Surace Chemistry
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Transcript of 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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