adsorption 12.10.9
TRANSCRIPT
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UNIT - 4. SURFACE CHEMISTRY
ADSORPTION
Introduction
Adsorption: Concentration or assimilation of a gas (or liquid) at the surface
of a solid (or liquid)
Occlusion. The adsorption of gases on the surface of solids
Adsorbent: The material providing the surface upon which adsorption
occurs
Adsorbate: The substance get adsorbed or attached on the surface ofadsorbent
Ex: Charcoal, Silica gel, Alumina gel, Clay, etc.
Desorption: The removal of adsorbed substance from the surface
Positive adsorption: Concentration of the adsorbate is more on the surface
of the adsorbent than in the solution (bulk).
e.g., In the concentrated solution of KCl, charcoal adsorbs KCl rather than
water and this leads decrease in concentration of KCl in solution.
Negative adsorption: Concentration of the adsorbate is less on the surface
of the adsorbent than in the solution (bulk)
e.g. In the dilute solutions of KCl, charcoal adsorbs water, thereby the salt
concentration is increased.
Exothermic nature of adsorption
Occurs spontaneously as the unbalanced or residual forces acting along the
surface
The adsorbent has a tendency to attract and retain molecules of other
species
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Results in decrease in the residual forces, thereby resulting decrease of
surface energy, which in turn appears in the form of heat.
The amount of heat evolved when 1 mole of any gas (or vapour) is adsorbed
on a solid adsorbent surface, is called enthalpy (or heat) of
adsorption.
Adsorption and Absorption:
Absorption: Substance distributed throughout the body of the solid or liquid
Adsorption Absorption
1. Concentration or assimilation of a
gas (or a liquid) at the surface of a
solid (or
liquid)
2. A surface phenomenon
3. A fast process
4. Equilibrium is attained easily
1. The substance assimilated is
uniformly distributed throughout the
body of the solid
or liquid.
2. A bulk phenomenon
3. A slow process
4. Attainment of equilibrium takes
some time
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5. Forces responsible for such
adsorption are very weak
6. The rate of adsorption increases
with the increase of pressure or
concentration of the adsorbate. Near
saturation pressure, multilayers are
formed
7. The amount of adsorption on a
surface is more function of the
adsorbate than the adsorbent
8. Such adsorption involves very
small or little activation energy
9. The equilibrium is established
rapidly
10. No surface compound formation
takesplace
11. It is not very specific in nature
Such adsorption, generally, involves
appreciable activation energy
responsible for such adsorption are
quite strong
The rate of adsorption decreases with
the increase of pressure or
concentration of the adsorbate. Near
saturation pressure, adsorption rate
decreases, since the adsorption is
confined only to upper surface layer
of adsorbent
The amount of adsorption is
characteristicof both adsorbate and
adsorbent.
Activation energy is involved in
chemical bond formation
Establishment of equilibrium
requires time
Actual surface compound formation
between the adsorbent and adsorbate
takes place.
It is highly specific in nature
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ADSORPTION OF GASES ON SOLIDS
Factors affecting the adsorption of gases on solid surfaces
(1) Nature of the gas:
Easily liquefiable gases (like HCl, NH3, Cl2, etc.) are adsorbed more easily
than the permanent gases (like H2, N2, O2, etc.).
The ease of liquefaction of a gas depends upon its critical temperature
The higher the critical temperature (Tc), the more easily the gas is liquefied
and consequently, more readily it is adsorbed.
The critical temperature increases the adsorption also increases
Adsorption of various gases on 1 g of activated carban
Gases SO2 NH3 CO2 N2 H2
C.T (K) 430 406 304 126 33
Amount of gas
adsorbed (ml)
380 180 48 8.0 4.5
(2) Nature of adsorbent:
Greater the surface area of the adsorbent, greater is its adsorption capacity.
Activated charcoal and silica gel are excellent adsorbents, since their
structure is highly porous and hence, possess large surface areas.
Activated charcoal and finely divide solid substances are better adsorbents.
Activation of adsorbent:
Activation leads to increase in the surface area i.e., adsorping power of the
adsorbent
i)Creation of roughness
Mechanical rubbing of metallic adsorbents,
Subjecting to some chemical reactions of metallic adsorbents
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ii) Increasing effective area
Sub-dividing the solid adsorbents into finer particles and hence, increasing
surface area
Strong heating in superheated steam of some adsorbents, e.g., when charcoal
is subjected to the action of superheated steam, its pores are opened, thereby
adsorption increases.
(3) Effect of pressure:
The extent of adsorption (x/m) or (S) (where x is the mass of adsorbate, gas,
and m is the mass of the adsorbent) depends upon the pressure.
Adsorption isotherm is a graph plotted between magnitude of adsorption
and pressure, at constant temperature.
Graph: refer class notes
The extent of adsorption (x/m) increases with increasing pressure (P) and
becomes maximum at Ps, called the saturation pressure.
(4) Effect of temperature:
Adsorption isobar is a graph plotted between magnitude of adsorption and
temperature, at constant pressure.
Since adsorption is an exothermic reaction, so with an increase in
temperature, the amount adsorbed (x/m) should decrease.
However, in case of chemisorption, the amount adsorbed (x/m) initially
increases and then decreases, because chemisorption (like an ordinary
chemical reaction) requires some activation energy.
Adsorption isobars can be used to distinguish between physical and
chemical adsorptions.
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Thus, in physical adsorption, there is a regular decrease in extent of
adsorption as temperature increases; whereas in chemisorption, there is
initial increase and then decrease in extent of adsorption as temperature
increases. (Ref. graph)
ADSORPTION OF SOLUTES FROM SOLUTIONS
Solid surfaces adsorb solutes from solutions in two ways
1) Solid substances adsorb dissolved substances (solutes) from solutions
Activated animal charcoal adsorbs Colouring matter present in sugar
solution, thereby making the latter colourless.
Activated charcoal adsorbs certain acids like acetic and oxalic present in
water, thereby acid concentration in water decreases.
Ammonia from solutions of NH4OH and phenolphthalein from solution of
acids or bases
2) An adsorbent adsorbs certain solute from solution in preference to
other solutes.
Charcoal adsorbs non-electrolytes more readily than electrolytes from a
solution.
Alumina adsorbs electrolytes in preference to non-electrolytes.
Factors influencing adsorption of solutes from solution
1) Effect of temperature and concentration
Adsorption from solution increases with rise of temperature and increase in
concentration of solution.
Freundlich adsorption equation is found applicable
Adsorption from solution decreases with rise of temperature and decrease in
concentration of solution.
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Positive adsorption: when conc. Of the solution is high, the adsorption of
solute is high and the conc. of solute is more on adsorbent surface than in the
solution (bulk)
Nagative adsorption: when conc. Of the solution is low, the adsorption of
solute will be low and the conc. of solute is less on the adsorbent surface
than in the solution (bulk)
2) Effect of surface area
Adsorption increases with increase in surface area of the adsorbent
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ADSORPTION ISOTHERMS
A graph plotted between the magnitude of adsorption and pressure at
constant temperature
Types of adsorption Isotherms
Type I: Monomolecular layer adsorption
Postulated by Langmuir
The rate of adsorption increases with the increase of pressure or
concentration of the adsorbate until it reaches the saturation pressure
Furthur increase in pressure will not increase the amt of adsorption
E.g., Adsorption of N2 or H2 on charcoal
Type II Multimolecular layer adsorption
Show large deviations from Langmuir model
The amt of adsorption increases with increase in pressure
Additional layer formation due to the extension of vander waals force
e.g., Adsorption of N2 on Pt at -195 C
e.g., Adsorption of Br2 on silica at 80C
Type III: Capillary condensation with multimolecular layer formation
Condensation of gases in the minute capillary pores of adsorbent
Multimolecular layer formation
e.g.: Adsorption of benzene on silica gel at 50o
C
e.g.: Adsorption of H2O vapour on activated Carbon at 100oC
REFER THE GRAPHS FROM NOTES
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FREEUNDLICHS ADSORPTION ISOTHERM
The Freundlich equation or Freundlich adsorption isotherm is an
adsorptionisotherm, which is a curve relating the concentration of a solute
on the surface of an adsorbent, to the concentration of the solute in the liquid
with which it is in contact.
For Adsorption of gases on solids
x/m = KP1/n
For Adsorption of solutes on solid in solutions
x/m = KC1/n
Where
x/m= extent of adsorption
where
x = mass of adsorbate
m = mass of adsorbent
p = Equilibrium pressure of adsorbate
c = Equilibrium concentration of adsorbate in solution.
K and 1/n are constants for a given adsorbate and adsorbent at a particulartemperature
From the adsorption isotherm, the following observations can easily be
made:
(i) At low pressure, the graph is almost straight line, 1/n = slop =
tan 45o
= 1
Log x/m = log K + 1 log P
thereby indicating x/m P or x/m = KP
(ii) At high pressure, the graph becomes almost parallel to X-axis,
thereby indicating 1/n = slop= tan 0 = 0
Log x/m = log K + 0 log P
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x/m = constant or x/m = K
(iii) At intermediate pressure, x/m depends on 0 to 1 power of pressure (i.e.,
fractional power of pressure). 1/n = 1 to 0
x/m P1/n
x/m = KP1/n
On taking the logarithm of both sides, the above expression assumes the
form:
Log x/m = log K + 1/n log P or
Log x/m = log K + 1/n log C
Thus, if log x/m is plotted against log P or log C, a straight line would be
obtained. The slope of the curve will give 1/n; while the intercepts on log
x/m axis at P = 0 or C=0 would give K
Limitations:
(i) Purely empirical basis
(ii) Valid upto certain pressure and invalid at high pressure
(iii) Fails when concentration of adsorbate is very high
(iv) K and n are temperature dependents, vary with temperature
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LANGMUIRS THEORY OF ADSORPTION
Assumptions or Postulates
1. Valencies at the surface of adsorbent atoms are not fully satisfied. Each
solid surface contains a fixed no. of active adsorption sites and each siter can
adsorb only one molecular or atomic species of the adsorbate
2. The residual valency force on the surface of adsorbent is effective only up
to a small distance (about 2 10-8
cm) and hence, the adsorbed gas layer is
only one molecule thick.
3. The phenomenon of adsorption consists of two opposing processes,
namely, condensation of the molecules of the adsorbate on the surface of the
adsorbent and evaporation or desorption of the adsorbed molecules from the
surface of the adsorbent.
4. A dynamic equilibrium is set up, when the rate of condensation becomes
equal to the rate of evaporation.
5. There is no interaction between the adjacent adsorbed molecules
6. The adsorbed gas molecule does not move around on the surface.
7. Each solid adsorbent surface has a fixed number of adsorption sites and
each site can adsorb only one atomic or molecular species
8. The heat of adsorption for all the adsorption sites are same irrespective of
the fraction of the surface covered with adsorbed molecule
Derivation Langmuirs adsorption equation.
Refer notes
= aP/1+aP (1)
x= amt of the adsorbate per gm of adsorbent
P = Pressure
a is adsorption co-efficient
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x
x =k (aP/1+aP)
x = ka P/ 1+ aP
x = bP / 1+ aP
1/b + aP / b = P/x
The equation (1) may be re-written as:
P/x = aP /b +1/b
Thus, if we plot P/x against P, we should get a straight line.
Case I: At very low pressures, aP becomes negligible in comparison with 1,
hence, equation (1) reduces to:
1/b>> aP/b (2)
P/x = 1/b
bP = x
x P
i.e., amount of adsorption per unit weight of adsorbent at a given
temperature is directly proportional to the pressure of the gas at low
pressures.
Case II: At high pressures, aP is very high as compared with 1 and,
therefore, (1) takes
the form
aP / b = P /x
x = (b/a) constant (3)
i.e., at high pressures, the extent of adsorption at a given temperature is
independent of
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pressure of the gas, because the surface becomes completely covered. Case
III: At intermediate pressure, equation 3 becomes
x/m = KP1/n
(4)
Where n lies between 0 and 1. Equation (4) is Freundlichs adsorption
isotherm
Limitations
1. This equation is also not valid at high pressure
2. Apparent saturation of a surface is observed even only a small
fraction of the area of the adsorbent is covered
3. The adsorption maximum is also variable on temperature and
prussure, This equation does not explain the multimolecular layer
formation
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APPLICATIONS OF ADSORPTION
(1) Activated charcoal
Gas masks in which all undesirable (toxic) gases are adsorbed selectively by
charcoal; while purified air passes through its pores.
Removing colouring matter of sugar solution and the decoloration of
vinegar.
Charcoal adsorption filters are used for removing organic matter from
drinking water and from industrial effleunts
Production of vacuum in Dewars flask.
(2) Silica and alumina gels
Removing moisture and for controlling humidity of room.
Silica gel has been employed for drying air, used in blast furnaces.
(3) Adsorption chromatography.
Selective adsorption by alumina, magnesia, etc., has been used for
separating different pigments
(4) Arsenic poisoning
Colloidal ferric hydroxide is administered which adsorbs the arsenic poison
and retains it and can thus be removed from the body by vomiting.
(5) Fullers earth
Refining petroleum and vegetable oils from unwanted materials, due to its
good adsorption capacity.
(6) Heterogeneous catalysis
Adsorption is the key process in catalysis
The adsorption of reactants on the catalyst surface provide close proximity
to the reactants and form products and products desorbed from the surface.
Contact process
Habers process
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Hydrogenation of oils.
(8) Mordants (aldsorbent)
Dying cloth, Mordants adsorb the dye particles, which otherwise do not stick
to the cloth.
(9) Measurement of surface area
Surface area of powder and rough surface can be measured (BET method)
(10) Water Conservation
The adsorbed stearic acid on the surface of water minimizes evaporation of
water
(11) Ore Dressing
Froath floation process: Low grade sulphide ores are freed from earthy
impurities
Role in Activated carbon
Activated carbon has very high surface area and it adsorb odorous, gaseous,
and liquid contaminants forming a strong chemical bond or attraction. Its
adsorption property is put to use in following
1. In producing high vacua - For this partly evacuated apparatus is connected
to a vessel containing activated carbon, cooled in liquid air. At this
temperature, carbon adsorbs residual air very effectively.
2. In gas masks or respirators - it adsorbs poisonous or foul smelling and
other harmful gases and vapour more readily than it adsorbs air.. Thus the air
gets filtered on passing thro the gas mask before breathing.
3. Remove offensive odor from the air (deodorizer) in air-conditioning
process in large restaurants, auditoriums and in refrigerators, in shoe insole
4. In room air purifiers, the activated carbon is often combined with zeolite
and thus acts as a filter for odour control, toxin removal and as a chemical
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sieve. In some units it is impregnated with potassium iodide or blended with
impregnated alumina to increase the adsorbent qualities. These home air
purifiers are particularly helpful to people with Multiple Chemical
Sensitivity (MCS) as well as for asthma patients
5. To remove impurities from gases such as hydrogen, nitrogen, Helium,
acetylene etc.
6. In cigarette filters used either as granule or powder in filters to remove
some harmful elements of cigarette smoke, or taste and flavour control
7. During waste disposal containing domestic, clinical, chemical waste etc.
by high temperature incineration, the flue gas is made to pass carbon to
remove heavy metals, dioxins and other harmful substances prior to release
in the air.
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ROLE OF ADSORPENTS
In Ion-Exchange Adsorption
The process of releasing the charged ion and adsorbing another ion with
similar charge by an adsorbent
Application: Water Softening
In Ion-Exchange Adsorption can be used in water softening process
1. Deionization or Demineralization
Demineralisation of water is done in an ion exchanger. In This process
anions and cations present in the hard water can be exchanged with the same
charged ions in the ion exchanger.
Ion Exchange resins are insoluble cross linked long chain macro polymer
with micro porous structure and the functional groups attached to the chains
are responsible for the ion exchanging properties.
The ion-exchanger commonly used is Styrene Divinyl Benzene Copolymer:
CH CH2 CH CH2
CH CH2
CH CH2
CH CH2CH CH2
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Cation Exchanger: Resins containing acidic functional group ( COOH,
SO3H) are capable of exchanging their H+
ions with other cations of hard
water. Cation exchange resin is represented as RH
CH CH2 CH CH2
CH CH2
CH CH2
CH CH2CH CH2
SO3 H- + SO3 H
- +
SO3 H- +
SO3 H- +
Capable of exchanging their cations with other cations
Polymers containing acidic functional gps like sulphonic (-SO3H) and
carboxylic(-COOH) acids
Sulphonated is more acidic because its pKa = 1, so completely ionized when
in contact with waterGenerally represented as R
-H
+
Exchange H+
ions with other cations
2R-H
++ Ca
+===== R2Ca
++ 2H
+
Cation exchange reaction is reversible, the original resin can be
regenerated on treatment with acid
Anion Exchanger: Resins containing basic functional groups ( NH2) orquaternary ammonium groups are capable of exchanging their OH
-ions with
other anions of hard water. Anion exchange resin is represented as ROH
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CH CH2 CH CH2
CH CH2
CH CH2
CH CH2CH CH2
-+CH2 N(CH3)3 OH
-+CH2 N(CH3)3 OH
-+CH2 N(CH3)3 OH
-+CH2 N(CH3)3 OH
Capable of exchanging their anion OH- with other anions (Cl-, SO42-, etc,..).
Polymers containing basic functional gps like quaternary Ammonium
hydroxide and amino gps
Quaternary Ammonium hydroxide is more basic in nature, since its pKb =
14, so completely ionized when in contact with water
Generally represented as R+OH
-
Exchange OH-
ions with other anionsR
+OH
-+ Cl
-===== R
+Cl
-+ OH
-
Anion exchange reaction is reversible, the original resin can be
regenerated on treatment with base
Process:
The hard water is first passed through a cation exchange column, H+
ions can be exchanged with the cations like Calcium, Magnesium ions
in hard water.
RH+
+ M+ RM
++ H
+
CaCl2 + RH2 RCa + 2HCl
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NaCl + RH2 RNa + HCl
The cation free water is then passed through an anion exchange
column, in which all the anions like chlorides, sulphates etc are
exchanged with OH- ions.
ROH-+ X
- RX
-+ OH
-
H+
+ OH- H2O
ROH + HCl RCl+ H2O
The water coming out is completely free from cations and anions.
This water is known as DM water or deionised water.
Regeneration: when the cation exchange resin is exhausted, it can be
regenerated by passing a solution of dil. HCl or H2SO4.
RNa + HCl RH + NaCl
Similarly for the exhausted anion exchanger dil.NaOH can be used.
RCl + NaOH ROH + NaCl
Advantages
1. This process can be used to soften highly acidic or alkalinewaters
2. It produces water of very low hardness (2 ppm)
Disadvantages
1. This equipment is costly and more expensive chemicals areneeded.
2. Turbid water clogs the pores in the ion exchange bed, so turbidwater cannot be used. Turbidity should be lower than 10 ppm
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2. Zeolite process
REFER NOTES:
ZeNa2 + Ca2+
- ZeCa + 2Na+
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Adsorption Chromatography
Based on the differences in the adsorption coefficients of substances on
solid
An analytical technique for identification and separation of components of a
mixture based on their differences in their adsorption coefficients of
substances on solid.
Column chromatography that in which the various solutes of a solutionare allowed to travel down an absorptive column, the individual components
being absorbed by the stationary phase
The mobile phase is allowed pass through the stationary phase. The
stationary phase retains the components of mobile phase at different points
depends on their adsorption.
Stationary phase:
Adsorbent packed in the column. Ex: silica gel, alumina.
Mobile phase:
The mixture of components is allowed to flow slowly over the stationaryphase.
Process
StepI: Separation of various compounds
When the solution containing different solutes is poured down a column,
filled with finely divided adsorbent. partial separation takes place due to the
difference in adsorption coefficient.
The components with higher adsorption tend to be retained at the top, theother components are adsorbed successively at various distance depends on
their adsorption.
A number of horizontal bands or zones or rings of different colours are
produced in the column. The colored zones or banded column of the
adsorbed substance is called as Chromatogram.
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StepII: Identification of compounds
Colored compounds produce colored rings, zones or bands.
Colorless compounds are observed by either exposing UV light on the
column or by spraying suitable chemical reagent in order to make them
colored.
The chemical reagents are called as developers.
The process of visualization of a colorless chromatogram is called as
development of chromatogram.
StepIII: Separation and estimation
The separation is improved by passing suitable solvent (developer) slowly
through the column.
The various zones are dissolved separately in suitable solvents and estimated
The process of recovery of various substances is elutionand the solvent is
called eluent.
*The adsorbate (solute) should possess sufficiently high solubility in the
solvent
*The competition between the solute and solvent molecules for the binding
sites (adsorption) in the adsorbent should also be considered.
*The solvent should not elute the solute more quickly and also should not
take a long time to elute, because leads to long retention time and band
broadening effects
*Some of the commonly employed solid adsorbents in the increasing order
of adsorptive power are,
powdered cellulose < starch < sucrose < CaCO3 < magnesia < silica gel