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Chapter 5: Surface Chemistry Adsorption: The accumulation of molecular species at the surface rather than in the bulk of a solid or liquid is termed as adsorption. Adsorption is essentially a surface phenomenon.

Adsorbate: The substance which is being adsorbed on the surface of another substance (adsorbent).

Adsorbent: The substance on the surface of which adsorption takes place is called adsorbent. Eg: Water vapours are adsorbed by silica gel.Water vapour –adsorbate; Silica gel – Adsorbent.

Desorption: The process of removing an adsorbed substance from a surface on which it is adsorbed..Sorption: The process in which both adsorption and absorption take place simultaneously.

Difference between adsorption and absorption: Adsorption Absorption

Adsorption is a surface phenomenon Absorption is a bulk phenomenonThe accumulation of molecular species at the surface rather than in the bulk of a solid or liquid.

It is the phenomenon in which a substance is uniformly distributed throughout the bulk of the solid.

In adsorption, the concentration of adsorbate increases only at the surface of the adsorbent

In absorption, concentration is uniform throughout the bulk of the solid.

Eg: Water vapours are adsorbed by silica gel. Eg: Water vapours are absorbed by anhydrous CaCl2

Enthalpy or heat of adsorption: Adsorption generally occurs with release in energy, i.e., it is exothermic in nature. The enthalpy change for the adsorption of one mole of an adsorbate on the surface of adsorbent is called enthalpy or heat of adsorption.

Mechanism of AdsorptionAdsorption arises due to the unbalanced or residual attractive forces on the surface of the adsorbent which attract adsorbate particles on its surface. During adsorption, there is decrease in surface energy which appears as heat. Therefore adsorption is an exothermic process. ΔH of adsorption is always negative. When a gas is adsorbed, the freedom of movement of its molecules becomes restricted, ΔS is negative. Adsorption is thus accompanied by decrease in enthalpy as well as decrease in entropy of the system. For a process to be spontaneous, at constant temperature and pressure, ΔG must be negative, i.e.,ΔG = ΔH – TΔS, ΔG can be negative if ΔH has sufficiently high negative value as – TΔS is positive.ΔG = ΔH – TΔS ; ΔS is - ve; ΔH of adsorption is always negative [surface energy↓, during adsorption].For an adsorption process to be spontaneous, ΔG must be negative, i.e., - ΔH > TΔS.Therefore adsorption is always exothermic.

Types of adsorption: a. Physical adsorption or physisorptionb. Chemical adsorption or chemisorption Difference between Physical and chemical adsorption:

Physisorption Chemisorption1. There are weak van der Waals’ forces between adsorbate and adsorbent.

1. There is strong chemical bond between adsorbate and adsorbent..

2. It is not specific in nature. 2. It is highly specific in nature.3. It is reversible in nature. 3. It is irreversible.4. Enthalpy of adsorption is low (20-40 kJ mol-1 ). 4. Enthalpy of adsorption is high (80-240 kJ mol-1).5.No appreciable activation energy is needed. 5. High activation energy is sometimes needed.6. Low temperature is favourable for physisorption. It decreases with increase of temperature(adsorption is exothermic)

6. High temperature is favouable for chemisorption. It increases with the increase of temperature (Activation energy is required) and then decreases (adsorption is

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exothermic).7.It forms multimolecular layers on adsorbent surface. 7. It forms unimolecular layer.8. It depends on the nature of gas. More easily liquefiable gases like NH3, CO2, are adsorbed readily.

8. It also depends on the nature of gas. Gases which can react with the adsorbent show chemisorption.

9. It depends on the surface area. It increases with an increase of surface area.

9. It also depends on the surface area. It too increases with an increase of surface area.

Factors affecting adsorption of gases on solids: a. Nature of adsorbate: In physical adsorption, easily liquefiable gases like NH3. HCl, CO2, etc. which have higher critical temperatures are absorbed to greater extent due to strong intermolecular forces whereas H2, O2, N2 etc. are adsorbed to lesser extent. b. Nature of adsorbent:

c. Specific area of the adsorbent: The greater the surface area of adsorbent, more will be the extent of adsorption. Solids, particularly in finely divided state, have large surface area and therefore, charcoal, silica gel, alumina gel, clay, colloids, metals in finely divided state, etc. act as good adsorbents. That is why porous or finely divided forms of adsorbents adsorb larger quantities of adsorbate.

d. Pressure of the gas: Physical adsorption increases with increase in pressure. Adsorption isotherm:The variation in the amount of gas adsorbed by the adsorbent with pressure at constant temperature can be expressed by means of a curve is termed as adsorption isotherm.

a. Freundlich Adsorption isotherm: The relationship between the quantity of gas adsorbed by unit mass of solid adsorbent (x/m) and pressure of the gas at constant temperature is called adsorption isotherm and is given by:

Where x- mass of the gas adsorbed on mass m of the adsorbent at pressure P. k and n depends upon the nature of the adsorbent and the gas at particular temperature. x/m first increases with increase in pressure at low pressure but becomes independent of pressure at high pressure.

Taking logarithm on both sides, we get,

If we plot a graph between log x/m and log P, we get a straight line.

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The slope of the line is 1/n and intercept = log k. The factor 1/n can have values between 0 and 1 (probable range 0.1 to 0.5). When 1/n = 0, x/m = constant, the adsorption is independent of pressure. When 1/n = 1, x/m = k P, i.e. x/m ∝ P, the adsorption varies directly with pressure.

b. Freundlich s equation for adsorption from solution: Freundlich s equation describes the behaviour of adsorption from solution with concentration of the solution. i.e.,

Where C is the equilibrium concentration when adsorption is complete. Taking logarithm on both sides,

Applications of Adsorption(i) Production of high vacuum: Traces of air can be adsorbed by charcoal from a vessel to give a very high vacuum.(ii) Gas mask, consists of activated charcoal or mixture of adsorbents adsorb poisonous gases, used for breathing in coal mines. (iii) Silica and aluminium gels adsorbs moisture and control humidity.(iv) Aqueous solution of raw sugar becomes colourless as the colouring substances are adsorbed by the charcoal.(v) Heterogeneous catalysis: Adsorption of reactants on the solid surface of the catalysts increases the rate of reaction. Manufacture of ammonia using iron as a catalyst, manufacture of H2SO4 by contact process and use of finely divided nickel in the hydrogenation of oils are excellent examples of heterogeneous catalysis.(vi) Separation of inert gases: Due to the difference in degree of adsorption of gases by charcoal, a mixture of noble gases can be separated by adsorption.(vii) In curing diseases: A number of drugs are used to kill germs by getting adsorbed on them.(viii) Adsorption indicators: Surfaces of certain precipitates such as silver halides have the property of adsorbing some dyes like eosin, fluorescein, etc. and thereby producing a characteristic colour at the end pt.(ix) Chromatographic analysis based on the phenomenon of adsorption finds a number of applications in analytical and industrial fields. Catalyst: Substances which alter the rate of a chemical reaction and themselves remain chemically and quantitatively unchanged after the reaction are known as catalysts and the phenomenon is known as catalysis. Promoters: Those substances which increase the activity of catalyst are called promoters eg: Mo is promoter whereas Fe is catalyst in Haber s Process

Catalytic poisons (Inhibitors): The substances which decrease the activity of catalyst are called catalytic poisons or inhibitors e.g., arsenic acts as catalytic poison in the manufacture of H2SO4 by contact process

Types of catalysis: a. Homogeneous catalysis: When the catalyst and the reactants are in the same phase, the catalytic process is

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known as homogeneous catalysis.

b. Heterogeneous catalysis: When the catalyst and the reactants are in different phases, the catalytic process is said to be heterogeneous catalysis.

(i)

(ii) Haber’s process.

(iii) Ostwald’s process.

(iv)

Adsorption theory of Heterogeneous catalysis: It explains the mechanism of heterogeneous catalyst. The mechanism involves 5 steps:- 1. Diffusion of reactants to the surface of catalyst. 2. Adsorption of reactant molecules on the surface of catalyst. 3. Chemical reaction on the catalyst surface through formation of an intermediate. 4. Desorption of reaction product from the catalyst surface. 5. Diffusion of reaction product away from the catalyst surface.

Activity of catalyst: The ability of a catalyst to increase the rate of a chemical reaction is called activity of a catalyst. Selectivity of catalyst: It is the ability of catalyst to direct a reaction to yield particular product. For example: CO and H2 react to form different products in presence of different catalysts as follows:

Shape selective catalysis: The catalytic reaction which depends upon the pore structure of the catalyst and molecular size and shape of reactant and product molecules is called shape- selective catalysis. E.g. Zeolites are shape selective catalysts due to their honey- comb structure. They are microporous aluminosilicates with three dimensional network of silicates in which some silicon

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atoms are replaced by aluminium atoms giving Al–O–Si framework. Zeolites are widely used as catalysts in petrochemical industries for cracking of hydrocarbons and isomerisation. 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 Enzymes: Enzymes are complex nitrogenous organic compounds which are produced by living plants and animals. They are actually protein molecules of high molecular mass. They are biochemical catalysts. Eg:- (i) Inversion of cane sugar:

(ii)

(iii) Conversion of starch into maltose:

(iv)

(v) (vi) In stomach, the pepsin enzyme converts proteins into peptides while in intestine, the pancreatic trypsin converts proteins into amino acids by hydrolysis.(vii) Conversion of milk into curd by lacto bacilli enzyme present in curd. Mechanism of enzyme catalysis: 1. There are a number of cavities of characteristic shape and possess active groups (active site} such as -NH2, -COOH, -SH, - OH, etc. present on the surface of colloidal particles of enzymes. 3. These are the active centres on the surface of enzyme particles. 4. The molecules of the reactant (substrate), which have complementary shape, fit into these cavities just like a key fits into a lock. 5. This specific bonding results in the formation of an activated complex which decomposes to yield the products.

Steps of enzyme catalysis: i) Binding of enzyme to substrate to form an activated complex.

ii) Decomposition of the activated complex to form product.

Characteristics of enzyme catalysis: 1. They are highly efficient. One molecule of an enzyme can transform 106 molecules of reactants per minute. 2. They are highly specific in nature, e.g., urease catalysis hydrolysis of urea only. 3. They are active at optimum temperature (298K - 310 K). 4. They are highly active at a specific pH called optimum pH (5-7). 5. Enzymatic activity can be increased in presence of coenzymes (non-protein vitamins) which can be called as promoters. 6. Activators are metal ions Na+, Co2+ and Cu2+ etc. that are weakly bonded to enzyme and increase its activity. [Amylase in presence of sodium chloride i.e., Na+ ions are catalytically very active].7. Enzymes can also be inhibited or poisoned by the presence inhibitors or poisons. [The inhibitors interact with the active functional groups on the enzyme surface and often reduce or completely destroy the catalytic activity of the enzymes].Eg: Drug action.

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Colloids: A colloid is a heterogeneous system in which dispersed phase is dispersed as very fine particles in dispersion medium called dispersed medium. Particle size is in between 1 and 1000 nm. Dispersed phase: The substance which is dispersed as very fine particles in dispersion medium. Dispersion medium: The substance present in larger quantity is called dispersion medium.

Distinction between true solution, colloids and Suspension: True solution Colloids Suspension

It is homogeneous It appears to be homogeneous but actually heterogeneous

It is heterogeneous

Its particle size is less than 1nm Its particle size is 1 – 1000 nm Its particle size is larger than 1000nm

It passes through filter paper It passes through ordinary filter paper but not through ultra-filters

It does not pass through filter paper.

It does not show Tyndall effect It shows Tyndall effect It does not show Tyndall effectIts particles cannot be seen by microscope

Its particles can be seen by powerful microscope due to scattering of light

Its particles can be seen even with naked eye.

Classification of colloids on the basis of the physical state of dispersed phase and dispersion medium: Dispersed phase Dispersion medium name Examples

Solid Solid Solid sol Some colored glasses and gem stonesSolid Liquid Sol paints , cell fluidsSolid Gas Aerosol Smoke, dust

Liquid Solid Gel Cheese, butter, jelliesLiquid Liquid Emulsion Milk , hair creamLiquid Gas Aerosol Fog, Mist, Cloud, insecticide sprays

Gas Solid Solid sol Pumice Stone, Foam RubberGas Liquid Foam Froth, whipped cream, soap lather

Classification of colloids on the basis of nature of interaction between dispersed phase and dispersion medium: (a) Lyophobic colloid (b) Lyophobic colloid

Lyophilic sols Lyophobic solsThese colloids are solvent loving. These colloids are solvent hating. In these colloids, the particles of dispersed phase have great affinity for the dispersion medium.

In these colloids the particles of dispersed phase have no affinity for the dispersion medium

They can be prepared by mixing the dispersed phase and the dispersion medium directly.

They cannot be prepared by mixing the dispersed phase and the dispersion medium directly. They are prepared only by special methods

They are stable, cannot be coagulated easily. They do not need stabilizing agents for their preservation.

They are not stable, hence coagulated easily. They need stabilizing agents for their preservation.

They are reversible sols. They are irreversible sols.Eg: starch, gum, gelatin, rubber sols e.g. metal (gold) sols, metal sulphide sols.

Classification of colloids on the basis of types of particles of the dispersed phase: a. Multimolecular colloids: b. Macromolecular colloids: c. Associated colloids (Micelles):On dissolution, a large number of atoms or smaller molecules of a substance aggregate together to form species of colloidal size. Eg;

These are the colloids in which the dispersed particles are themselves large molecules (usually polymers) of colloidal dimensions.

Those colloids which behave as normal, strong electrolytes at low concentrations, but show colloidal properties at higher concentrations

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gold sol, Sulphur sol. e.g., Naturally occurring macromolecules - starch, cellulose, proteins and enzymes; and man-made macromolecules - polythene, nylon, polystyrene, synthetic rubber, etc.

due to the formation of aggregated particles of colloidal dimensions. Such substances are also referred to as associated colloids. The aggregate particles are called micelles. Eg: Soaps and synthetic detergents.

Kraft Temperature (Tk): The temperature above which Micelles are formed is called Kraft temperature. Critical Micelle Concentration (CMC): The concentration above which Micelles are formed are called critical micelle concentration. Soaps: Soaps are sodium or potassium salts of higher fatty acids e.g., sodium stearate CH3(CH2)16COO-Na+, Sodium palmitate CH3(CH2)14COO-Na+. Cleansing action of soaps and detergents: Explain the cleansing action of soap.

The cleansing action of soap is due to emulsification and micelle formation. Soaps are basically sodium and potassium salts of long chain fatty acids, R-COO-Na+. A soap molecule consists of two parts – polar COO- is hydrophilic and a long hydrocarbon chain (non-

polar) is hydrophobic. When soap is added to water containing dirt, the soap molecules forms a micelle around the oil

droplet, in such a manner that their hydrophobic parts get attached to the dirt molecule and the hydrophilic parts point away from the dirt molecule (soluble in water). This is known as micelle formation.

Thus, the polar group (COO- ) dissolves in water while the non-polar group (hydrocarbon) dissolves in the dirt particle.

Now, as these micelles are negatively charged, they do not come together and a stable emulsion is formed. These ionic micelles are then washed with excess of water.

Methods of preparation of colloids: a. Chemical methods: 1. Oxidation: SO2 +2H2 S → 3S (Sol) + 2H2 O 2. Hydrolysis: FeCl3 +3H2O → Fe(OH)3(sol) + 3HCl 3. Double decomposition: As2O3 + 3H2S → As2S3(sol) + 3H2O 4. Reduction: 2 AuCl3 + 3 HCHO + 3H2O → 2Au(sol) + 3HCOOH + 6HCl

b Electrical disintegration or Bredig s Arc method -Preparation of metal sols like gold, silver, platinum.It involves dispersion as well as condensation. In this method, electric arc is struck between metal electrodes immersed in the dispersion medium. The intense heat produced vaporizes the metal which then condenses to form particles of colloidal size.

c. Peptization: Peptization may be defined as the process of converting a precipitate into colloidal sol by shaking it with dispersion medium in the presence of a small amount of electrolyte. The electrolyte used for this purpose is called peptizing agent.

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Purification of colloidal sols: The process used for reducing the amount of impurities to a requisite minimum.a. Dialysis: It is a process of removing a dissolved substance from a colloidal solution by means of diffusion through a suitable membrane. Particles of true solution can pass through animal membrane but not the colloidal particles. The apparatus used for this purpose is called dialyser. A bag of suitable membrane containing the colloidal solution is suspended in a vessel through which fresh water is continuously flowing. The molecules and ions diffuse through membrane into the outer water and pure colloidal solution is left behind.

b. Electro dialysis: It is a process of removing a dissolved substance from a colloidal solution by means of diffusion through a suitable membrane under an electric field.The process of dialysis is quite slow. It can be made faster by applying an electric field if the dissolved substance in the impure colloidal solution is only an electrolyte. The ions of the electrolyte migrate towards the oppositely charged electrodes.

c. Ultrafiltration: It is the process of separating the colloidal particles from the solvent and soluble solutes present in the colloidal solution by ultra- filters, which are permeable to all substances except the colloidal particles. Colloidion is a 4% solution of nitro- cellulose in a mixture of alcohol and ether. An ultra-filter paper may be prepared by soaking the filter paper in a colloidion solution, hardening by formaldehyde and then finally drying it.

Properties of colloids: a. Colour: The colour of colloidal solution depends upon the wavelength of light scattered by the colloidal particles which in turn depends upon the nature and size of particles. The colour also depends upon the manner in which light is received by the observer.

b. Brownian movement: It is the zig - zag motion of colloidal particles due to the unbalanced bombardment of the dispersed particles with the molecules of the dispersion medium. The Brownian movement has a stirring effect which does not permit the particles to settle and thus, is responsible for the stability of sols.

c. Colligative properties: Colloidal particles being bigger aggregates, the number of particles is small, hence, the values of colligative properties are small as compared to values shown by true solutions at same concentrations..

d. Tyndall effect: It is the scattering of light by colloidal particles due to which path of beam becomes visible. The bright cone of light is called the Tyndall cone.

Tyndall effect is observed only when (a) The diameter of the dispersed particles is not much smaller than the wave length of the light used (b) The refractive indices of the dispersed phase and the dispersion medium differ greatly in magnitude.

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e. Charge on colloidal particles: Colloidal particles always carry an electric charge either positive or nega-tive. The colloidal particles develop charge due to the following reasons (a) Electron capture by sol particles during electro dispersion of metals (b) Due to preferential adsorption of ions from solutions (c) Due to formulation of electrical double layer.

When AgNO3 solution is added to KI solution, the precipitated AgI adsorbs I- ions from the dispersion medium and negatively charged colloidal solution results, AgI/I– .

When KI solution is added to AgNO3 solution, positively charged sol results due to adsorption of Ag+ ions from dispersion medium, AgI/Ag+

If FeCl3 is added to excess of hot water, a positively charged sol of hydrated ferric oxide is formed due to adsorption of Fe3+ ions, Fe2O3.xH2O/Fe3+ .

When FeCl3 is added to NaOH a negatively charged sol is obtained with adsorption of OH- ions,Fe2O3.xH2O/OH–

f. Helmholtz electrical double layer: When the colloidal particles acquire negative or positive charge by se-lective adsorption of one of the ions, it attracts counter ions from the medium forming a second layer. The combination of fixed and diffused layers of opposite charges around colloidal particles is called Helmholtz electrical double layer. Eg: AgI/I- K+  & AgI/Ag+I-

g. Electrokinetic potential or zeta potential: The potential difference between the fixed layer and the dif-fused layer of opposite charges is called electrokinetic potential or zeta potential.

h. Electrophoresis: The movement of colloidal particles towards oppositely charged electrodes under an ap-plied electric potential is called electrophoresis.

Coagulation or precipitation: The process of settling of colloidal particles as precipitate is called coagulation.

The coagulation of the lyophobic sols can be carried out in the following ways:(i) By electrophoresis: The colloidal particles move towards oppositely charged electrodes, get discharged and precipitated.(ii) By mutual coagulation by mixing two oppositely charged sols: Oppositely charged sols when mixed in almost equal proportions, neutralise their charges and get partially or completely precipitated. Mixing of hy-drated ferric oxide (+ve sol) and arsenious sulphide (–ve sol) bring them in the precipitated forms. (iii) By boiling: When a sol is boiled, the adsorbed layer is disturbed due to increased collisions with the mole-cules of dispersion medium. This reduces the charge on the particles and ultimately gets precipitated.(iv) By persistent dialysis: On prolonged dialysis, traces of the electrolyte present in the sol are removed al-most completely and the colloids become unstable and ultimately coagulate.(v) By addition of electrolytes: When excess of an electrolyte is added, the colloidal particles are precipitated. The reason is that colloids interact with oppositely charged ions causes neutralisation and coagulation.The ion responsible for neutralisation of charge on the particles is called the coagulating ion. A negative ion causes the precipitation of positively charged sol and vice versa.

a. Hardy Schulze Rules: i) Opposite charged ions are effective for coagulation. ii) Greater the valence of the flocculating/ coagulating ion , the greater is its coagulating power.

e.g. for negatively charged colloids.

for positively charged colloids.

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b. Coagulating value of an electrolyte: The minimum concentration of an electrolyte in millimoles per litre required to cause precipitation of a sol in two hours.

Protection of colloids: Lyophilic sols are more stable than lyophobic sols due to extensive solvation.Protective colloids: When a lyophilic sol is added to the lyophobic sol, the lyophilic particles form a layer around lyophobic particles and thus protect the lyophobic particles from electrolytes. Lyophilic colloids used for this purpose are called protective colloids.

Emulsion: colloidal system in which dispersed phase and dispersion medium are liquids. [a liquid is dispersed in another liquid]. Types of emulsions: a. Water dispersed in oil: When water is the dispersed phase and oil is the dispersion medium. E.g. butter, cold cream. b. Oil dispersed in water: When oil is the dispersed phase and water is the dispersion medium. E.g. milk, vanishing cream. Emulsification: The process of stabilizing an emulsion by means of an emulsifier. Emulsifying agent: The substances which are added to stabilize the emulsions are called emulsifying agents or emulsifiers. E.g. soaps, gum Demulsification: The process of breaking an emulsion into its constituent liquids is called demulsification. This can be done some process like freezing, boiling, centrifugation or by some chemical methods.

Colliods Around Us(i) Blue colour of the sky: Dust particles along with water suspended in air scatter blue light which reaches our eyes and the sky looks blue to us.(ii) Fog, mist and rain: When a large mass of air containing dust particles, is cooled below its dewpoint, the moisture from the air condenses on the surfaces of these particles forming fine droplets. These droplets being colloidal in nature continue to float in air in the form of mist or fog. Clouds are aerosols having small droplets of water suspended in air. On account of condensation in the upper atmosphere, the colloidal water droplets grow bigger and bigger in size, and come down in the form of rain. Sometimes, the rainfall occurs when two oppositely charged clouds meet.It is possible to cause artificial rain by throwing electrified sand or spraying a sol carrying charge opposite to the one on clouds from an aeroplane.(iii) Food articles: Milk, butter, halwa, ice creams, fruit juices, etc., are all colloids in one form or the other.(iv) Blood: It is a colloidal solution of an albuminoid substance. The styptic action of alum and ferric chloride solution is due to coagulation of blood forming a clot which stops further bleeding.(v) Soils: Fertile soils are colloidal in nature in which humus acts as a protective colloid. On account of colloidal nature, soils adsorb moisture and nourishing materials.(vi) Formation of delta: River water is a colloidal solution of clay. Sea water contains a number of electrolytes. When river water meets the sea water, coagulation take place. Deposition of coagulated clay results in delta formation.

Applications of colloids(i) Electrical precipitation of smoke: Smoke is a colloidal solution of solid particles such as carbon, arsenic compounds, dust, etc., in air. In Cottrell precipitator, charged smoke particles are passed through a chamber containing plates with charge opposite to the smoke particles. Smoke particles lose their charge on the plates and get precipitated. (ii) Purification of drinking water: Alum is added to water to coagulate the suspended impurities and make water fit for drinking purposes.

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(iii) Medicines: Most of the medicines are colloidal in nature. Colloidal medicines are more effective because they have large surface area and are therefore easily assimilated. For example, argyrol is a silver sol used as an eye lotion. Colloidal antimony is used in curing kalaazar. Colloidal gold is used for intramuscular injection. Milk of magnesia, an emulsion, is used for stomach disorders. (iv) Tanning: Animal hides are colloidal in nature. When a hide, which has positively charged particles, is soaked in tannin, which contains negatively charged colloidal particles, mutual coagulation takes place. This results in the hardening of leather. This process is termed as tanning. Chromium salts are also used in place of tannin.(v) Cleansing action of soaps and detergents: (vi) Photographic plates and films: Photographic plates or films are prepared by coating an emulsion of the light sensitive silver bromide in gelatin over glass plates or celluloid films.(vii) Rubber is obtained by coagulation of latex which is a negatively charged colloidal particles.(viii) Paints, inks, synthetic plastics, rubber, graphite lubricants, cement, etc., are all colloidal solutions.============================================================================== Chapter 5: Surface Chemistry 1. Adsorption is always exothermic. Why? Adsorption decreases the surface energy of the adsorbent, which appears as heat. So it is exothermic.

2. The enthalpy of adsorption of chemisorption is high. Why? Chemisorption involves chemical bond formation.

3. What is meant by co-enzyme? The non- protein substance present along with enzymes, which enhances the activity of enzymes

4. 1 gm of activated charcoal adsorb more SO2(g) (critical temperature 630K) than methane (critical temperature 190K ).Why? Easily liquefiable gases (higher critical temperature) are easily adsorbed by a solid, because near the critical temperature the van der Waal forcess between the gas molecules and the solid adsorbents is stronger.

5. Why is FeCl3 preferred over KCl in case of a cut leading to bleeding? FeCl3 helps in coagulation of blood more effectively than KCl. Greater the valency of coagulating ion, more will be coagulating power.

7. What are aerosols? Give an eg. Colloids of a solid in gas. eg :- smoke

8. Why are powdered substances more effective adsorbents than their crystalline forms? Because powdered form provides more surface area due to which extent of adsorption increases.

9. Give 2 examples of positively charged colloids. (i) haemoglobin (blood) (2) Fe(OH)3 sol

10. Write the suitable adsorbent for the following adsorbate (a) chlorine gas (b) moisture (c) polluting gases like NO2& SO2 (d) gases like H2,O2,CO etc (a)Charcoal (b) silica gel (c) charcoal (d) transition metals like Ni, Co etc

11. What happens when? (a)A beam of light is passed through As2S3 sol - Tyndall effect -The path of the light becomes visible due to the scattering of light by colloidal particles.(b) KCl is added to Fe(OH)3sol - Fe(OH)3 sol gets coagulated. (c) Gelatin is added to gold sol- gold sol gets stabilized. (d) Colloidal sol of Fe2O3 and As2S3 are mixed- Mutual coagulation i.e,The oppositely charged colloids get neutralized and gets coagulated. (e) Alum is added to water for purification- For coagulating suspended impurities like sand & soil particles.

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(f) The sky appears blue- Due to scattering of blue light by dust particles. (g) Freshly prepared Fe(OH)3 is shaken with little amount of dil.FeCl3 – A colloidal solution of Fe(OH)3 is obtained (peptization)(h) Electrodes connected to a battery are dipped into a sol- Electrophoresis i.e, colloidal particles move to-wards oppositely charged electrodes.(i) an electrolyte added to a sol – sol get coagulated.(j) an emulsion is subjected to high speed centrifugation.- Demulsification.(k) electric current is passed through a colloidal solution – electrophoresis.

12. Explain why hydrophilic sols are relatively more stable than hydrophobic sols? This is because hydrophilic sols are extensively hydrated and there is strong interaction between dispersed phase and dispersion medium

13. Account for the following: (a)Fe(OH)3 sol is positively charged- A deep red sol of Fe(OH)3 is obtained by the hydrolysis of FeCl3 . The sol particles are positively charged because of preferential adsorption of Fe3+ ions. (b)The extent of physical adsorption decrease with rise in temperature- Adsorption is an exothermic process. So the rate of physisorption decreases with the rise in temperature in accordance with the Le-Chatlier principle.

14. Name two industrial heterogeneous catalytic processes. Haber s process for the manufacture of ammonia iron is used as catalyst Contact process for the manufacture of sulphuric acid, V2O5is used as catalyst.

15. Explain why deltas are formed where river & sea water meet. River water (colloidal solution of clay) gets coagulated by electrolytes in sea water so as to form deltas.

16. Cottrell smokes precipitator is fitted at the mouth of the chimney used in factories. It removes poisonous gases by adsorption & smoke free from poisonous gases comes out. It works on the prin-ciple of electrophoresis.

17. Which one of the following electrolytes is most effective for the coagulation of ferric hydroxide sol (+ve) & why? (i) NaCl, Na2SO4, Na3PO4. Na3PO4 is more effective because phosphate ion has highest negative charged ion, greater coagulating power.

18. Which one of the following electrolytes is most effective for the coagulation of As2S3 sol (-ve) & why? AlCl3, BaCl2, NaClAlCl3

19. What is collodion? What is its use? Collodion is 4% solution of nitro-cellulose in a mixture of alcohol and ether. Filter paper is soaked in it and hardened by formaldehyde and dried. Used for making membranes for ultrafiltration, which stops the flow of colloidal particles. \ 20. The colloidal solution of gold prepared by different methods have different colours. Why? Due to difference in the size of colloidal particles.

21. A sol is prepared by addition of excess AgNO3 solution to KI solution. What charge is likely to de-velop on the colloidal particles? Positive [AgI/ Ag+]

22. What are the physical states of dispersed phase and dispersion medium of froth? Dispersed phase is gas, dispersion medium is liquid.

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23. What is the cause of Brownian movement among colloidal particles? Due to collision between colloidal particles and molecules of dispersion medium.

24. Arrange the solutions: True solution, colloidal solution, suspension in decreasing order of their parti-cles size? Suspension > colloidal > true solution. 25. Give an example of micelles system? Sodium stearate (C17 H35 COO- Na+)

26. How is adsorption of a gas related to its critical temperature? Higher the critical temperature of the gas, greater is the ease of liquefaction. i.e. greater Vander walls forces of attraction and hence large adsorption will occur.

27. Write down the Example of Negative Sol? Arsenic sulphide.

28. What is difference between Sol. & Gel? Both are colloidal solutions. Sol has solid as dispersed phase & liquid as dispersion medium. While Gel has liquid as dispersed phase and solid as dispersion medium.

29. Name the enzyme which converts milk into curd. - Lactobacilli

30. Arrange the gases CO, N2 and CH4 in increasing order of adsorption on the surface of charcoal in a closed vessel. Give reasons also. N2 < CO < CH4 as critical temp↑, intermolecular force of attractions ↑, Extent of adsorption↑.

31. A small amount of silica gel and a small amount of anhydrous calcium chloride are placed separately in two beakers containing water vapour. Name of phenomenon that takes place in both the beakers. Silica gel - Adsorption, Anhydrous CaCl2 - Absorption, as it forms CaCl2. 2H2O

Intext Questions5.1 Write any two characteristics of Chemisorp-tion.[notes – pg -1]]

[notes – pg -1]]

5.2 Why does physisorption decrease with the in-crease of temperature?

Physisorption is an exothermic process. According to Le-chatelier’s principle, it decreases with an increase in temperature. This means that physisorption occurs more readily at a lower temperature.

5.3 Why are powdered substances more effective adsorbents than their crystalline forms?

Because powdered form provides more surface area due to which extent of adsorption increases.

5.4 Why is it necessary to remove CO when ammo-nia is obtained by Haber’s process?

CO acts as catalyst poison for Haber s process therefore it will lower the activity

5.5 Why is the ester hydrolysis slow in the begin-ning and becomes faster after sometime?

CH3COOC2H5 + H2O → CH3COOH + C2H5OHThe acid produced in the reaction acts as autocatalyst and makes the reaction faster.

5.6 What is the role of desorption in the process of catalysis.

It makes the surface of the solid catalyst free for the fresh adsorption of the reactants on the surface.

5.7 What modification can you suggest in the Hardy Schulze law?

Hardy-Schulze law states that ‘the greater the valence of the coagulating ion added, greater is its coagulating power’.This law takes into consideration only the charge carried by an ion, not its size. The smaller the size of an ion, the more will be its polarizing power. Thus Hardy-Schulze law can be stated as ‘the greater the polarizing power of the coagulating ion added, the greater is its coagulating power’.

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5.8 Why is it essential to wash the precipitate with water before estimating it quantitatively?

It is done so as to remove soluble impurities adsorbed on the surface of the ppt.

Reader back exercises5.1 Distinguish between the meaning of the terms adsorption and absorption. Give one ex.

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5.2 What is the difference between physisorption and chemisorption?

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5.3 Give reason why a finely divided substance is more effective as an adsorbent.

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5.4 What are the factors which influence the adsorption of a gas on a solid?

(i) Nature of gas (ii). Nature of adsorbent (iii). Temp. (iv). Pressure (v). Activation of adsorbent (vi). Surface area of adsorbent.

5.5 What is an adsorption isotherm? Describe Freundlich adsorption isotherm.

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5.6 What do you understand by activation of adsorbent? How is it achieved?

Activation of adsorbent means increasing the adsorbing power of the adsorbent which can be achieved by making adsorbent more porous & rough, by breaking into small pieces or powdering it, thereby increasing surface area.

5.7 What role does adsorption play in heterogeneous catalysis?

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5.8 Why is adsorption always exothermic ? Refer5.9 How are the colloidal solutions classified on the basis of physical states of the dispersed phase and dispersion medium?

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5.10 Discuss the effect of pressure and temperature on the adsorption of gases on solids.

Adsorption increases with increase in pressure. x/m = k P1/n. Adsorption is exothermic process. Thus in accordance with Le-Chatelier’s principle, the magnitude of adsorption decreases with an increase in temperature.

5.11 What are lyophilic and lyophobic sols? Give one example of each type. Why are hydrophobic sols easily coagulated ?

Hydrophobic sols are easily coagulated because they are unstable due to least force of attraction between dispersed phase and dispersioin medium.

5.12 What is the difference between multimolecular and macromolecular colloids? Give one ex. of each. How are associated colloids different from these two types of colloids?

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5.13 What are enzymes ? Write in brief the mechanism of enzyme catalysis.

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5.14 How are colloids classified on the basis of(i) physical states of components(ii) nature of dispersion medium [refer Q. 26](iii) interaction between dispersed phase and dispersion medium?

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5.15 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.

(i)Tyndall effect

(ii)Coagulation, positive hydrated ferric oxide sol attracts Cl- ion, neutralized and coagulated.

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(iii) electric current is passed through a colloid (iv) Electrophoresis5.16 What are emulsions? What are their different types? Give example of each type.

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5.17 What is demulsification? Name two demulsifiers.

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5.18 Action of soap is due to emulsification and micelle formation. Comment.

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5.19 Give four ex. of heterogeneous catalysis. Refer5.20 What do you mean by activity and selectivity of catalysts?

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5.21 Describe some features of catalysis by zeolites.

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5.22 What is shape selective catalysis? Refer5.23 Explain the foll. terms:(i) Electrophoresis (ii) Coagulation (iii) Dialysis (iv) Tyndall effect.

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5.24 Give four uses of emulsions. i. Cleansing action of soaps is based on the formation of emulsions.ii. Antiseptics and disinfectants when added to water form emulsions.iii. The process of emulsification is used to make medicines.iv. Digestion of fats in intestine takes place by the process of emulsification

5.25 What are micelles? Give an example of a micellers system.

Micelle formation is done by substances such as detergents when dissolved in water. Soap molecules contain a hydrophobic and hydrophilic part. When present in water, these substances form a spherical structures in such a manner that their hydrophobic part are present towards the centre, while the hydrophilic parts are pointing towards outside (water). This is known as micelle formation.Eg: Sodium stearate C17H35COONa

5.26 Explain the terms with suitable examples:(i) Alcosol (ii) Aerosol (iii) Hydrosol.

(i) Alcosol : A colloidal solution having alcohol as the dm and a solid as the dp. Eg: colloidon – cellulose nitrate in ethanol.(ii) Aerosol : A colloidal solution having gas as the dm and a solid/ liquid as the dp. Eg: smoke , fog.(iii) Hydrosol: A colloidal solution having water as the dm and a solid / liquid as the dp. Eg: Starch sol, gold sol.

5.27 Comment on the statement that “colloid is not a substance but a state of substance”.

Common salt (a typical crystalloid in an aqueous medium) behaves as a colloid in a benzene medium. Hence, we can say that a colloidal substance does not represent a separate class of substances. When the size of the solute particle lies in between 1 nm - 1000 nm, it behaves as a colloid.Hence, we can say that colloid is not a substance but a state of the substance which is dependent on the size of the particle. A colloidal state is intermediate between a true solution and a

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suspension.