surace chemistry

8/8/2019 Surace Chemistry

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SURACE CHEMISTRY

the material providing the

surface upon which

adsorption occurs is

known as the adsorbent The substance adsorbed

or attached is called

adsorbate

ADSORPTION :

THE PHENOMENON OF CONCENTRATION OR ASSIMILATION

OF A GAS OR LIQUID AT THE SURFACE OF A SOLID OR LIQUID

WITH WHICH IT IS IN CLOSE PROXIMITY OR CONTACT IS

CALLED


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Absorption

It is a bulk phenomenon in which the substance assimilated

is uniformly distributed throughout the body of the solid orliquid.

This is slow process compared to absorption


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Classification of adsorption:

1) Physical adsorption:

In which the adsorbed molecules are held to thesurface of the adsorbent by week physical forces

This is reversible


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2) Chemical adsorption :

The force which holds the adsorbed molecules is of

valence type. It involves interaction of the free valencies at the surface

molecules.

The heat evolved is considerably higher than evolved in

physical adsorption


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Langmuirs adsorption isotherm

Valencies at the surface of adsorbent atoms are not fully satisfied,they can share electrons with atoms present in the adsorbate to form

bonds resembling a covalent linkage

If the bond is weak a physical adsorption takes place

If the bond is strong chemisorption occurs.

It consist 2 opposing forces, condensation and evoporation(desorption ) of the molecules.

When the adsorption starts condunsation is highest, as the surface

becomes progresively covered the rate of condensation gradually

decreases.

Ultimately a dynamic equilibrium is set up when the rate ofcondensation becomes equal to rate of evoporation.


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Consider a surface on which the molecules of a gas are

condensing and evaporating

let = fraction of the total covered at a particular instant

1- =fraction of surface still bare

Rate of condensation r1=k1(1- )P

Rate of evaporation r2=k2

at equilibrium r1=r2

k1(1- )P=k2

=k1P /k2+k1P

but quantity of gas adsorbed per unit mass of the adsorbent w

is proportional to the surface covered

w=k

w=bP / (1+aP)


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At low pressure aP IS becomes negligible in coparision

with has found in all cases

w=bPAt high pressure aP is very high as compared with 1 and

w= constant


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Colloidal state

Colloids, which diffuse very slowly in solution and whosesolution cant pass through animal or veg membrane.

Eg: starch, glue, albumin, gelatin, proteins etc

Colloidal state:

a substance is said to be in the colloidal state, when itis dispersed in another medium in the form of very small

particles having diameter210-4 to 110-7 cm.


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Characteristics of colloidal state:

It is a heterogeneous system, intermediatly between true

solution and suspension. Due to their unique size they dont settle down under the

force of gravity even on long keeping.

Colloidal particles are charged either all +vely or vely

They posses optical and colligative properties. The particles of the dispersed substance in internal or

discontinuous phase are called DISPERSED PHASE

While insoluble medium or external phase, in which they are

dispersed, is called DISPERSION MEDIUM


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Dispersed

phase

Dispersed

medium

Colloidal system Examples

Solid Solid Solid sols gem stones,

pearlsSolid Liquid Sols starch, proteins,

Paints

Liquid Solid Gels Jellies, cheese

butter, hoot polish

Liquid Liquid Emulsion Milk, hair cream,medicines

Solid Gas Aerosols of solids Smoke, dust in

air, smog

Liquid Gas Aerosols of liquids Mist,Fog,clouds,in

secticide sprays

Gas Solid Solid foam Foam, pumice

stone, ice-cream,

rubber

Gas liquid foam Soda water,

whipped cream

froth


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Optical properties or tyndal effect:

Ifa powerful beam of light is passed through a colloidal

solution placed in a dark room, the path of beam becomesvisible, when viewed through a microscope placed at right

angles to the path of light.

The colloidal particles appears as pin-points of light moving

against the dark background. This phenomenon is known as

tyndal effect.


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Cause of tyndal effect:

It is believed to be due to scattering of light by the coloidal

particles.

The particles absorb the incident light energy, becomes

self-luminous and scatter this absorb light from their

surfaces.

As the intensity of scattering is maximum in the plane at

right angles to the direction of incident beam, so path isvisible, hen viewed from the sides.

electrical properties

Colloids have three electrical properties, they are

1) Electrophoresis

2) Electro-osmosis

3) Iso electric point


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Electrophoresis:

Colloidal particles are electrically charged

either positive or negative.

When a high potential gradient is appliedbetween a U-tube, filled partly with a

colloidal solution and rest with distilled

water.

The colloidal particles move towards

oppositely charged electrode with aspeed of the order of about 1 micron per

second per unit potential gradient.

On reaching the electrode, they lose their

charge and get precipitated.

The movement of the colloidal particles

under the influence of an electric field, is

known as electrophoresis.

If the movement of colloidal particles is

towards cathode, the phenomenon, is

called cataphoresis as in case of ve

charged sols like As2S3


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Electro-osmosis:

when an electric current is passed through a colloidal

solution in such a way that the dispersed particles areprevented from movement, it is observed that the dispersion

medium moves.

This phenomenon of movement of the dispersion medium of

a colloidal solution, under the influence of an electric field,when the dispersed particles are prevented from moving, it

called ELECTRO-OSMOSIS.


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Isoelectric point:

Lyophilic colloids are +vely charged in strongly acidic

solution and vely charged in alkaline solution. However at a certain H+ion concentration is called the

iso-electric point.

At which the dispersed particles are neutral and hence

they don't migrate even when subjected to an electricfield.

At the iso electric point, coagulation of colloidal particles

starts.


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MICELLES:

Molecules of substances like soap

and detergents are smaller than thecolloidal particle size.

These substances behave as normalstrong electrolytes at lowconcentrations.

In concentrated solutions, thesesubstances exhibit colloidal properties

The colloidal size aggregates of soapor detergent molecules formed in aconcentrated solution, are referred to

as MICELLES. The minimum concentration at which

the micelle formation starts isdesignated as C.M.C


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APPLICATIONS OF COLLOIDS:

DUE TO ELECTROPHORESIS

SMOKE PRECIPITATION

REMOVAL OF DIRT FROM SEWAGE

PURIFICATION OF WATER

ELECTROPLATING OF RUBBER

LEATHER TANNING

ARTIFICIAL RAINS

IN WARFARE


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