Role of Adsorption 2011

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Engineering Chemistry Unit III: Surface Chemistry

Material prepared by : Dr.S.L Branch: CSE B & CIVIL 1

Lecture Notes : Summary

Role of Adsorption

1. In heterogeneous catalysis ( Contact Theory)

Mechanism

The reaction in which the catalyst is in a different physical phase from the reactants

and products is termed heterogenous catalysis. The most important of such reactionsare those in which the reactants/products are in gas phase, while the catalyst is in solidstate. This process is called contact catalysis since the reaction occurs by contact of

reactants on the solid catalyst surface and not in gaseous phase.Ni

Eg: CH2 = CH2 + H2 → CH3 – CH3

Ethylene

Eg: Hydrogenation of ethylene in presence of Ni

The various steps involved are:

1. Diffusion of ethylene and hydrogen towards surface

2. Adsorption of ethylene and hydrogen at the active sites of nickel by vanderwaalsforce; The reactant molecules get crowded on the catalyst surface thereby increasing

the concentration on the surface.( fig 2)

3. Formation of activated complex:a. Dissociation of physisorbed hydrogen molecule into H- atoms and forms

chemisorbed surface bond with Nickel . The heat of adsorption evolved duringchemisorption provide necessary activation energy for the adsorption reaction

and forms intermediate activated complex with ethyleneb. Linking of H - atoms to ethylene to form activated complex which then

decomposes to form ethane.(fig 3a , 3b)





4. Desorption of ethane from the surface-(fig 4)

Ni Ni Ni Ni

Ni Ni Ni Ni

Ni Ni Ni Ni

H H

H C C H

Fig : 2

Fig : 3a

Fig : 3b: Activated

complex formation

Physisorbed

hydrogen

Chemisorbed

hydrogen





Ni Ni Ni Ni

H H

H

C

C

H

H H

Fig : 4: Desorption of products(ethane) from

catalyst surface





2. In Column Chromatography

Adsorption chromatography is based on the differences in the adsorption of substances on solid.

Principle:

It is found that closely related substances with almost similar and physical and

chemical properties which cannot be separated from one another by ordinary meansare adsorbed to different extents on the surface of adsorbent. This facilitates separation

and purification.

If a solution containing different solutes is poured down a column filled with finelydivided adsorbent, the solute most readily adsorbed is retained on the top layer along

with smaller amounts of other constituents. The less readily adsorbed constituents areheld on lower portions of the column.

A partial separation of the constituents of the mixture is thus easily achieved. The

separation is further enhanced by repeating and modifying the process. This process isknown as column chromatography first developed by a Polish botanist, M.S.Tswett,

to separate plant pigments.

Some common adsorbents used are : aluminium oxide, silica gel, MgO, MgCO3,

CaCO3,Charcoal, cellulose, Fuller’s earth.

Solvents used are: benzene, cyclohexane, chloroform, ethylalcohol, water , CCl4,

pyridine etc.

Process: The various stages are

1. Separation of mixture into different components : When a mixture of

components dissolved in a suitable solvent is passed slowly through the columnpacked with alumina (say), a number of horizontal bands of different colors are

produced in the column. This gives only a partial separation of various constituents.

2. Development of chromatogramThis initial separation of constituents can be improved by passing either the

original or some other suitable solvent slowly through the column. The solvent used iscalled Eluent.





The various colored zones now becomes more sharply defined. The process of improving the separation of a mixture by passing the original solvent or some othersuitable solvent is known as “development of the chromatogram”.

The process of separation of a mixture of two components X and Y isillustrated below (fig a – e next page)

3. Isolation & Estimation of components :

In order to separate or to estimate the various constituents, the chromatogram after

development is pushed out of the glass tube and the various zones are cut with a knifeat the boundaries. The colored components are dissolved separately in suitable

solvents and estimated quantitatively. The process of recovery of various substancesfrom the chromatogram is known as Elution.

Colorless compounds are observed by either exposing UV light on the column or by

spraying suitable chemical reagent in order to make them colored.

Applications:a. Quantitative separation of two or more components of a mixture (separation of

metal ions)b. Purification of substances from their contaminants

c. Concentration of solutes from dilute solutionsd. Identification of products.(eg : isomers of organic compounds





Schematic Representation ogf separation of components in a Mixture

Sample

X+Y

AdsorbentSam le X+Y

Adsorbent

Eluent

Y

Adsorbent

Eluent

Y

Sam le X+Y

X

Adsorbent

Eluent

Y

X

Adsorbent

Fig : ( a ) : Mixture of X

& Y Fig : ( b )

Fig : ( c ) Fig : ( d )

Sample

X+Y

Adsorbent





Eluent

Y

X

Adsorbent

Eluent

Eluent

Fig : ( e ) : Distinct

bands of X & Y

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Role of Adsorption 2011