Adsorption:- The formation of a layer of
gas on the surface of solid eg.gas like
O₂,H₂,NH₃ get adsorbed on the surface
of charcoal.
Adsorbate:- The substance adsorbed is
known as adsorbate.
Adsorbent:-The substance on which
adsorption takes place is known as
adsorbent.
There are two types of adsorption:-
Physisorption:-Adsorbate is
held to the adsorbent by weak
Van der waal’s forces.
Chemisorption:-adsorbate is
held to the adsorbent by strong
chemical bond.
Adsorption Isotherm:- The relation between extent
of adsorption (x/m) and pressure of a gas at
constant temperature is known as adsorption
isotherm.
When extent of adsorption x/m(x is the amount of
adsorbate , m is mass of adsorbent is plotted
against pressure at constant tempt.).
Curve thus obtained is known as adsorption
isotherm..
An adsorption isotherm. The
isotherm show that at the start,
there is linear increase in the
extent of adsorption as the pressure
is increase .so x/m is proportional to
p.
At very high pressure ,the
saturation point is reached and x/m
does not change much with increase
of pressure . However for moderate
range of pressure the variation of
x/m with p is intermediate of two
extreme case .
Freundlich adsorption isotherm:- Freundlich in 1909 gave an empirical relationship
between the quantity of a gas adsorbed by unit mass of
solid adsorbent and pressure at particular tempt.
The relationship can be expressed by
x/m=k.p1/n(n>1)
where n=mass of the adsorbed .
m=mass of adsorbent at pressure p
k&1/n=constant which depend on the nature of the adsorbent
(Evaluated experimentally)
The value if 1/n is between 0 and 1.The variation x/m with
pressure is shown in figure.The curve is also known as Freundlich
isotherm.
•When 1/n=1, the friundlich equation explain the first part of the
isotherm x/m=kp for x/m proportional to p(adsorption varies
directly at p)
*
when 1/n=0,the equation explain the last part of the
isotherm x/m=k.p0 =k
i.e;saturation point reached when x/m becomes maximum after
which there is no change in x/m at pressure is further increased (so
that adsorption is independent of p)
• When 1/n is between 0to1, the equation accounts for the
middle portion of the isotherm.
• Evaluation of k and 1/n of a Freundlich Isotherm:- On taking
logarithm of Freundlich equation we get
logx/m=1/nlogp+logk.This is similar to an equation of straight
line y=c+mx. Thus a plot of log x/m v/s logp hives a straight line
shown in the figure from the plot we find that :
Intercept =logk
Slope =1/n
“
“Freundlich Isotherm
fails in case of high
concentration of adsorbate.
It is also fails at high
pressure.”
Application of adsorption:Washing of cloths – The cleaning action of
shoaps and detergent is governed by adsorption
of grease and oil by hydrocarbon.
froth floatation process:- In the froth floatation
process the desire sulphide ore is separated
from their gangue. The metal sulphide is
adsorbed on the surface.
Silica gel and
Alumina gel are used to
adsorbed water vapour and control humidity.
Evaluation:1. What is adsorption isotherm?
2. Describe Freundlich Isotherm.
3. Discuss the effect of pressure
and temperature on the adsorption of gases
on solid.
4. Write a mathematical expression showing
relationship between amount of solute
adsorbed pr unit mass of the solid adsorbent
and concentration of the solute in the solution.