) Slower
2) Requires more energy
3) More complicated and expensive setup

6. 3. References
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2. Kelly, Ryan T., et al. "The ion funnel: theory, implementations, and applications." Mass spectrometry reviews 29.2
(2010): 294-312.
3. Odebunmi, E. O., Ogunsakin, E. A., & Ilukhor, P. E. P. (2002). Characterization of crude oils and petroleum
products:(I) Elution liquid chromatographic separation and gas chromatographic analysis of crude oils and
petroleum products. Bulletin of the Chemical Society of Ethiopia, 16(2), 115-132.
4. Experimental and theoretical applications of thermodynamics to chemistry / by Dr. Walther Nernst.
5. Definition of separating funnel | Dictionary.com. (2020). Retrieved 16 April 2020, from
https://www.dictionary.com/browse/separating-funnel
6. www.dictionary.com. 2020. Definition Of Separating Funnel | Dictionary.Com. [online] Available at:
<https://www.dictionary.com/browse/separating-funnel> [Accessed 16 April 2020].
7. Nernst Distribution law. (2020). Retrieved 16 April 2020, from
https://fch.upol.cz/skripta/fcc_and_zvem_english/FCH/Nernst%20Distribution%20law.htm
8. GCSE CHEMISTRY - What is a Separating Funnel? - How can Liquids be Separated using a Separating Funnel?
- GCSE SCIENCE. (2020). Retrieved 16 April 2020, from https://www.gcsescience.com/e-separating-funnel.htm
9. http://sites.science.oregonstate.edu/~gablek/CH361/bare_sepfunnel.htm
10. Barkan, Diana Kormos, and Diana Kormos Buchwald. Walther Nernst and the transition to modern physical
science. Cambridge University Press, 1999.
11. https://www.meritnation.com/ask-answer/question/what-are-the-applications-of-separating-funnel/is-matter-around-us-
pure/9540049
12. https://itstillworks.com/advantages-disadvantages-fractional-distillation-8513450.html
13. https://connectusfund.org/6-advantages-and-disadvantages-of-fractional-distillation
14. https://www.qsstudy.com/chemistry/applications-distribution-law
15. Cantwell, Frederick F., and Manon Losier. "Liquid–liquid extraction." Comprehensive Analytical Chemistry 37
(2002): 297-340.


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