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