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ADSORPTIONS
Adsorption :- The binding and accumulation
of molecules at the surface more than bulk
Adsorbate :- The substance, accumulates at
the surface
Adsorbent :- the material on the surface of
which the adsorption takes place
Distinction between Adsorption and
Absorption
In adsorption, the substance is concentrated
only at the surface
while in absorption, the substance is uniformly
distributed throughout the bulk of the solid
SORPTION is used to describe both Both
adsorption and absorption simultaneously
Adsorption in action
(i) If a gas like O2, H2, CO, Cl2, NH3 or SO2 is
taken in a closed vessel containing powdered
charcoal, it is observed that the pressure of the
gas in the enclosed vessel decreases. The gas
molecules concentrate at the surface of the
charcoal, i.e., gases are adsorbed at the surface.
How adsorption take palce
Surface has unbalanced or residual attractive forces. These forces of the adsorbent are responsible for attracting the adsorbate particles on its surface
During adsorption, there is always a decrease in residual forces of the surface, which make surface most stable
Thermodynamics of adsorptions
new bonds and attraction forces between adsorbent and adsorbate , are formed,which relase energy hence adsorption is Exothermic
ΔH of adsorption is always negative.
When a gas is adsorbed, the freedom of movement of its molecules become restricted. This leads to decrease in the entropy of the gas after adsorption, i.e., ΔS is negative
Adsorption is accompanied by decrease in enthalpy as well as decrease in entropy of the system.
For a process to be spontaneous, the thermodynamic requirement is that, at constant temperature and pressure, ΔG must be negative,
, ΔG = ΔH – TΔS, ΔG can be negative if ΔH has sufficiently high negative value as – TΔS is positive.
Types of Adsorption
Desorption. The process of removing an adsorbed substance
from a surface on which it is adsorbed is called
desorption.
Factors affecting adsorption
Effect of adsorbate: The amount of gas
adsorbed by a solid depends upon the nature of
gas.
In general, more easily liquefiable a gas is
(i.e. higher its critical temperature), the more
readily will it be adsorbed. Thus 1gm of
activated charcoal adsorbs 380 ml of sulphur
dioxide (critical temperature 157°C), 16 ml of
methane (critical temperature –83°C) and 4.5
ml of hydrogen (critical temperature –240°C).
This is valid for physical adsorption only.
Since Chemical Adsorption is
Specific in nature, it occurs only if the gas can
form a chemical bond with the solid.
1 1DAV CENTENARY PUBLIC SCHOOL, PASCHIM ENCLAVE, NEW DELHI-87
Effect of surface area [specific area] of
the absorbent: The greater the surface area
of the solid, the greater would be its adsorbing
capacity.
Effect of temperature: Physical adsorption
decreases with increase in temperature.
Effect of Pressure
as physical adsorption is reversible in nature and is
accompanied by decrease in pressure. Therefore, it
is expected that extent of adsorption increases with
increase in pressure and decrease in pressure
causes desorption.
The extent of adsorption is generally
expressed as x/m where ‘m’ is mass of adsorbent
and ‘x’ is mass of adsorbate when equilibrium has
attained.
The graph between extent of adsorption
(x/m) and the pressure ‘P’ of gas at constant
temperature is calledAdsorption Isotherm.
This is a simple type of adsorption isotherm in
which at equilibrium pressure Ps, reaches its
maximum value and no more adsorption takes
place even if the pressure is further increased.
This state is also called SaturationState and the corresponding pressure (Ps) is
called Saturation Pressure.
Such isotherms are obtained in cases where
adsorbing gases forms unimolecular layers on
the surface of adsorbent and adsorbing gas
behaves ideally in vapour phase.
x/m = k.p1/n(n>1)………………………(i) x/m= extent of adsorbtion
where x is the mass of the gas adsorbed on mass m of the adsorbent at pressure P, k and n are constants which depend on the nature of the adsorbent and the gas at a particular temperature. The relationship is
log x/m = log k+ 1/n log (ii)
The following adsorption isotherms are at
different temperature
Activation”of adsorbent increasing the surface area of solid adsorbent
by converting into fine powder To make it more efficient in adsorbing the
molecules Activated charcoal is used in gas masks as it
adsorbs all the toxic gases and vapours and purifies the air for breathing.
Adsorption from Solution Phase Solids can adsorb solutes from solutions also
When a solution of acetic acid in water is shaken with charcoal, a part of the acid is adsorbed by the charcoal
and the concentration of the acid decreases in the solution
(i) x/m decreases with an increase in temperature. (ii) x/m increases with an increase of surface area
of the adsorbent. (iii) x/m depends on the concentration of the
solute in solution. (iv) x/m adsorption depends on the nature of the adsorbent and the Applications of Adsorption
• (i) Production of high vacuum • (ii) Gas masks • (iii) Control of humidity • (iv) Removal of colouring matter from
solutions • (v) Heterogeneous catalysis • (vi) Froth floatation process • (vii) Chromatographic analysis • (ix) Separation of inert gases
2 2DAV CENTENARY PUBLIC SCHOOL, PASCHIM ENCLAVE, NEW DELHI-87
Adsorptions COLLOIDS
Colloids :-It is a heterogenous mixture with Size of
particles between 1nm to 1000 nm
Solute and solvent are replaced by dispersed
phase & dispersion medium Its not a type of substance but a state of
substance
D I S P E R S E D
P H A S E
D I S P E R S I O N
M E D I U M
T Y P E O F
C O L L I D E E X A M P L E S
Solid Solid Solid Sol
Some colored glasses and gem stones
Solid Liquid Sol paints , cell fluids
Solid Gas Aerosol Smoke, dust
LiQuid Solid Gel Cheese, butter, jellies
Liquid Liquid Emulsion Milk , hair cream
Liquid Gas Aerosol
Fog, Mist, Cloud, insecticide sprays
Gas Solid Solid Sol Pumice Stone, Foam Rubber
Gas Liquid Foam
Froth, whipped cream, soap lather
Classification based on nature of
interaction Lyophilic colloids (solvent attracting)
Directly formed by on mixing with a suitable
liquid
like gum, gelatine rubber etc.).
self-stabilizing
reversible sols
example, gums, gelatin, starch, albumin in water
Lyophobic colloids (solvent hating colloids )
Can not be formed on simply mixing with a
suitable liquid
e.g. When metals and their sulphides simply
dispersion medium, they don’t form colloids.
need stabilizer to preserve them.
Irreversible i.e easy separation but difficult to re
formation .
colloidal solutions of gold,silver, Fe(OH)3, As2S3,
etc.
Classification based on type of particles of
the dispersed phase
Macromolecular colloids:
the single molecules is large enough to have sizes
of a colloidal particles.e.g,proteins, starch,
cellulose
Multimolecular colloids : ,
a large number of atoms or smaller molecules of a
substance aggregate together to form species
having size in the colloidal range Consists of
a gold sol , Sulphur sol consists of particles of S8
sulphur molecules.
Associated colloids:-
At low concentrations, behave as normal, strong
electrolytes solutions But At higher
concentrations exhibit colloidal state properties
due to the formation of aggregated particles
(micelles)
Critical Micelle Concentration
The formation of micelles takes place only
and above a particular conc. called CMC 3 3
DAV CENTENARY PUBLIC SCHOOL, PASCHIM ENCLAVE, NEW DELHI-87
formation of micelles takes place only
at a particular temperature called
Kraft temperature (Tk)
Properties of colloids
Brownian movement: Zig- zag movement of
colloidal particles in a colloidal sol
(1)
Tyndall effect:When a beam of light is allowed
to pass through a colloidal solution, the path of
light is visible and illuminated due to scattering
of light by colloids
Electrical properties of Colloidals
Electrophoresis The movement of colloidal
particles under an applied electric potential is
called electrophoresis
The existence of charge on colloidal particles is
confirmed by electrophoresis experiment
Electroosmosis movement of the dispersion
medium in an electric field.
Theory of charge formation on colloids
particles
(i) Due to the dissociation of the surface
molecules
C15H31COONa → C15H31COO– + Na+
Sodium palmitate
(ii) Due to selective adsorption of ions
particles adsorb the ions …..
• common with their own lattice ions and
• present in excess
e.g colloid of AgI can adsorb either Ag+ or I-
which ever is in excess
Examples of charged colloidal particles
(-)ve collod particles :-Metal sulphides: As2S3,
CdS
Metal dispersions : Ag, Au, P
(+)ve colloid particles :- Metal hydroxides:
Al(OH)3, Fe(OH)3 Metal oxide : TiO2
Stability of lyophobic colloidal system
The stability of the lyophobic sols is due to the
presence of charge on colloidal particles.
(dispersed phase) because this charge do not let
the particles come closer and aggregates to settle
down to form ppt
Coagulation or precipitation: it is
aggregation of colloidal particle which leads to
settle down as Ppt
If, somehow, the charge is removed, the particles
will come nearer to each other to form aggregates
(or coagulate) and settle down under the force of
gravity
Methods of coagulations
All methods help in removing or neutralize the
charge from colloidal particle so they can come
together and form Ppt
1) By electrophoresis
(2) By mixing two oppositely charged sols
(3) By addition of electrolytes
(4) By persistent dialysis
The ions carrying the charge opposite to that of
sol neutralise the charge of colloid particles and
causes coagulation of the sol
Hardy-Schultz Rule The coagulation of a colloidal solution by an electrolyte does not take place until the added electrolyte has certain minimum concentration in the solution. The minimum amount of an electrolyte (millimoles) that must be added to one litre of a colloidal solution so as to bring about complete coagulation or flocculation is called the Coagulation or Flocculation Value of the Electrolyte. Thus smaller is the flocculation value of electrolyte, greater is its coagulation or precipitating power. Different electrolytes have different coagulation values. The coagulation behaviour of various electrolytes was studied by Hardy and Schultz. They gave a generalisation known as Hardy – Schultz Law, which states, “Greater the valency of oppositely charged ions of the electrolyte being added, the faster is coagulation”. So, for coagulation of sols carrying negative charge Al3+ ion is more effective than Ba2+ions or Na+ ions. Similarly for coagulation of positively charged sols ion is more effective than or ions.
4 4DAV CENTENARY PUBLIC SCHOOL, PASCHIM ENCLAVE, NEW DELHI-87
Thus in case of positively charged sol the coagulating power of anions is in the order of
and in case of negatively charged sols, the coagulating power of cations is in the order of Al3+ > Ba2+ > Na+
Coagulation or flocculation value
The number of millimoles of an electrolyte
required to bring about the coagulation of one
litre of a colloidal solution is called its flocculation
value.”
Coagulation value or flocculating value ∝
1/Coagulating power
Peptization : Process of converting a precipitate
into colloidal particles on adding suitable
electrolyte is known as peptisation
e.g. Fe(OH)3 solution is formed from FeCl3.
Electrol-disintegration (Bredig’s arc
method)
Preparation of colloid , when alternating
current passed through a metal( gold) electrode
,it gives Electrical disintegration to form colloids
of metals like gold
Dialysis :- It is a Purification method of
colloids
The process of separating the colloidal particles
from those of crystalloids by diffusion of mixture
through a parchment or animal membrane is
known as dialysis”.
Electrodialysis
This is a special type of dialysis process, which is
accelerated by the application of a potential
difference across the membrane. So ions
migrate faster than the colloids .
Applications of colloids
1. Rubber plating (2)
2. Sewage disposal
3. Smoke screen
4. Purification of water
5. Cleaning action of soap
6. In medicine
7. Formation of delta
8. Photography
9. Artificial rain The styptic action of alum and ferric chloride solution
is due to coagulation of blood forming a clot which
stops further bleeding
River water is a colloidal solution of clay. Sea water
contains a number of electrolytes. When river water
meets the sea water, the electrolytes present in sea
water coagulate the colloidal solution of clay resulting
in its deposition with the formation of delta
Dust particles along with water suspended in air
scatter blue light which reaches our eyes and the sky
looks blue to you
EMULSION
“An Emulsion is a colloidal solution in
which both dispersion medium as well as
dispersed phase are liquids. (The liquids
should be Immiscible)”.
Preparation
The substances thus to stabilize the emulsions are
calledEmulsifiers or Emulsifying Agents
The commonly used emulsifying agents are soaps,
proteins, long chain sulphonic acids,
Types of Emulsion
Emulsion is colloidal solution of two immiscible
liquids. So there are two types of Emulsions.
i). Oil-in-water type (o/w type)
ii). Water-in-oil type (w/o type)
i). Oil-In-Water Emulsions
In this emulsions, oil acts as disperse phase and
water acts as dispersion medium. Eg . are milk,
vanishing cream etc.
ii). Water-In-Oil Emulsions
In this water acts as dispersed phase and oil acts
as dispersion medium. For example, butter, cod
liver oil, cold cream etc.
Identification of Emulsions
The following tests may be employed to
distinguish between two types of emulsions;
(i). Dye Test
To the emulsion some oil soluble dye is added. If
the background becomes coloured, the emulsion
is water-in-oil type and if the droplets becomes
coloured the emulsion is oil-in-water type.
(ii). Dilution Test
If the emulsion can be diluted with water, this
indicates that water is dispersion medium and the
emulsion is of oil-in-water type. In case the water
added forms a separate layer than emulsion is
water-in-oil type 5 5
DAV CENTENARY PUBLIC SCHOOL, PASCHIM ENCLAVE, NEW DELHI-87
Applications of emulsions
(i) Cleansing action of soaps.
(ii) Milk, which is an important constituent of our
diet an emulsion of fat in water.
(iii)Digestion of fats in intestine is through
emulsification
Demulsification :-The process of
decomposition of an emulsion into its constituent
liquids is called demulsification. Examples of
demulsifiers are surfactants, ethylene oxide, etc.
Protective collloids
Covering up of lyophobic particles by lyophilic
particles is known as its protective action and
such colloids are called protective colloids
Protection of lyophobic colloids increase their
stability
Gold Number The lyophilic colloids differ widely in their power
of protection. The protective action of different colloids is measured in terms of the Gold number introduced by Zsigmondy. The gold number is defined as: “The number of milligrams of a lyophilic colloid that will just prevent the precipitation of 10ml of standard gold sol (containing 0.5 to 0.6gm of gold per litre) when 1 ml of 10% sodium chloride solution is added”.
Smaller the gold number,higher is protective power
The gold number of few protective colloids is as follows Geletine = 0.005 – 0.01 Haemoglobin = 0.03 – 0.07 Egg Albumin = 0.08 – 0.10 Gum Arabic = 0.10 – 0.15 Potato Starch = 25 Casein = 0.01 – 0.02
Method of Preparation of Colloids 1. Condensation or Aggregation Methods.
2. Dispersion Methods.
1. Condensation or Aggregation Methods
These methods consists of chemical reactions or change of solvent whereby the atoms or molecules of the dispersed phase appearing first, aggregate to form colloidal particles. The important chemical methods for preparing lyophobic sols are as follows;
2. Double Decomposition:
AS2O3 + 3 H2S-------->AS2S3(Yellow Sol) + 3H2O Sols of silver halide are obtained by mixing dilute solutions of silver salts and alkali halides in equivalent amounts. Silica gel sol is prepared by
this method between dilute solutions of sodium silicate and HCl.
a. Oxidation:
SO2 + 2H2S-------->3S + 2H2O H2S + (O)-------->S + H2O
b. Reduction:
2AuCl3 + 3SnCl2----->2Au(Gold sol.)+ 3 SnCl4 or AuCl3 + Tannic acid-------->Au (Sol.)
c. Hydrolysis: The method is used to prepare hydroxides and oxides of weakly electropositive metals like Fe, Al or Sn. FeCl3 + 3H2O------>Fe(OH)3(Red Sol.)+ 3HCl. The important physical methods for preparing lyophobic sols are:
d. By Exchange of Solvent:
For Example, when a solution of sulphur in alcohol (ethanol) is added to an excess of water, a colloidal solution of sulphur is obtained due to decrease in solubility.
Dispersion Methods In this method large particles of the substances are broken, into particles of colloidal dimensions in presence of dispersion medium. Since the sols formed are highly unstable. They are stabilized by
adding some suitable stabilizer. Some of the methods employed for carrying out dispersion are as follows:
e. Bredig’s Arc Method:
This process involves dispersion as well as aggregation. Colloidal solutions of metals such as gold, silver, platinum etc. can be prepared by this method. In this method electric arc is struck between electrodes of metal immersed in the dispersion medium. The intense heat produced vapourises some of metal, which then condenses to form particles of colloidal size.
Column I
Column II
(i) Protective colloid (a) FeCl3 + NaOH
(ii) Liquid – liquid colloid (b) Lyophilic colloids
(iii) Positively charged colloid (c) Emulsion
(iv) Negatively charged colloid (d) FeCl3 + hot water
6 6DAV CENTENARY PUBLIC SCHOOL, PASCHIM ENCLAVE, NEW DELHI-87
f. Peptisation:
Peptisation may be defined as the process of converting a precipitate into colloidal form by shaking it with dispersion medium in the presence of small amount of electrolyte. The electrolyte used for this purpose is called Peptizing Agent. This method is applied, generally, to convert fresh precipitate into colloidal solution because such precipitates are simply clusters of particles of colloidal size held by weak forces. IMP Question: 1 . Explain what is observed (i) When a beam of light is passed through a colloidal sol. (ii) An electrolyte, NaCl is added to hydrated ferric oxide sol. (iii) Electric current is passed through a colloidal sol? Answer (i) When a beam of light is passed through a colloidal solution, then scattering of light is observed. This is known as the Tyndall effect. This scattering of light illuminates the path of the beam in the colloidal solution. (ii) When NaCl is added to ferric oxide sol, it dissociates to give Na+ and Cl- ions. Particles of ferric oxide sol are positively charged. Thus, they get coagulated in the presence of negatively charged Cl- ions. (iii) The colloidal particles are charged and carry either a positive or negative charge. The dispersion medium carries an equal and opposite charge. This makes the whole system neutral. Under the influence of an electric current, the colloidal particles move towards the oppositely charged electrode. When they come in contact with the electrode, they lose their charge and coagulate. 2. A colloidal solution of AgI is prepared by 2 different methods as shown: (i) What is the charge of AgI colloidal particles in the two test tubes (A) and (B)? (ii) Give reasons for the origin
of charge ANS (I) AgI/I- , because KI is in excess (B) AgI/Ag+ 3 Arrange the following electrolytes will have maximum coagulating value for AgI/Ag+sol? (i)Na2S(ii) Na3PO4(iii) Na2SO4 (iv) NaCl 4. Match the items given in Column I and Column II
CATALYSIS Catalysts :- . Substances, which increase the rate of a chemical reaction and themselves remain chemically and quantitatively unchanged after the reaction
Catalysts increase the rate of a reaction by decreasing the activation energy Catalysts change the mechanism
Catalysts do not change the equilibrium constant Catalysts are not consumed A small quantity of catalyst should be able to
affect Heterogeneous catalysis- the reactants and the catalyst are in
different phases. catalyst = solid reactants = li quid/gas
Homogeneous catalysis- catalyst and reactants are in the same phase, usually liquid.
Shape- Selective Catalyst Zeolites The catalytic reaction that depends upon the pore
,structure of the catalyst and the size of the
reactant and product molecules
Zeolites are good shape-selective catalysts
because of their honeycomb-like structures
. An important zeolite catalyst used in the
petroleum industry is ZSM-5. It converts alcohols
directly into gasoline (petrol) by dehydrating
them to give a mixture of hydrocarbons.
MECXHANISM OF CATALYSIS
1. Adsorption and activation of the reactants
7 7DAV CENTENARY PUBLIC SCHOOL, PASCHIM ENCLAVE, NEW DELHI-87
2. Migration of the adsorbed reactants on the
surface
3. Reaction of the adsorbed substances
4. Escape, or desorption, of the products
Enzymes Enzymes are catalysts in biological
systems.
Mechanism of enzymes Step 1: Binding of enzyme to substrate to form an activated complex. E + S → ES≠ Step 2: Decomposition of the activated complex to form product ES≠→E + S
The substrate fits into the active site of the enzyme much like a key fits into a lock.
Enzyme catalysis is unique in its efficiency and high degree of specificity. characteristics are : (i) Most highly efficient: (ii) Highly specific nature: (iii) Highly active under optimum temperature:. The optimum temperature range for enzymatic activity is 298-310K. . (iv) Highly active under optimum pH: which is between pH values 5-7. (v) Increasing activity in presence of activators and co-enzymes:
1 What role does adsorption play in heterogeneous catalysis? Heterogeneous catalysis:A catalytic process in which the catalyst and the reactants are present in different phases is known as a heterogeneous catalysis. This heterogeneous catalytic action can be explained in terms of the adsorption theory. The mechanism of catalysis involves the following steps: (i) Adsorption of reactant molecules on the catalyst surface. (ii) Occurrence of a chemical reaction through the formation of an intermediate. (iii) De-sorption of products from the catalyst surface (iv) Diffusion of products away from the catalyst surface. What is the difference between multimolecular and macromolecular colloids? Give one example of each. How are associated colloids different from these two types of colloids 2 Explain what is observed (i) When a beam of light is passed through a colloidal sol. (ii) An electrolyte, NaCl is added to hydrated ferric oxide sol. (iii) Electric current is passed through a colloidal sol?
3 Explain the following terms: (i) Electrophoresis (ii) Coagulation (iii) Dialysis (iv) Tyndall effect.
4 Out of PO43–,, SO4
2–,, Cl–,, which wil act as the
best coagulating agent for for Fe (OH)3
5 Out of NaCl, MgSO4, Al2 (SO4)3, K4[Fe(CN)6],
which one will bring about the coagulation of a gold sol quickest and in the least of concentration ?
6 (i)Why the sun looks red at the time of setting ? Explain on the basis of colloidal properties. (ii)What is the purpose of adding gelatin to ice cream ? (iii)Bleeding is stopped by the application of alum to
a wound. Why 3. 7 What happens when freshly precipitated Fe (OH)3
is shaken with a little amount of dilute solution of FeCl3 ?
8 If we add equimolar amounts of ferric hydroxide sol and arsenic sulphide sol, what will happen ? 9 On passing H2S through dilute HNO3 the colourless
solution becomes turbid. Why 10 Out of CO and NH3 which is adsorbed on
activated charcoal to a large extent and why 11 Why CO is removed from habers process of ammonia
8 8DAV CENTENARY PUBLIC SCHOOL, PASCHIM ENCLAVE, NEW DELHI-87