Introduction Surface chemistry is the branch of chemistry, which deals with the study of phenomena occurring at the surface separating the two bulk phases. These two bulk phases can be pure compounds or solutions.

Adsorption

The phenomenon of attracting and retaining the molecules of the substance on the surface of a liquid or a solid resulting into a higher concentration of molecules on the surface is called adsorption. The substance thus adsorbed on the surface is called the adsorbate and the substance on which it is adsorbed is called adsorbent. The reverse process, i.e. removal of the adsorbed substance from the surface is called desorption (which can be brought about by heating or reducing the pressure). The adsorption of gases on the surface of metals is called occlusion.

Difference between adsorption and absorption:

Adsorption AbsorptionIt is a surface phenomenon, i.e. it occurs only at the surface of the adsorbent.

It is a bulk phenomenon, i.e. occurs throughout the body of the material.

In this phenomenon the concentration on the surface of adsorbent is different from that in the bulk.

In this phenomenon, the concentration is same throughout the material.

Positive and negative adsorption

When the concentration of the adsorbate is more on the surface of the adsorbent than in the bulk, it is called positive adsorption. If the concentration of the adsorbate is less, relative to its concentration in the bulk, it is called negative adsorption.

Types of adsorption

An experimental study of the adsorption of various types on solids shows that there are two main types of adsorption. Physical adsorption & chemical adsorption.

Physical adsorption & chemical adsorptionDifference between physical adsorption and chemisorption

Physical Adsorption Chemisorption1. The forces operating in these cases are

weak van der Waal’s forces.1. The forces operating in these cases are

similar to those of a chemical bond.2. The heats of adsorption are low, i.e.

about 2040 kJmol1. 2. The heats of adsorption are high, i.e.

about 40 – 400 kJmol1 3. The process is reversible, i.e.

desporption of the gas occurs by increasing the temperature or decreasing the pressure.

3. The process is irreversible.

4. It does not require any activation energy. 4. It requires activation energy.5. This types of adsorption usually takes

place at low temperature and decreases with increases of temperature.

5. This type of adsorption first increases with increase of temperature. The effect is called activated adsorption. But on further increasing temperature beyond a limit, it decreases.

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6. The amount of the gas absorbed is related to the ease of liquification of the gas.

6. There is no such correlation.

7. It forms multimolecular layer. 7. It forms unimolecular layer.

Factors Affecting the Adsorption of Gases

Almost all solids adsorb gases to some extent, however, the exact amount of a gas adsorbed depends upon the following factors.

(i) Nature and surface area of the adsorbent: The greater the surface area of the adsorbent, greater is the volume of the gas adsorbed. Therefore, charcoal and silica gel are excellent adsorbents because they have highly porous structures and hence large surface areas.

(ii) Nature of the gas: Different gases are adsorbed to different extents by the same adsorbent at the same temperature. As critical temperature increases, ease of liquefaction increases and hence adsorption increases.

(iii) Effect of temperature: Adsorption is generally temperature dependent. Mostly, adsorption is an exothermic process and therefore, adsorption decreases with increasing temperature. However, as expected for endothermic adsorption processes, adsorption increase with increase in temperature.

(iv) Pressure: At constant temperature, the adsorption of a gas increases with increase of pressure. It is observed that at low temperature, the adsorption of a gas increases very rapidly as the pressure is increased.

(v) Activation of the solid adsorbent: This is usually done by increasing the surface area of the adsorbent by any of the following ways, (a) making the surface of the adsorbent rough. (b) subdividing the adsorbent into smaller piece or grains. (c) removing the gases already absorbed.

Exercise 1.What do you understand by activation of adsorbent? How is it achieved?

Freundlich’s Adsorption Isotherm

In case of adsorption of gases on solids, the relation

between x/m and the pressure p of the gas at constant

temperature is given by the equation.

where K and n are the parameter of the equation

depending upon the nature of the gas and the solid.

According to this, x/m increases with increase of p but

since n > 1, x/m does not increase as rapidly as P, as can

be seen from the isotherm.

Freundlich adsorption isotherm

Taking logarithms on both sides of equation (i) we get,

Thus, if we plot a graph between log(x/m) and log p, a straight line is obtained. The slope of the line is equal to 1/n and the intercept on log (x/m) axis will correspond to logK. Therefore, value of K and n can be determined.

Graph of log x/m Vs log P

Langmuir Adsorption Isotherm

One of the drawbacks of the Freundlich adsorption isotherm is that it fails at high pressure. Langmuir’s adsorption isotherm is based on kinetic theory of gases. Langmuir considered adsorption to consist of the following two opposing processes: (i) adsorption of the gas molecules on the surface of the solid. (ii) desorption of the adsorbed molecules from the surface of the solid. Langmuir believed that eventually a dynamic equilibrium is established between the above two opposing processes. He also assumed that the layer of the adsorbed gas wasunimolecular. Such type of adsorption is obtained in the case of chemisorption, hence isotherm works particularly well for chemisorption. The Langmuir adsorption isotherm is represented by the relation,

Where a and b are two Langmuir parameters. At very high pressure, the above isotherm acquires the limiting from,

At very low pressure, equation (iii) is reduced to x/m = ap (at very low pressure) 1+ bp 1 …(iv b) In order to determine the parameters a and b, equation (iii) may be written in its inverse form.

A plot of m/x against 1/p gives a straight line with slope and intercept equal to 1/a and b/a, respectively, thus both parameters can be determined.

Langmuir isotherm indicates that at low pressure x/m increases linearly with p. At high pressure, x/m becomes constant, i.e. the surface is fully covered and change in pressure has no effect and no further adsorption takes place, as is evident from figure.

Langmuir adsorption isotherm

Catalysis

Catalyst is a substance which can change the speed of a chemical reaction without being used up in that reaction and the phenomenon is known as catalysis. If a catalyst increases speed of a

reaction, it is called a positive catalyst and the phenomenon is called positive catalysis. While if a catalyst decreases the speed of a reaction, it is called a negative catalyst and the phenomenon is called negative catalysis. For example, oxidation of SO2 to SO3 in presence of NO (lead chamber process) or in presence of V2O5 (contact process) are examples of positive catalysis, decomposition of H2O2 in presence of phosphoric acid and oxidation of chloroform in presence of alcohol are examples of negative catalysis. A catalyst lowers the activation energy for the forward reaction as well as for the backward reaction. As a result, the reaction follows an alternate path and the rate of forward reaction as well as that of the backward reaction are accelerated to the same extent. Hence, equilibrium constant of the reaction remains unaffected. Similarly, it may be noted that the enthalpy change of the reaction also remains unaffected. In the presence of catalyst, the equilibrium is however attained quickly.

Exercise 2.What role a catalyst play in establishing the equilibrium in reversible process.

Types of Catalysis

Catalysis can be broadly classified into two types.

1. Homogeneous catalysis: If the catalyst is present in the same phase as the reactants, it is called a homogeneous catalyst and this type of catalysis is called homogenous catalysis. Two common examples of homogenous of catalysis are (i) Oxidation of sulphur dioxide to sulphur trioxide in presence of nitric oxide as catalyst (in

lead chamber process for manufacture of H2SO4).

Here, all substances are present in the gaseous phase. Similarly, oxidation of CO by O2

takes place in presence of NO as catalyst.

(ii) Decomposition of ozone in presence of Cl atoms acting as catalyst.

2. Heterogeneous catalysis: If the catalyst is present in a different phase than that of the reactant, it is called a heterogeneous catalyst and this type of catalysis is called heterogeneous catalysis. The catalyst in heterogeneous catalysis is generally solid and the reactants are mostly gases and sometimes liquids. In heterogeneous catalysis the reaction starts at the surface of the solid catalyst that is why it is known as surface catalysis. Some examples of heterogeneous catalysis are: (i) Manufacture of ammonia from N2 and H2 by Haber’s process using iron as catalyst. (ii) Synthesis of methyl alcohol (CH3OH) from CO and H2 using a mixture of Cu, ZnO and

Cr2O3 as catalyst. (iii) Manufacture of sulphuric acid by the oxidation of SO2 to SO3 using V2O5 as catalyst.

Illustration 1. Write down the heterogeneous catalyst involved in polymerization of ethylene.

Solution:Heterogenous catalyst: Ziegler – Nata catalyst or (R3Al + TiCl4)

Exercise 3.Why is a catalyst more effective in finely grinded state?

Nature of solid catalysts

Solid catalysts may be metals, metal oxides, metal sulphides, clays etc. There materials may be used in their pure form or in the form of their mixture. Further they may be crystalline, microcrystalline (in the form of fine particles) or amorphous.

Important Features of Solid Catalysts

(i) Activity: Activity of the catalyst is its capacity to increase the speed of the chemical reaction. It may be increased upto 1010 times. Combination of H2 and O2 in the presence of platinum (catalyst) to form water with explosive violence is an example of catalytic activity. In the absence of the catalyst platinum, H2 and O2 do not combine and can be stored as such for an indefinite period.

The activity depends upon the extent of chemisorption. The adsorption should be reasonably strong but not so strong that the absorbed molecules become immobile and no space is available for other reactants to get adsorbed.

Illustration 2. Why it is advantage to use a catalyst for a reaction having endothermic nature?

Solution: For an endothermic reaction (H = +ve) and thus heat is required to get better yield which raised the cost appreciably. In presence of catalyst, the heat of reaction is lowered.

(ii) Selectivity: By selectivity of a catalyst we mean its ability to direct the reaction to form particular products excluding others. For example, CO and H2 react to form different products in presence of different catalysts as follows,

Action of a catalyst is highly selective in nature, i.e. a given substance can act as a catalyst only in a particular reaction and not for all the reactions.

(iii) Shape selective catalysis by zeolites: The catalytic reaction that depends upon the pore structure of the catalyst and the size of the reactant and product molecules is called shape selective catalysis. Zeolites are good shape selective catalysts because of their honeycomblike structures. The reactions taking place in zeolites depend upon the size and shape of reactant and product molecule as well as upon the pores and cavities of the zeolites. That is why these types of reactions are called shape selective catalysis reactions. An important zeolite catalyst used in petroleum industry is ZSM -5. It converts alcohol directly into gasoline (petrol), by dehydration of alcohols, a mixture of hydrocarbons is formed.

Illustration 3. A little amount of HCl or H2O increase the activity of AlCl3 in the isomerisation of paraffins; why?

Solution: HCl or H2O acts as promoters.

Exercise 4.H2 produces from Zn + dil. H2SO4 does not discharge yellow colour of FeCl3, but if FeCl3 is added in Zn + dil. H2SO4, the colour is discharge. Is it an example of catalysis.

Enzyme catalysis: All biological reactions are catalysed by special catalysts called enzymes. Enzymes are proteins with high molar mass. They increase rates by 108 to 1020 times. Enzymes are also extremely specific. Each reaction is generally catalysed by a particular enzyme. Urease for example, catalyses only the hydrolysis of urea and none of the several thousand other enzymes present in the cell catalyses that reaction.

Illustration 4. The rate of fruit fermentation increases with time.

Solution: In fermented fruit an increase in concentration of enzymes with time speeds up enzyme catalysed reactions.

Mechanism of Enzyme Action

The two most accepted mechanisms are given below

(i) Lock and key model

The specificity of the enzymes is due to the presence of some specific regions, called the active sites which are associated with some functional groups, which form weak bond such as H – bonds, van der Waal’s attraction, with the substrate (reactant) molecules. The shape of the active site of any given enzyme is such that only a specific substrate can fit into it, in the same way as one key can open a particular lock. Once the proper orientation has been achieved, substrate molecules react to

form the products in two steps as shown in the figure.

`Lock key and mechanism of enzyme action

(ii) Induced fit model:

According to this model, the enzyme can change its shape when the substrate comes in contact with the active site so that there is a perfect fit rather than rigidly shaped lock and key.

Kinetics of enzyme catalysis Enzyme catalysed reactions take place in two steps as follows:

Step I: Formation of enzyme – substrate complex.

Step II: Dissociation of enzyme – substrate complex to form the products.

The rate of formation of product depends upon the concentration of EP.

Illustration 5. The colour of KMnO4 discharge slowly in the beginning during its reaction with oxalic acid but fastens after some time. Why?

Solution: Mn2+ ions formed during the reaction acts as auto catalyst.

Illustration 6. Why is necessary to remove arsenic impurities in the manufacture of SO3 by contact process?

Solution: Arsenic poisons the catalyst Pt and thus it should be removed.

Exercise 5.Is it necessary to remove CO when ammonia is obtained by Haber process?

Exercise 6.Acetyl chloride on reduction with H2 in presence of Pd as catalyst gives ethanol, whereas in presence of Pd-BaSO4, it gives ethanal.

Colloids

Solutions are homogenous systems e.g. sugar solution. If sand is stirred in water, it slowly settles down and is called a suspension. Between the extremes of suspensions and solutions a large group of systems called colloidal dispersion or simply colloids exist. A colloidal is a heterogeneous system in which one substance is dispersed (dispersed phase) as very fine particles in another substance called dispersion medium. The essential difference between a solution and a colloidal is one of particle size. In a solution, the particles or ions are small molecules. In a colloid, the dispersed phase may consist a particles of a simple macromolecule (such as protein or synthetic polymer) or an aggregate of many atoms, ions or molecules. Colloidal particles are larger than simple molecules but small enough to remain suspended. They have a range of a diameter between 1 and 1000 nm (109 to 106m). Illustration 7. Comment on the statement that “colloid is not a substance but a state of

substance”. Solution: Any substance (solid, liquid or gas) using special method can be brought into

colloidal state. For example, NaCl in water forms true solution but in benzene forms colloidal solutions in alcohol but colloidal in water.

Classification of colloids

Colloids are classified on the basic of the following criteria:

(a) Physical state of dispersed phase and dispersion medium. (b Nature of interaction between dispersed phase and dispersion medium. (c) Type of particles of the dispersed phase.

1. Classification based on physical state of dispersed phase and dispersion medium

Depending upon whether the dispersed phase and the dispersion medium are solids, liquids or gases, eight types of colloidal systems are possible. A gas mixed with another gas forms a homogenous mixture and hence is not a colloidal system. The examples of the various of colloids along with their typical names are listed in table:

Types of colloidal systems

Dispersed phase

Dispersion medium

Type of colloid Example

Solid Solid Solid sols Allows, ruby glass, gems, marbles etc. Solid Liquid Sols Gold sol, protein, starch, Agar, gelatin in

water etc.Solid Gas Aero sols of solid Smoke, stormLiquid Solid Gels Cheese, gems, gellies, curd etc.Liquid Liquid Emulsions Butter, milk, blood, shampoo etc.Liquid Gas Aero sols of liquid Fog, clouds, mist etc.Gas Solid Solid foams Pumice stone, styrene foam, porous pot,

mattresses etc.Gas Liquid Foams Froths, air bubble, whipped cream etc.

Out of various types of colloids given above, the most common are sols (solid in liquids), gels (liquids in solids) and emulsions (liquid in liquids). Further, it may be mentioned that depending upon the dispersion medium, the sols are given special names as follows:

Dispersion medium Name of the sol Water Aquasol or hydrosol Benzene BenzosolGases Aerosol

Illustration 8. System Special Name

(a) Liquid dispersed in gas (i) Solid sol (b) Gas dispersed in liquid (ii) Gel (c) Liquid dispersed in liquid (iii) Solid foam (d) Solid dispersed in liquid (iv) Sol (e) Gas dispersed in solid (v) Emulsion (f) Liquid dispersed in solid (vi) Foam (g) Solid dispersed in solid (vii) Aerosol

Solution: (a) – (vii), (b) – (vi), (c) – (v), (d) - (iv), (e) – (iii), (f) – (ii), (g) – (i)Exercise7.

Name of colloidal solution Example

(a) Emulsion (i) Pumic stone(b) Aerosol (ii) Rocks salts(c) Sol (iii) Cheese (d) Solid sol (iv) Protein (e) Foam (v) Milk (f) Gel (vi) Mist (g) Solid foam (vii) Whipped cream

2. Based on the nature of interaction between dispersed phase and dispersion medium: On this basis, colloidal sols are divided into two categories namely, lyophilic and lyophobic. If water is the dispersion medium, the terms are hydrophilic and hydrophobic. (i) Lyophilic colloids: The word ‘lyophilic’ means solvent loving. Colloidal sols directly formed

by substances like gum, gelatine, starch, rubber etc., on mixing with a suitable liquid (the dispersion medium) are called lyophilic sols. If the dispersion medium is separated from the dispersed phase (by evaporation), the sol can be reconstituted by simply remixing with the dispersion medium. That is why these sols are also called reversible sols.

(ii) Lyophobic colloids: The word ‘lyophobic’ means solvent hating substances. When substances are simply mixed with the dispersion medium they do not form the colloidal sol. These sols are readily precipitated (or coagulated) on the addition of small amounts of electrolytes, by heating or by shaking and hence are not stable. Further, once precipitated, they do not give back the colloidal sol by simple addition of the dispersion medium. Hence these sols need stabilizing agents for their preservation.

Exercise 8.Property Name

(a) Water loving colloids (i) Hydrophobic colloids (b) Water hating colloids (ii) Reversible colloid (c) Prepared easily from a precipitate (iii) Irreversible colloid (d) Can not be prepared easily from

precipitate(iv) Hydrophilic colloid

3. Classification based on types of particles of the dispersed phase: Multimolecular macromolecular and associated colloids Depending upon the type of the particles of the disperised phase, colloids are classified as: multimolecular, macromolecular and associated colloids.

(i) Multimolecular colloids When on dissolution, a large number of atoms or smaller molecules of a substance aggregate together to form species having size (with diameters less than 1 nm) in the colloidal range, the species thus formed are called multimolecular colloids. For example sulphur sol consist of particles containing a thousand or more of S8 sulphur molecules.

(ii) Macromolecular colloids: Macromolecules have large molecular masses. These on dissolution in a suitable solvent form a solution in which the size of the macromolecules may be in the colloidal range. Such systems are called macromlecular colloids. These colloids are quite stable and resemble true solutions in many respects.

Examples of naturally occuring macromolecules are starch, cellulose, proteins and enzymes. Examples of man – made macromolecules are polyethene, nylon, polystyrene, synthetic rubber etc.

(iii) Associated Colloids (Micelles): There are some substances which at low concentrations behaves as normal, strong electrolytes but at higher concentrations exhibit colloidal behaviour due to the formation of aggregated particles. The aggregated particles thus formed are called micelles. These are also known as associated colloids. The formation of micelles take place only above a particular temperature called kraft temperature (Tk) and above a particular concentration called critical micelle concentration (CMC). On dilution, these colloids revert back to individual ions. Surface active agents such as soaps and synthetic detergents belong to this class. For soaps, the CMC is 104 to 103 molL1. These colloids have both lyophobic and lyophilic parts. Micelles may contains as many as 100 molecules or more.

Illustration 9. Why medicines are more effective in colloidal state?

Solution: A colloidal state has larger surface area of sol particles and this shows more effective adsorption. Thus medicines in colloidal state are more effectively absorbed and give better results.

Exercise 9.Fatty deposits on pans can be removed by boiling pans with washing detergency.

Preparation of colloidal sols For the preparation of lyophobic and lyophilic sols different methods are employed.

(A) Preparation of lyophobic sols Lyophobic sols can be prepared by two methods (i) Condensation methods (ii) Dispersion methods

(i) Condensation methodIn these methods particles of atomic or molecular size are induced to combine to form aggregates having colloidal dimensions. For this purpose chemical as well as physical methods can be applied.

(a) Chemical methodColloidal solutions can be prepared by chemical reactions leading to formation of molecules by double decomposition, oxidation, reduction or hydrolysis. These molecules then aggregate leading to formation of sols.

(b) Physical methods

(i) Exchange of solvent: When a true solution is mixed with an excess of the other solvent in which the solute is insoluble but solvent is miscible a colloids sol is obtained. For

example, when a solution of sulphur in alcohol is poured in excess of water, a colloidal sol of sulphur is obtained.

(ii) Excessive cooling: The colloidal sol of ice in an organic solvent such as CHCl3 or ether can be obtained by freezing a solution of water in the solvent. The molecules of water which can no longer be held in solution separately combine to form particles of colloidal size.

2. Dispersion methods

In these methods large particles of the substance are broken into particles of colloidal dimensions in the presence of dispersion medium. These are stabilized by adding some suitable stabilizer. Some of the methods employed for carrying out dispersion are given below: (a) Mechanical dispersion

In this method, the coarse suspension of the substance is brought into a colloidal state in the dispersion medium by grinding it in a colloid mill, ball mill or ultrasonic disintegrator the colloid mill consists of two metal discs. Close together, rotating at high speed (7000 revolution per minute) in opposite directions. The suspension particles are form to the colloidal size. (b) Electrical disintegration or Bredig’s arc methods

This process involves dispersion as well as condensation. Collodial sols of metals such as gold, silver, platinum etc can be prepared by this method. In this method electric arc is struck between electrodes of the metal immersed in the dispersion medium. The intense heat produced vaporizes the metal, which then condenses to form particles of colloidal size.

Bredig’s arc method

(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. This method is applied, generally, to convert a freshly prepared precipitate into a colloidal sol. For example, when freshly precipitated Fe(OH)3 is shaken with aqueous solution of FeCl3(peptizing agent) it adsorbs Fe+3 ions and thereby breaks up into small – sized particles.

(B) Preparation of lyophilic sols

Lyophilic sols are quite stable and can be easily prepared by shaking the lyophilic material with dispersion medium. Some examples colloidal sols of gelatin, gum, starch, egg.

Purification of colloidal sols

The colloidal sols obtained by various methods are impure and contain impurities of electrolytes and other soluble substances. These impurities may destabilize the sol. Hence, they have to be

removed. A very important method of removal of soluble impurities from sols by a semipermeable membrane is known as dialysis.

(i) Dialysis

Particles of true solutions can pass through parchment paper or cellophone membrane, sol particles can not pass through these membranes. A bag made up of such membrane is filled with the colloidal solution and is then suspended in fresh water. The electrolyte particles pass out leaving behind the colloidal sol. Movement of ions across the membrane can be expendited by applying electric potential through two electrodes. This method is faster than simple dialysis and is known as electrodialysis. (ii) Ultra –filtration

In this method, colloidal sols are purified by carrying out filtration through special type of graded filters called ultra – filter. These filter papers allow only the electrolytes to pass through. These filter papers are made of particular pore size by impregnating ordinary filter paper with colloidal particles. In order to accelerate the filtration through such filter papers, increased pressure or section is employed.

(iii) Ultra-centrifugation

In this method, the colloids sol is taken in a tube which is placed in an ultra – centrifuge. On rotation of the tube at high speeds, the colloidal particles settle down at the bottom of the tube and the impurities remain in the solution called the centrifugate. The settled colloidal particles are mixed with an appropriate dispersing medium to regenerate the sol.

Important properties of colloidal sols

(i) Colligative properties: Colloidal sols show the colligative properties, viz., relative lowering of vapour pressure, elevation in boiling point, depression in freezing point and osmotic pressure.

(ii) Optical propertiesTyndall effect: If a strong beam of light is passed through a colloidal sol placed in a dark place, the path of the beam gets illuminated. This phenomenon is called tyndall effect. True solutions do not exhibit tyndall effect because the particles in them are too small in size and do not cause any scattering.

Illustration 10. Sky appear blue. Why?

Solution: Colloidal particles of dust, dirt in air scatter blue light to the maximum extent.

(iii) Mechanical properties

Brownian movement

The colloidal particles are seen to be in constant zig-zag motion. This zig – zag motion is called Brownian movement. Brownian movement arises because of the impact of the molecules of the dispersion medium with colloidal particles. It has been postulated that the impact of the molecules of the dispersion medium with the colloidal particles. As the size of the particle increases, Brownian movement becomes slow. Ultimately, when the dispersed particles become big enough to acquire the dimensions of suspension, no Brownian movement is observed.

(a) Brownian movement provides a direct demonstration of ceaseless motion of molecules as postulated by kinetic theory.

(b) It counters the forces of gravity acting on colloidal particles and hence helps in providing stability to colloidal sols by not allowing then to settle down.

(iv) Electrical properties (Electrophoresis): The particles of the colloids are electrically charged and carry positive or negative charge. The dispersion medium has an equal and opposite charge making the system neutral as a whole due to similar nature of the charge carried by the particles. They repel each other and do not combine to form bigger particles. That is why a sol is stable and particles do not settle down. Arsenious sulphide, gold silver and platinum particles in their respective colloidal sols are negatively charged while particles of ferric hydroxide, aluminium hydroxide are positively charged. The existence of the electric charge is shown by the phenomenon of electrophoresis. It involves the movement of colloidal particles either towards the cathode or anode, under the influence of the electric field.

(iv) Coagulation of colloids: The presence of small amounts of appropriate electrolytes is necessary for the stability of the colloids. However, when an electrolyte is added in larger concentration, the particles of the sol take up the ions, which are oppositely charged and thus get neutralized. The neutral particles then start aggregating giving particles of larger size which are then precipitated. This process of aggregation of colloidal particles into an insoluble precipitate by the addition of some suitable electrolyte is known as coagulation. At lower concentration of electrolytes, the aggregation of particles is called flocculation. The minimum amount of an electrolyte (in milli moles) that must be added to one litre of a colloidal solution so as to cause its compete coagulation is called the precipitation or coagulation value of the electrolyte.

Different electrolytes have different coagulation value.

(i) The ions carrying charge opposite to that of sol particles are effective in causing the coagulation of the sol.

(ii) Coagulation power of an electrolyte is directly proportional to the fourth power of the valency of the ions causing coagulation.

Illustration 11. Which of the following can act as a protective colloid? (A) gelatin (B) silica gel (C) oil-in-water emulsion (D) all correct

Solution: (A)

Illustration 12. The coagulation of 100 ml of a colloidal sol of gold is completely prevented by addition of 0.25 g of starch to it before adding 1 ml of 10 % NaCl solution. Find out the gold number of starch.

Solution: Starch added to 100 ml of gold sol to completely prevent coagulation by 1 ml of 10% NaCl sol = 0.25 g = 250 mg. Starch required to be added to 10 ml of gold sol to completely prevent coagulation by 1 ml of 10 % NaCl sol = 25 mg By definition, gold number of starch = 25

Illustration 13. For the coagulation of 100 ml of arsenious sulphide sol, 5 ml of 1 M NaCl is required. What is the Flocculation value of NaCl?

Solution: 5 ml of 1 M NaCl contains = of NaCl.

Thus 100 ml of As2S3 sol require NaCl for complete coagulation = 5 millimoles 1 L i.e. 1000 ml of the sol require NaCl For complete coagulation = 50 millimoles By definition, Flocculation value of NaCl = 50.

Illustration 14. How rubber is obtained by latex?

Solution: Latex is a colloidal solution of rubber having – vely charged rubber particles. These are coagulated to give rubber.

Exercise 10.Artificial rain is made by spraying salt over clouds.

Exercise 11.What is electrostatic precipitation of carbon?

Exercise 12.Gold number for different lyophilic colloids (A, B, C, D) are in the order A < B < C < D. Which one is more powerful protective colloid and why?

Exercise 13.Which of the following will have the highest coagulation power for As2S3 colloid? (A) (B)(C) (D)

EMULSIONS

Emulsions are colloids in which both dispersed phase and dispersion medium are liquids. Emulsion can be broadly classified into two types.

(i) Oil in water emulsions: In this type of emulsions, oil acts as (organic solvent) dispersed phase and water acts as dispersion medium. Some examples of this type of emulsions are milk, vanishing cream, etc. In milk, liquid fat is dispersed in water.

(ii) Water in oil emulsions: In this type of emulsions, water acts as dispersed phase and oil (organic solvent) acts as dispersion medium. Cold cream, butter etc, are examples of oil emulsions.

Identification of Emulsion

The following tests may be employed to distinguish between the two types of emulsions:

(i) Dye test: Some oil soluble dye is added to the emulsion. If the background becomes coloured, the emulsion is water – in – oil type and if the coloured droplets are seen, the emulsion is oil – in – water type.

(ii) Dilution test: If the emulsion can be diluted with water, this indicates that water is the dispersion medium and the emulsion is of oil – in – water type. In case the added water forms a separate layer, the emulsion is water – in – oil type.

Exercise 14. Butter is a colloid formed when (A) fat is dispersed in water (B) fat globules are dispersed in water (C) water is dispersed in fat (D) none of the above

Preparation of Emulsion

The process of making an emulsion is known as emulsification. Emulsion may be obtained by vigorously mixing both the liquids. The dispersed drops at once come together and form separate layers. To stabilize an emulsion, the addition of a small quantity of a third substance known as emulsifying agent or emulsifier is essential. Soaps and detergents are most frequently used as emulsifiers.The other common stabilizing agents are proteins, gum and agar-agar.

Illustration 15. Why gelatin is added to ice cream?

Solution: To stabilize to prevent coagulation of ice cream particles.

Demulsification

The separation of an emulsion into its constituent liquids is called demulsification. The various techniques applied for demulsification are freezing, boiling, centrifugation, electrostatic precipitation or chemical methods, which destroy the emulsifying agents.

APPLICATION OF COLLOIDS

Colloids including emulsions find a number of uses in our daily life and industry. Some of the uses are given below:

(i) Rubber plating: Latex is a colloidal solution of negatively charged rubber particles. Rubber plated articles are prepared by depositing negative charged particles over the article to be rubber plated by making that article an anode in a rubber plating bath.

(ii) Medicines: Medicines in colloidal form are easily absorbed by the body tissues and hence are more effective.

(iii) Sewage disposal: Colloidal particles of dirt, mud etc. carry electric charge. Hence when sewage water is passed through the plates kept at a high potential, the colloidal particles are coagulated due to electrophoresis and the suspended matter gets removed.

(iv) Purification of water: The precipitation of colloidal impurities present in water can be done by adding certain electrolytes like alum etc. The negatively charged colloidal particles of impurities get neutralized by the Al3+ ions and settle down and pure water can be decanted off.

(v) Formation of delta: River water contains charged colloidal particles of clay, sand and many other materials. Sea water is a very big store – house of a variety of electrolytes dissolved in it. As soon as river water comes in contact with sea water, the electrolytes present in sea water coagulate the suspended colloidal particles which ultimately settle down at the point of contact and thus the level of the river bed rises. As a result, water adopts a different course and delta is formed in due course of time.

(vi) Smoke screen: In warfare, smoke screens are used which are colloidal dispersion of certain substance in the air.

Illustration 16. Property Reason (a) Brownian movement (i) Due to the scattering of light (b) Tyndal effect (ii) Due to the presence of charge (c) Electrophoresis (iii) Due to the neutralization of charge (d) Coagulation (iv) Due to unequal bombardment by

solvent molecules

Solution. (a) – (iv), (b) – (i), (c) – (ii), (d) – (iii)

Exercise 15.Property Application

(a) Tyndal effect (i) Smoke precipitator(b) Eectrophoresis (ii) Ultra microscope (c) Coagulation (iii) Medicine (d) Adsorption (iv) Sewage water disposal

Exercise 16.System Special Name

(a) Bohr (i) Vapour pressure (b) Roentgen (ii) Colloids (c) Tyndal (iii) Chemical bond (d) Raoult (iv) Frequency conditions(e) Pauling (v) X- rays

ANSWER TO EXERCISES

Exercise 1:Activation of adsorbent refers to its clean surface and fine particle size. Cleaning can be done by ultrahigh vacuum while large surface area can be achieved by grinding.

Exercise 2:Catalyst increases the rate of forward reaction and backward reaction, which ultimately helps in attaining the equilibrium earlier.

Exercise 3:On grinding, an increase in surface area results in an increase in active centres which leads to more degree of adsorption.

Exercise 4:No. The colour in later case is discharge by nascent H, the reactive form of H2

Exercise 5:CO acts as poison for catalyst Fe and thus it should be removed.

Exercise 6:The later is Rosenmund’s reaction where activity of Pd is reduced by BaSO 4 (a poison).

Exercise 7:(a) – (v), (b) – (vi), (c) – (iv), (d) – (ii), (e) – (vii), (f) – (iii), (g) – (i)

Exercise 8:(a) – (iv), (b) – (i), (c) – (ii), (d) – (iii)

Exercise 9.Na2CO3 (washing soda) on hydrolysis form NaOH which on boiling with fat give rise to Saponification.

A part of fat is thus used and rest fat is removed by the soap so formed.

Exercise 10:Clouds are water dispersed in gas systems. Spray of salt causes their coagulation and thus water molecules drop down to bring in rain.

Exercise 11:Smoke contains – vely charged carbon particles dispersed in air. The smoke, before it comes out from the chimmey, is passed through a chamber containing charged plates at high tension i.e. high electrode potentials. The vely charged particles lose their charge at anode and get precipitated. The method is used to control air and soil pollution. The apparatus is known as Cotrell’s precipitation.

Exercise 12:D is most powerful protective colloid. Lesser is gold number more is protecting power.

Exercise 13:(C)

Exercise 14: (C)

Exercise 15:(a) – (ii), (b) – (i), (c) – (iv), (d) – (iii)

Exercise 16: (a) – (iv), (b) – (v), (c) – (ii), (d) – (i), (e) – (iii)

MISCELLANEOUS EXERCISES

Exercise 1: What are micelles? Give an example of a miscellar system.

Exercise 2: Write a note on multimolecular colloids.

Exercise 3: Explain adsorption and absorption with one example each.

Exercise 4: Describe how adsorption of a gas on a solid depends on (i) gas pressure (ii) temperature

Exercise 5: What is adsorption? Why adsorption takes place only at the interface?

Exercise 6: Compare characteristics of physical adsorption and chemisorption.

Exercise 7: Give an account on important applications of the phenomenon of adsorption.

Exercise 8: Write short notes on (i) homogenous catalysis (ii) heterogenous catalysis

Exercise 9: When acidic solution of KMnO4 is added to a hot solution of oxalic acid, the colour is decolorised slowly in the beginning but after sometime it disappears rapidly.

Exercise 10: Explain the term dialysis.

SOLVED PROBLEMS

Subjective:

Board Type Questions

Prob 1. What happens when a colloidal sol of Fe(OH)3 is mixed with that of As2O3?

Sol. Their mutual precipitation occurs.

Prob 2. What type of solution NaCl form in benzene?

Sol. Colloidal solution

Prob 3. What is the difference in the nature of a dilute soap solution and a concentration soap solution?

Sol. Dilute soap solution behave like a true solution whereas concentrated soap solution behave like a colloidal solution.

Prob 4. What happens when a freshly prepared Fe(OH)3 is shaken with a little amount of dilute solution of FeCl3?

Sol. Peptization takes place forming a positively charged colloidal sol of Fe(OH)3.

Prob 5. What happens to a gold sol if gelatin is added to it?

Sol. Gold sol which is lyophobic starts behaving like a lyophilic sol.

IIT Level Questions

Prob 6. What is the difference between a colloidal sol, gel and emulsion?

Sol. In a colloidal sol, the dispersed phase is a solid and the dispersion medium is a liquid, in a gel, it is opposite. In an emulsion, both the dispersed phase and dispersion medium are liquids.

Prob 7. Why is a colloidal sol stable?

Sol. All the particles in a colloidal sol carry the same charge and hence keep repelling each other and can not aggregate together to form bigger particles.

Prob 8. What happens when persistent dialysis of a colloidal solution is carried out?

Sol. The stability of a colloidal sol is due to the presence of a small amount of the electrolyte. On persistent dialysis, the electrolyte is completely removed. As a result, the colloidal sol becomes unstable and gets coagulated.

Prob 9. What is demulsification? Name two demulsifiers.

Sol. The process of separation of the constituent liquids of an emulsion is called demulsification. Demulsification can be done by centrifuging or boiling.

Prob 10. The conductance of an emulsion increases on adding common salt. What type of emulsion is this?

Sol. Oil in water type.

Prob 11. Alum is used in cleaning town water supply. Explain.

Sol. The water obtained from natural sources is often contaminated with bacteria and dispersed impurities. Alum is added to such water to destroy bacteria (antibacterial nature) as well as to coagulate undesired impurities (coagulating nature) as to make water fit for drinking purpose.

Prob 12. Why hard water consumes more soap?

Sol. Soap on dissolution in water give RCOO ions which undergoes to micelle formation

Responsible for cleaning. In hard water, RCOO ions are used up by Ca+2 or Mg2+ ions to form insoluble (RCOO)2Ca or (RCOO)2Mg and thus micelle formation starts only when whole of Ca2+ and Mg2+ ions are precipitated out by soap.

Prob 13. What is the difference between multimolecular and macromolecular colloids? Give an example of each. How are associated colloids different from these two types of colloids?

Sol. Multimolecular colloids contain dispersed particles made of aggregates of many molecules. These are lyophobic colloids e.g. As2S3 sol. Macromolecular colloids are molecularly dissolved solutions of a polymer (due to large size of polymer molecule, it take colloidal state) and are lyophilic colloids e.g. starch solution. Association colloids are also lyophilic colloids but are formed by the aggregation of soap or detergent molecules in solution above CMC (these are miceller colloids).

Prob 14. Describe a chemical method each for the preparation of sols of sulphur and platinum in water.

Sol. S-sol: By bubbling H2S gas in cold solution of an oxidising agent. (i)

(ii)

(iii)Pt-sol: By reducing its salt solution by suitable reducing agent like formaldehyde, SnCl2, hydrazine etc.

Prob 15. Action of soap is due to emulsification and micelle formation. Comment.

Sol. Soap molecules adsorb on to the (oily) surface and try to emulsify it. Soap micelles to solubilize the insoluble dirt / greasy material.

Objective:

Prob 1. Electricity is passed through a colloidal solution then it is called as (A)Brownian movement (B) cataphoresis (C) electro-osmosis (D) tyndall effect

Sol. (B)

Prob 2. As2O3 sol is (A) positive colloid (B) negative colloid (C) neutral colloid (D) none of the above

Sol. (B)

Prob 3. Which one of the following is a lyophilic colloid? (A)milk (B) gum (C) fog (D) blood

Sol. (B)

Prob 4. Cellulose dispersed in ethanol is called (A) emulsion (B) micelle (C) collodion (D) hydrophilic sol

Sol. (C)

Prob 5. Action of catalyst depends on (A) mass (B) solubility (C) particle size (D) none

Sol. (C)

True and False

Prob 6. The colligative properties of colloidal solutions are always lower.

Sol. True

Prob 7. Fog is an example of colloidal solution depends upon size and shape of sol particles.

Sol. True

Prob 8. Zymase is needed to catalyse the hydrolysis of glucose.

Sol. False

Prob 9. Enzyme catalysed reactions are highly specific.

Sol. True

Prob 10. A catalyst increases the rate of forward reaction without influencing that of reverse reaction.

Sol. False

Fill in the blanks

Prob 11. Lower is the critical micelle concentration of surfactant,……………is the detergency.

Sol. More

Prob 12. More is the coagulation value of electrolyte, lesser is its……………………….

Sol. Coagulation power

Prob 13. The charge on sol particles of +ve AgI sol is due to preferential adsorption of……….ion.

Sol. Ag+ ion

Prob 14. The phenomenon when a reaction influences the rate of other reaction is…………….

Sol. Induced catalysis

Prob 15. The oxidation of CHCl3 is retarded by adding……………………

Sol. Alcohol

ASSIGNMENT PROBLEMS

Subjective:

Level – O

1. Explain the role of adsorption in heterogeneous catalysis.

2. Explain the cause of peptization.

3. Explain the variation of adsorption with pressure.

4. Why the sun looks red at the time of setting? Explain on the basis of colloidal properties.

5. What do you understand by activity and selectivity of catalysts? Describe some features of catalysis by zeolites.

6. Why physical adsorption is multimolecular whereas chemisorption is unimolecular?

7. Write michalis – menten equation. Deduce the conditions under which the reaction is of first order or second order.

8. What do you understand by “isoelectric point of colloid”.

9. What do you understand by “induced catalysis”? Explain with a suitable example.

10. What do you understand by kraft temperature and CMC in colloids? What is the CMC for soap solution?

Level – I

1. Frequently, preparation of a colloid such as a protein can be made more stable if the colloid is dialyzed. Why is this so?

2. Which will be adsorbed more readily on the surface of charcoal and why NH3 or CO2?

3. Why silica gel in used as a dehumidizer?

4. Which one of the following electrolytes is more effective for the coagulation of Fe(OH)3 sol and why? NaCl, Na2SO4, Na3PO4

5. What type of building blocks are present in the structure of zeolites? What is this structure called?

6. Why adsorption is always exothermic?

7. What is the significance of reciprocal of ‘gold number’?

8. A small amount of silica gel and that of anhydrous CaCl2 are placed separately in two comes of a vessel containing water vapour. What phenomena will occur?

9. A catalyst lowered the activation energy by 25 KJ/mol at 25C. By how many times will the rate grow?

10. The decomposition of ether takes place in presence of I2. The rate law of decomposition may be given as

What is the nature of reactant and the role of I2?

Level – II

1. Give six difference between physical adsorption and chemisorption.

2. What do you mean by an adsorption isotherm and an adsorption isobar? Explain Freundlich adsorption isotherm for the adsorption of gases on solids.

3. Comment on the adsorption of nitrogen on iron.

4. A particle of radius 1 cm is broken to form colloidal particles of radius 1000A . Calculate the number of colloidal particles produced and their total surface area.

5. How does the rate of enzyme – catalysed reactions vary with (i) temperature (ii) pH? Represent diagrammatically.

6. Compare the coagulating power of AlCl3 with that of NaCl. Given that this coagulation values are 0.093 and 52 respectively.

7. SnO2 forms a positively charged colloidal sol in acidic medium and a negatively charged sol in the basic medium. Why? Explain.

8. Give the mechanism of following reactions (a) In lead chamber process, NO(g) is used as catalyst in the oxidation of SO2 to SO3. (b) NO(g) catalyses the decomposition of ozone to oxygen. (c) Ozone layer depletion by Freon or Teflon.

9. A solution of palmitic acid (M = 256) in benzene contains 4.24 g of acid per litre. When this solution is dropped on the water surface, benzene evaporates and palmitic acid form monomolecular film of the solid type. If we wish to cover an area of 500 cm 2 with a monolayer, what volume of solution should be used? The area occupied by one palmitic acid molecule may be taken to be .

10. Explain the use of the following as catalyst. (i) zeolites (ii) enzymes

Objective:

Level – I

1. Absorption is generally accompanied by (A) decrease in entropy of system (B) decrease in enthalpy (C) making the value of TS negative (D) all

2. The function of zymase is to (A) change starch into sugar. (B) conversion of glucose to alcohol and carbon dioxide. (C) change starch into malt sugar and dextrin. (D) change malt sugar into glucose.

3. Cationic surfactant is (A) C17H35COONa (B)

(C) C16H33N(CH3)3Cl (D) none

4. None – ionogenic surfactants are (A) (B) C17H35COONa

(C) CnH2n+1 (OCH2CH2)xOH (D) all

5. Which can absorbs larger volume of hydrogen gas? (A) colloidal solution of palladium (B) finely divided nickel (C) finely divided platinum (D) colloidal Fe(OH)3

6. Which forms multimolecular layer during adorption? (A) physical adsorption (B) Vander-waal’s adsorption (C) freundlich adsorption (D) all

7. In which of the following reaction is a catalyst required? (A) (B)

(C) (D) all of the above

8. Which acts as poison for Pd charcoal in lindlar catalyst? (A) BaSO4 (B) Quinoline (C) both (A) and (B) (D) none

9. In the following which one of the following is a natural colloid (A) sodium chloride solution (B) cane sugar solution (C) urea solution (D) blood

10. Which is an example of auto catalysis? (A) hydrolysis of methyl acetate (B) decomposition of tri nitro glycerin(C) oxidation of oxalic acid by KMnO4 (D) all

11. Brownian movement is ………………property (A) electrical (B) mechanical (C) optical (D) colligative

12. The tyndall effect is not observed in (A) suspension (B) true solution (C) emulsion (D) colloidal solution

13. Smoke has generally blue tings. It is due to (A) scattering (B) coagulation (C) Brownian motion (D) electro-osmosis

14. Collodial system is a two phase system. Point out which of the following combination can not be considered as colloidal system (A) solid – solid (B) liquid – liquid (C) gas – gas (D) liquid – gas

15. Rate of physisorption increases with (A) decrease in temperature (B) increase in temperature (C) decrease in pressure (D) decrease in surface area

Level – II

1. Which plot is the adsorption isobar for chemi-sorption? (A) (B)

(C) (D)

2. Which of the following is an example of zeolite? (A) ZSM-5 (B) AgNO3 (C) Mg(OH)2 (D) Co(OH)3

3. Which of the following types of materials acts as effective catalysts? (A) alkali metals (B) transition metals (C) alkaline earth metals (D) radioactive metals

4. An example of bio catalysis is (A) enzyme (B) minerals (C) plants (D) all proteins

5. Catalyst increases the rate (A) by decreasing Ea (B) by increasing Ea(C) by increasing entropy (D) all of these

6. Zeolites are used as catalyst in (A) petrochemical industries during cracking (B) In the preparation of H2SO4 (C) In the hydrolysis of ester (D) all

7. In the adsorption of oxalic acid by activated charcoal, the activated charcoal is known as (A) adsorbent (B) adsorbate (C) absorber (D) none

8. The enzyme ptyalin used for digestion of food is present in (A) saliva (B) blood (C) Intestine (D) adrenal glands

9. In acid – base catalysis, the real catalysis is (A) acid (B) base (C) H+ ions (D) acid or base

10. The formation of diethyl ether form ethanol is catalysed by

(A) H2SO4 (B) AlCl3 (C) Cu (D) Ni

11. In Zeigler Natta polymerization of ethylene, the active species are (A) AlCl3 (B) Et3Al (C) C2H4 (D) Ti+3

12. The heats of adsorption in physio-sorption lie in the range (KJ/mole) (A) 40 – 400 (B) 40-100 (C) 10-40 (D) 1 – 10

13. The simplest way to check whether a system is a colloid is by (A) tyndall effect (B) Brownian movement (C) electro dialysis (D) finding out particles size

14. Which of the following ions can cause coagulation of proteins?(A) Ag+ (B) Fe3+

(C) Cu+ (D) Ni2+

15. Which of the following is less than zero during adsorption? (A) G (B) S (C) H (D) All the above

ANSWERS TO ASSIGNMENT PROBLEMS

Subjective:

Level – I

1. This is because dialysis helps in removing undersiable ions form a colloidal preparation in order to stabilize the colloid.

2. NH3 has higher critical temperature than that of CO2 i.e. NH3 is more easily liquefiable than CO2. Hence NH3 has greater intermolecular forces of attraction and hence will be adsorbed more readily.

3. Silica gel has stronger adsorption for humidity i.e. moisture of the air. Hence it is used as a dehumidizer.

4. Fe(OH)3 is a positively charged sol. According to hardy-schulz rule, greater the charge on the oppositely charged ion of the electrolyte added, more effective it is in bringing about coagulation. Hence Na3PO4 (containing ions) is most effective.

5. The building blocks in the structure of zeolites are truncated octahedral (cubooctahedral). This structure is called ‘b - cage’ or ‘sodalite (age).

6. Adsorption is accompanied by decrease of radomness i.e. this factor opposes the process i.e. S is ve. For the process to be spontaneous, G must be ve. Hence according to equation G = H TS, G can be ve only if H is ve and T is low.

7. Smaller the gold number, greater is its protective power. Hence reciprocal of gold number is direct measure of the protective power of the colloid.

8. Adsorption of water vapour on silica gel and absorption on CaCl2.

9. 24069 times

Level – II

9. 0.0239 cm3

Objective:

Level - I

1. D 2. B 3. C4. C 5. A 6. D7. C 8. C 9. D10. D 11. B 12. B13. A 14. C 15. A

Level - II

1. C 2. A 3. B4. A 5. A 6. A7. A 8. A 9. D10. A 11. D 12. C13. A 14. A 15. D