It has been observed that solid substance(s) have characteristic behaviour as in many cases the solid has the tendency to attract and retain the molecules of the other species in contact only at the surface and not deeper into the bulk. Such tendency becomes more prominent if the substance is in finely divided form or it’s surface is rough. Under these conditions, the surface area increases and also the residual forces acting along the surface increase.

The phenomenon of higher concentration of any molecular species at the surface than in the bulk is known as adsorption. The substance adsorbed is known as adsorabate and the substance on which adsorbate it is adsorbed is known as adsorbent. The removal of the adsorbed sustance from a surface is called desorption. When equilibrium is reached

Rate of adsorption $=$ Rate of desorption

Whereas, a physical or a chemical phenomenon in which atoms, molecules or ions enter some bulk phase - gas, liquid or solid is called absorption.

When adsorption and absorption occur simultaneously and it is difficult to measure their relative extent then the phenomenon is referred to absorption

Difference between Adsorption and Absorption:

Factors affecting adsorption

i) Nature of the adsorbent: Same gas may be adsorbed to different extents on different adsorbents.

ii) Surface area of the adsorbent: Greater the surface area, greater is the adsorption. That is why adsorption increases if the adsorbent is subdivided into smaller particles.

iii) Nature of the gas being adsorbed: Greater is the critical temperature of a gas, greater are the vanderwaals forces of attraction, greater is the adsorption.

iv) Temperature: On increasing the temperature, extent of adsorption, being exothermic process decreases.

v) Pressure: Adsorption increases with pressure at constant temperature.

vi) Activation of the solid adsorbent: This means increasing the adsorbing power of the solid adsorbent. This can be done by subdividing the solid adsorbent or by removing the gases already adsorbed by passing superheated steam.

vii) Liquefacation of gases: Gases that get liquefied easily such as $\mathrm{HCl}, \mathrm{NH} _{3}$, and $\mathrm{CO} _{2}$ get adsorbed to a greater extent than like $\mathrm{H} _{2}, \mathrm{~N} _{2}, \mathrm{O} _{2}$, etc.

Types of adsorption

Depending upon the nature of forces between the adsorbate and the adsorbent molecules, adsorption can be classified into two groups namely physical adsorption and chemical adsorption (chemisorption)

A physical adsorption at low temperature may pass into chemisorption as the temperature is increased. e.g., $\mathrm{H} _{2}$ is first adsorbed on the surface of nickel by vander waal forces, it then dissociates to form ’ $\mathrm{H}$ ’ atom which are held to the surface by chemisorption.

Adsorption Isotherm

• Adsorption Isotherm: It is the plot of the mass of gas adsorbed per gram of adsorbent $(\mathrm{x} / \mathrm{m})$ versus equilibrium pressure at constant temperature where $\mathrm{x}$ represents the mass of the gas adsorbed by $m$ gram of the adsorbent.

• Freundlich Adsorption Isotherm: Freundlich adsorption isotherm at room temperature is shown in the Fig: 1.

Fig 1: Plot of $\mathrm{x} / \mathrm{m} \mathrm{v} / \mathrm{s} P$ for Freundlich adsorption isotherm

From the above plot, we observe that

i) For low pressures, $\dfrac{x}{m} \propto P$

ii) For high pressures, $\dfrac{x}{m} \propto P^{0}$

iii) For intermediate pressures,

$$ \dfrac{x}{m} \propto P^{1 / n} \text { or } \dfrac{x}{m}=k P^{1 / n} \hspace{5cm}\cdots \cdots \text{(i)} $$

where $\mathrm{n}$ is an integer and it is a constant.

The factor $\dfrac{1}{n}$ can have value between 0 and 1 .

When $\dfrac{1}{n}=0, \dfrac{x}{m}=$ constant, the adsorption is independent of pressure.

When $\dfrac{1}{n}=1, \dfrac{x}{m}=k p$, i.e. the adsorption varies directly with pressure.

Taking logarithm of equation (i)

$\log \dfrac{x}{m}=\log k+\dfrac{1}{n} \log P$

Plot of $\dfrac{x}{m}$ vs $\log \mathrm{P}$ is a straight line with slope and intercept on $\mathrm{Y}$-axis $=\log \mathrm{k}$. (Fig.: 2)

Limitations of Freundlich adsorption

Fig. 2: Plot of $\log x / m$ v/s $\log P$ for Freundilich adsorption isotherr

• It is unable to explain deviation from linearity at high pressure.

• It is applicable only for physical adsorption as it considers multimolecular layer of adsorption.

• It doesn’t consider the role of surface area of adsorbent in the process of adsorption The Freundlich isotherm was modified by Langmuir in 1916.

Langmuir adsorption isotherm: Assuming the formation of a monolayer of the adsorbate on the surface of the adsorbent, it was derived by Langmuir that the mass of the gas adsorbed per gram of the adsorbent is related to the equilibrium pressure by the equation:

$$ \dfrac{x}{m}=\dfrac{a P}{1+b P} $$

where $a$ and $b$ are constants whose values depend upon the nature of the gas adsorbed, the nature of the adsorbent and temperature. The above equation can be rewritten as

$$ \begin{aligned} & \dfrac{x / m}{P}=\dfrac{a}{1+b P} \\ & \text { or } \dfrac{P}{x / m}=\dfrac{1+b P}{a}=\dfrac{1}{a}+\dfrac{b}{a} P \end{aligned} $$

Thus, a plot of $\dfrac{P}{x / m}$ versus $\mathrm{P}$ is a straight line (Fig. 3) with slope $=$ b/a and intercept on $y$-axis $=\dfrac{1}{a}$

Fig. 3: Plot of $\mathrm{P} /(\mathrm{x} / \mathrm{m}) \mathrm{v} / \mathrm{s} \mathrm{P}$ for Langmuir adsorption isotherm

Alternatively, writing the eqn. in the inverse form, we have

$\dfrac{m}{x}=\dfrac{1+b P}{a P}=\dfrac{b}{a}+\dfrac{1}{a P}$

Fig. 4: Plot of $\mathrm{x} / \mathrm{m} / \mathrm{s} 1 / \mathrm{P}$ for Langmuir adsorption isotherm

Thus a plot of $\mathrm{m} / \mathrm{x}$ vs $1 / \mathrm{P}$ will be a straight line (Fig.: 4 ) with slope $=1 / \mathrm{a}$ and intercept on $Y$ - axis $=b / a$

Adsorption Isobars

These are plots of $\mathrm{x} / \mathrm{m}$ vs temperature at constant pressure for physical and chemical adsorption, (Fig.: 5 & 6).

Adsorption of solutes from solutions by solid adsorbent

Solids can adsorb solutes from solution also. When a solution of acetic acid in water is shaken with charcoal, the concentration of acetic acid in the solution is found to decrease. This phenomenon is found to be similar to the adsorption of a gas on the surface of a solid. Therefore, Freundilich adsorption isotherm is applicable in the adsorption of solutes from solutions by solid adsorbent

$$ \text { i.e. } \dfrac{x}{m}=k c^{1 / n} $$

where $\mathrm{c}$ is the concentration of a solution in $\mathrm{mol} \mathrm{L}^{-1}$.

Taking logarithm on both sides of above equation

$$ \log \dfrac{x}{m}=\log k=\dfrac{1}{n} \log \alpha $$

Plot of $\log x / m$ vs/ $\log c$ is a straight line with slope of $(1 / n)$.

Applications of adsorption:- A few applications are:-

• Purification of many organic compounds by chromatographic adsorption.

• Separation of a mixture of inert gases by difference in degree of adsorption of gases by charcoal.

• The catalytic effect of a number of catalyst is based on the principle of adsorption.

• Removal of poisonous gases like $\mathrm{CO}, \mathrm{CH} _{4}$ using activated charcoal in gas masks.

Colloids

Depending upon the rate of diffusion Thomas Graham, classified the soluble substances into two categories: crystalloids (e.g. sugar, urea etc.) and colloids (e.g. starch, gelatine etc.). It was later observed that crystalline substance can be converted into colloidal form by suitable means. For e.g., the colloidal form of sodium chloride which is a crystalloid can be obtained in benzene.

Therefore, a new classification, based on the size of solute particles has be introduced

Colloids are classified on the basis of dispersed phase and dispersed medium

Dispersed phase is the substance which has been dispersed. It is a discontinuous phase, the particle size of which lies the range $1 \mathrm{~nm}-1000 \mathrm{~nm}$.

Dispersion medium is the medium in which the substance has been dispersed. It is a continuous phase.

There are eight different types of colloidals depending on the physical state of dispersed state and dispersion medium

Colloids are also classified on the basis of the nature of the dispersion medium.

Depending upon the nature of interaction between phase and dispersion medium, colloids can be classified as

Classifification on the basis of nature of particle of the dispersed phase.

On this basis, the colloids are classified into three types namely multimolecular colloids, macromolecular colloids and associated colloids.

• Multimolecular colloids: They are formed by the aggregation of a large number of atoms or molecules which generally have diameter less than 1nm, e.g., sols of gold, sulphur etc. Their atoms or molecules are held together by weak van der waals forces and their molecular masses are not high

• Macromolecular colloids: They are molecules of large size, e.g., polymers like rubber, nylon, polythene, starch, cellulose, proteins, enzymes, etc. These substances when dissolved in a suitable liquid, directly form the colloidal solution. They have high molecular masses and have lyophobic character.

• Associated colloids: The substances which when dissolved in a medium at low concentration behave as normal strong electrolytes but at higher concentration exhibit colloidal state properties due to the formation of aggregated particles are called associated colloids. The aggreagated particles thus formed are called micelles. Their formation takes place above a particular temperature called Kraft temperature $\left(T _{k}\right)$ and above a particular concentration called Critical Micelle Concentration (CMC). These molecules contain both lyophilic and lyophobic groups.

Preparation of Colloids

Different methods are used for the preparation of lyophobic and lyophilic sols.

• Preparation of Iyophilic sols: by dissolving these substances (e.g. starch, gelatine, glue etc.) in water either in cold or warm. Solutions of colloidal electrolytes (e.g. soaps, dyes) are also prepared in the similar manner.

• Preparation of lyophobic sols: To obtain a substance in colloidal form either the substance in bulk is broken down into particles of colloidal dimension ( $1 \mathrm{~nm}$ to $1000 \mathrm{~nm}$ ) or the size of molecular particles is increased to colloidal dimensions. In some cases, another substance is added to increase the stability of sol. Such substances are known as stabilizer. Thus, there are two ways by which lyophobic sols can be prepared.

$\qquad$I) Disintegration/Dispersion method

$\qquad$II) Aggregation/Condensation method

I) Disintegration methods:

These involve the breaking down of bigger particles into colloidal particles. This is done by the following methods.

i) Mechanical disintegration: The substance is grinded virtually to the size of colloidal particles and are then dispersed in the liquid. A stabilizer is often added to stabilize colloidal solution e.g. colloidal graphite and printing ink are prepared by this method.

ii) Electro-disintegration (Bredig’s arc method.): This method is employed to prepare colloidal solutions of metals like gold, silver, platinum etc. The method involves passing of an electric current (arc) through electrodes made of desired metal, suspended in a trough of water kept cooled by the surrounding ice. Due to the intense heat at the arc, the metal changes into vapours which condense to form the colloidal particles.

iii) Peptisation: It is the process of converting a freshly precipitated substance into colloidal particles by shaking with a suitable electrolyte, e.g., $\mathrm{Fe}(\mathrm{OH}) _{3}$ pptwith $\mathrm{FeCl} _{3}$ solution, $\mathrm{AgI}$ ppt with $\mathrm{AgNO} _{3}$ or $\mathrm{KI}$ solution and $\mathrm{Al}(\mathrm{OH}) _{3}$ ppt. with insufficient quantity of very dil. $\mathrm{HCl}$ solution. During peptization, the precipitate adsorbs one of the ion of the electrolyte on its surface resulting in development of positive or negative charge on the precipitate, which ultimately breaks into particles of colloidal dimensions

II) Aggregation methods:

These involve the joining together of a large number of smaller particles to form colloidal particles. This is done by the following methods:

i) Chemical methods, eg.,

by double decomposition:

$$ \mathrm{As} _{2} \mathrm{O} _{3}(\mathrm{aq})+3 \mathrm{H} _{2} \mathrm{~S}(\mathrm{aq}) \longrightarrow \underset{\text { Colloidal sol }}{\mathrm{As} _{2} \mathrm{S} _{3}}+\mathrm{H} _{2} \mathrm{O} $$

by reduction:

$$ 2 \mathrm{AuCl} _{3}(\mathrm{aq})+3 \mathrm{SnCl} _{2}(\mathrm{aq}) \longrightarrow \underset{\text { Gold sol }}{2 \mathrm{Au}}+3 \mathrm{SnCl} _{4} $$

By oxidation:

$$ \mathrm{Br} _{2}(\mathrm{aq})+\mathrm{H} _{2} \mathrm{~S}(\mathrm{aq}) \longrightarrow \underset{\text { Colloidal sol }}{\mathrm{S}+2 \mathrm{HBr}} $$

By hydrolysis:

$$ \mathrm{FeCl} _{3}(\mathrm{aq})+3 \mathrm{H} _{2} \mathrm{O} \longrightarrow \underset{\text { Colloidal sol }}{\mathrm{Fe}(\mathrm{OH}) _{3}}+3 \mathrm{HCl} $$

ii) Exchange of solvent: If a solution of sulphur or phosphorus in alcohol is poured into water, a colloidal solution of sulphur or phosphorus is obtained due to its low solubility in water.

iii) By change of physical state: By passing the vapours of elements like $\mathrm{Hg}, \mathrm{S}$ through cold water the colloidal sol can be prepared.

Purification of Colloids

Colloids prepared by above methods contain excessive amount of electrolytes and other substances which may coagulate it. Therefore, colloids are purified by the following methods:

i) Dialysis: This is the process of separating the particles of the colloids from those of the crystalloids by diffusion of the mixture through a parchment paper or animal membrane.

ii) Electrodialysis: In this method, the movement of ions across the membrance (as in case of dialysis) is expedited by applying electric current through two electrodes. This method is very fast.

iii) Ultra filtration: This is the process of separating the particles of the colloids from those of the crystalloids by filtration through ultra filter papers (obtained by treating ordinary filter paper with collodion or gelatine solution to narrow down the pores suitably).

iv) Ultra cetrifugation: In this method, the colloidal solution is placed in a high speed centrifugal machine. The colloidal particles settle down while impurities remain in the centrifugate. The settled colloidal particles are mixed with water to form the colloidal solution again.

Properties of colloidal sols

The characteristic properties are:

• Heterogeneous nature : Colloids are heterogeneous in nature and consists of two phases, the dispersed phase and the dispersion medium. The phenomena of Tyndall effect, electrophoresis and electroosmosis confirm heterogeneity of colloidal systems.

• Visibility : The partictes are too small to be seen with naked eye but become visible when viewed through ultramicroscope due to scattering of light by them.

• Colligative properties : Colloidal sols have low values as compared to true solution, having same concentration. It is because the mole fraction of disprced phase due to high average molecular mass of colloidal particles. The molecular mass of colloidals particles can be determined by osmotic pressure measurements.

• Surface tension and Viscosity : The surface tension and viscosity of lyophobic sols are not very different from that of dispersed medium. But, lyophillic sols show higher viscosity and lower surface tension in comparison to the dispersion medium.

Tyndall effect: It is due to the scattering of light by colloidal particles. (Fig. 7 & Fig. 8) Tyndall effect is observed only when the following two conditions are satisfied:

i) The diameter of the dispersed particles is not much smaller as compared to the wavelength of light used.

ii) There is a large difference in the refractive index of the dispersed phase and the dispersion medium.

• Brownian movement : Brownian movement is the continuous zig-zag movemnt of the colloidal particles in a colloidal sol. Brownian movement arises due to the unequal impacts

from different directions by the particles of the dispersion medium on the colloidal particles. (Fig. 9)

When the dispersed particles acquire the dimensions of suspension no Brownian movement is observed. Thus, motion depends on size of the particles and the viscosity of the solution.

Fig. 9: Brownian movement.

Brownian movement counters the force of gravity acting on colloidal particles and does not allow it to settle down and thus provides stability to colloidal sol.

• Electrophoresis - The process of electrophoresis or cataphoresis which involves the movement of colloidal particles either towards the cathode or anode under the influence of an electric field. (Fig. 10)

Fig. 10: Electrophoresis

• Electro-osmosis - Electro-osmosis or called elctro-endosmosis is a phenomenon in which the molecules of the dispersion medium are allowed to move towards oppositely charged electrode under the influence of an electric field whereas colloidal particles are not allowed to move.

• Stability of colloidal sols : All the dispersed particles in a colloid carry the same charge (either positive or negative) while the dispersion medium has an equal and opposite charge. The particles, therefore, repel one another and do not come close together to form large non-colloidal particles.

The charge on colloidal particles is due to preferential adsorption of ions, e.g., $\mathrm{Fe}(\mathrm{OH}) _{3}$ adsorbs $\mathrm{Fe}^{3+}$ ions from $\mathrm{FeCl} _{3}$ solution and forms positively charged sol. Similarly, $\mathrm{AgCl}$ particles can adsorb $\mathrm{Cl}^{-}$ions from chloride solutions or $\mathrm{Ag}^{+}$ions from solutions having silver ions. The sol will be negatively charged in the first case and positively charged in the second case.

Negatively charged colloidal sols: Metallic particles like $\mathrm{Cu}, \mathrm{Au}, \mathrm{Pt}, \mathrm{Ag}$ etc., starch, clay, silicic acid, metal sulphides like $\mathrm{As} _{2} \mathrm{~S} _{3}, \mathrm{CdS} _{2} \mathrm{Sb} _{2} \mathrm{~S} _{3}$ etc., acidic dyes like congo red, eosin etc.

Positively charged colloidal sols : Metal hydroxides like $\mathrm{Fe}(\mathrm{OH}) _{3}, \mathrm{Al}(\mathrm{OH}) _{3}, \mathrm{Cr}(\mathrm{OH}) _{3}$, $\mathrm{Ca}(\mathrm{OH}) _{2}$, oxides like $\mathrm{TiO} _{2}$ etc., haemoglobin and basic dyes like methylene blue.

Coagulation of colloidal solutions

The stability of the lyophobic sols is due to the presence of charge on colloidal particles. If the charge is removed, the particles aggregate or flocculate and settle down under the foce of gravity.

• Coagulation or flocculation: It is the process of aggregating the colloidal particles together so as to form bigger particles which may settle down as a precipitate. It is usually done by addition of suitable electrolytes.

• Difference between Flocculation and Coagulation - When a small amount of the electrolyte is added, i.e., when the concentration of the electrolyte added is low, the process is called flocculation. It can be reversed on shaking. However, at higher concentration, coagulation takes place and the process cannot be reversed simply by shaking.

• Coagulation value or Precipitation value or Flocculation value: It is the minimum amount of the electrolyte in millimoles that must be added to one litre of the colloidal sol to bring about complete coagulation or precipitation. Thus. smaller is the flocculation value of an electrolyte, greater is its coagulating or precipitating power.

$$\dfrac{\text { Coagulating power of electrolyte } 1}{\text { Coagulating power of electrolyte } 2}=\dfrac{\text { Coagulating value of electrolyte } 2}{\text { Coagulating value of electrolyte } 1}$$

• Hardy-Schulze rule: It states that “Greater is the valency of the oppositely charged ion of the electrolyte being added, faster is the coagulation. Further, the coagulating power of an electrolyte is directly proportional to the fourth power of the valency of the effective ion.” Thus, for a negatively charged sol, like $\mathrm{As} _{2} \mathrm{~S} _{3}$, the order is $\mathrm{Al}^{3+}>\mathrm{Ba}^{2+}>\mathrm{Na}^{+}$and for a positively charged sol, like $\mathrm{Fe}(\mathrm{OH}) _{3}$, the order is

$$ \left.\left[\mathrm{Fe}(\mathrm{CN}) _{6}\right]^{4-}>\mathrm{PO} _{4}{ }^{3-}>\mathrm{SO} _{4}^{2-}>\mathrm{Cl}^{-}\right] $$

Coagulation can also be caused by electrophoresis, mutual precipitation (mixing colloidal sols of opposite charge), prolonged dialysis or by heating or cooling the sol.

Protection of Colloids

Lyophillic sols being extensively solvated are more stable than lyophobic sols. Therefore, lyophillic sols have unique property of protecting lyophobic sols. The process is known as protection and the lyophilic colloids which are used to make lyophobic colloids stable, i.e., to protect them from coagulation are called protective colloids.

GELS AND EMULSIONS

GELS: They are colloidal systems in which liquids are dispersed phase and solids act as dispersion medium

Types of Gels

i) Elastic gels: These gels posses the property of elasticity, these are reversible, i.e., they give solid on dehydration which can be changed back into gel by adding water and heating. When placed in contact with water, they absorb water and swell. This property is called imbibition. Examples include gelatin, agar-agar, starch etc. When gels are allowed to stand for long the, they give out some quantity of trapped liquids, which accumulates on the surface. This is called syneresis or weeping of the gel.

ii) Non elastic gels: These gels do not possess the property of elasticity. These are irreversible, i.e., when dehydrated, these change into powder which cannot be changed back by addition of water. These do not show the phenomenon of imbibition. Example includes silicic acid.

EMULSIONS: -

It is a colloidal dispersion in which both the dispersed phase and the dispersion medium are liquids (which are otherwise immiscible). Substances like soaps which help in making the emulsions stable are called emulsifiers or emulsifying agents.

Types of Emulsions:

Emulsions are of two types:

i) Oil in water $(0 / \mathrm{w})$ : emulsions in which oil is the dispersed phase and water is the dispersion medium. For example, milk and vanishing cream.

ii) Water in oil ( $\mathrm{w} / \mathrm{o})$ : emulsions in which water is the dispersed phase and oil is the dispersion medium. For example, cod liver oil, butter and cream.

• Demulsification: The process of breaking an emulsion to yield constituent.

APPLICATION OF COLLOIDS

Most of the substances, we come across in our daily life are colloids. From the food that we eat to clothes that we wear or the things that we use are mainly composed of colloids.

• Food items: Milk, butter, fruit juices are examples of colloids

• Blood: Our blood is a colloidal solution of an albuminoid substance.

• Soil: Fertile soils are colloidal in nature in which humus acts as protective colloid.

• In purification of drinking water: Alum is added to water obtained from natural sources to destroy bacteria and coagulate the suspended impurities and make it fit for drinking purpose.

• In medicines: Most of the medicines are colloidal in nature as these are easily assimilated and hence are more effective. Milk of magnesia (an emulsion) is used in stomach disorders. Colloidal antimony is used in curing kala-azar. The disinfectants like dettol and lyzol are also emulsions.

• Colloids are also used extensively in rubber industry, photographic platch, soaps and detergents, paints, lubrecants etc.

CATALYST AND CATALYSIS

Catalyst is a substance which can change the speed of a chemical reaction without being used up in that reaction and the process is called catalysis. A catalyst is said to lower the activation energy and thus increase the rate of the reaction. Promotors are substances that increase the catalytic activity even though they are not catalysts by themselves. For e.g. in Haber’s process, molybedenum acts as a promotor for iron which is used as a catalyst.

$$ \mathrm{N} _{2}(\mathrm{~g})+3 \mathrm{H} _{2}(\mathrm{~g}) \xrightarrow[\mathrm{Mo}(\mathrm{s})]{\mathrm{Fe}(\mathrm{s})} 2 \mathrm{NH} _{3}(\mathrm{~g}) $$

A catalyst may act as Positive catalyst or inhibitor (negative catalyst)

Positive catalyst

For example, oxidation of $\mathrm{SO} _{2}$ to $\mathrm{SO} _{3}$ in presence of $\mathrm{NO}$ (lead chamber process) or in presence of $\mathrm{V} _{2} \mathrm{O} _{5}$ (contact process) are examples of positive catalysts

Inhibitors (Negative catalyst)

If a catalyst decreases (retards) the speed of a reactions, it is called an inhibitor or a negative catalyst. e.g. decomposition of $\mathrm{H} _{2} \mathrm{O} _{2}$ in presence of phosphoric acid and oxidation of chloroform in presence of alcohol are examples of negative catalysts.

Types of Catalysis

There are two types of catalysis:

• Homogeneous catalysis: When the catalyst is present in the same phase as the reactants, it is called a homogeneous catalyst and the process is called homogeneous catalysis.

$$ \begin{aligned} & \mathrm{CH} _{3} \mathrm{COOC} _{2} \mathrm{H} _{5}(\mathrm{l})+\mathrm{H} _{2} \mathrm{O}(\mathrm{I}) \xrightarrow{\mathrm{H} _{2} \mathrm{SO} _{4}(\mathrm{aq})} \mathrm{CH} _{3} \mathrm{COOH}(\mathrm{I})+\mathrm{C} _{2} \mathrm{H} _{5} \mathrm{OH}(\mathrm{l}) \\ & 2 \mathrm{SO} _{2}(\mathrm{~g})+\mathrm{O} _{2}(\mathrm{~g}) \xrightarrow{\mathrm{NO}(\mathrm{g})} 2 \mathrm{SO} _{3}(\mathrm{~g}) \end{aligned} $$

• Heterogeneous catalysis: When the catalyst is present in a different phase than that of the reactants, it is called a heterogeneous catalyst and the process is called heterogeneous catalysis.

i) Manufacture of $\mathrm{NH} _{3}$ from $\mathrm{N} _{2}$ and $\mathrm{H} _{2}$ using $\mathrm{Fe}$ as catalyst (Haber’s process)

ii) Manufacture of $\mathrm{H} _{2} \mathrm{SO} _{4}$ by oxidation of $\mathrm{SO} _{2}$ to $\mathrm{SO} _{3}$ using platinised asbestos or $\mathrm{V} _{2} \mathrm{O} _{5}$ as catalyst (contact process)

Features of Solid Catalysts

i) Activity: It depends upon the extent of chemisorption. The adsorption should be reasonably strong but not so strong that they become immobile and no space is available for other reactants to get adsorbed.

ii) Selectivity: This is the ability of a catalyst to direct the reaction to yield a particular product.

$$ \begin{aligned} & \text { e.g., } \mathrm{CO}(\mathrm{g})+3 \mathrm{H} _{2}(\mathrm{~g}) \xrightarrow{\mathrm{Ni}} \mathrm{CH} _{4}(\mathrm{~g})+\mathrm{H} _{2} \mathrm{O}(\mathrm{g}) \\ & \mathrm{CO}(\mathrm{g})+2 \mathrm{H} _{2}(\mathrm{~g}) \xrightarrow{\mathrm{Cu} / \mathrm{ZnO}-\mathrm{Cr} _{2} \mathrm{O} _{3}} \mathrm{CH} _{3} \mathrm{OH}(\mathrm{g}) \\ & \mathrm{CO}(\mathrm{g})+\mathrm{H} _{2}(\mathrm{~g}) \xrightarrow{\mathrm{Cu}} \mathrm{HCHO}(\mathrm{g}) \end{aligned} $$

ZEOLITES AS SHAPE SELECTIVE CATALYSTS

Zeolites are microporous alumino silicates with three dimensional network in which some Si atoms are replaced by $\mathrm{Al}$ atoms giving Al-O-Si frame work.

They are heated in vacuum so that water of hydration is lost and they become porous. The size of pores varies between $260 \mathrm{pm}-740 \mathrm{pm}$ which allows only those molecules to be adsorbed whose size is small enough to enter these pores.

Thus, the reactions taking place in zeolites depend upon the size and shape of the reactant and product molecules and upon the pores and cavities of the zeolites.

ZSM-5 which converts alcohols directly into gasoline (petrol) by dehydrating them to give a mixture of hydrocarbons.

ENZYME CATALYSIS

All biological reactions are catalysed by enzymes, i.e., enzymes are biological catalysts. They are all globular proteins with high molar mass ( 15,000 to $1,000,000 \mathrm{~g} / \mathrm{mol}$ ). Some of them also contain a non-protein component called cofactor.

Properties of Enzymes

i) Efficiency: They are highly efficient catalysts. They increase rate by $10^{8}$ to $10^{20}$ times.

ii) Specificity: Each enzyme catalyses only one chemical reaction.

Enzyme invertase hydrolyses sucrose but not maltose which is hydrolysed by the enzyme maltase.

$$ \underset{\substack{\text { Sucrose }}}{\mathrm{C} _{12} \mathrm{H} _{32} \mathrm{O} _{11}}+\mathrm{H} _{2} \mathrm{O} \xrightarrow{\text { invertase }} \underset{\substack{\text { Glucose }}}{\mathrm{C} _{6} \mathrm{H} _{12} \mathrm{O} _{6}}+\underset{\text { Fructose }}{\mathrm{C} _{6} \mathrm{H} _{12} \mathrm{O} _{6}} $$

$$ \mathrm{C} _{12} \mathrm{H} _{12} \mathrm{O} _{11}+\mathrm{H} _{2} \mathrm{O} \xrightarrow{\text { maltase }} 2 \mathrm{C} _{6} \mathrm{H} _{12} \mathrm{O} _{6} $$

The reason for specificity is that each enzyme has a specific active site on its surface where it binds to the substrate (reactant) through intermolecular forces (H-bonds, dipole forces).

Optimum temperature and $\mathrm{pH}$

In fact, as the temperature or pH is increased, rate rises to a maximum and then falls off. (Fig. 11) The optimum temperature range for enzymatic activity is $298-310 \mathrm{~K}$ and $\mathrm{pH}$ range is $5-7$.

Fig. 11: Effect of pH on enzyme activity

• Enzyme activators (co-enzymes): The activity of certain enzymes is increased in the presence of certain substances, called co-enzymes, e.g., vitamins present as non-protein part. The activators are generally metal ions like $\mathrm{Na}^{+}, \mathrm{Mn}^{2+}, \mathrm{Cu}^{2+}, \mathrm{Co}^{2+}$ etc.

• Mechanism of enzyme catalysis (Lock and key mechanism).

Step I : Formation of Enzyme-Substrate complex

$$ \mathrm{E}+\mathrm{S} \rightleftharpoons \mathrm{ES} \text { (Fast and Reversible) } $$

Step II : Dissociation of Enzyme-Substrate complex to form the products

$\mathrm{ES} \longrightarrow \mathrm{EP} \longrightarrow \mathrm{E}+\mathrm{P} \quad$ (Slow and Rate determining)

Solved Questions

Question 1. The addition of $4 \mathrm{~mL}$ of $0.005 \mathrm{M} \mathrm{BaCl} _{2}$ to $16.0 \mathrm{~mL}^{2}$ of $\mathrm{As} _{2} \mathrm{~S} _{3}$ sol just causes complete coagulation in $2 \mathrm{hrs}$. What is the flocculating value of the effective ion?

Question 2. What type of sol is obtained by adding $\mathrm{FeCl} _{3}$ solution to slight excess of $\mathrm{NaOH}$ ? What type of electrolyte will be effective in its coagulation?

Question 3. What is the gold number of starch when the coagulation is just prevented by adding $1 \mathrm{~mL}$ solution of $10 \% \mathrm{NaCl}$ to $10 \mathrm{~mL}$ gold sol containing $0.0250 \mathrm{~g}$ of starch.

Question 4. For a given system, plot of $\log \mathrm{x} / \mathrm{m} \mathrm{v} / \mathrm{s} \log \mathrm{P}$ is a straight line with an angle $45^{\circ}$ and intercept 0.3010 . What is the amount of gas adsorbed per gram of adsorbent when the initial pressure is $0.5 \mathrm{~atm}$.

Question 5. During adsorption of hydrogen gas over a sample of charcoal, $1.12 \mathrm{~cm}^{3}$ of $\mathrm{H} _{2}(\mathrm{~g})$ measured over STP $(298,1 \mathrm{~atm})$ was found to adsorb per gram of charcoal. The density of $\mathrm{H} _{2}$ is found to be 0.07 $\mathrm{g} / \mathrm{cm}^{3}$. Considering formation of monolayer, calculate

i) number of molecules of $\mathrm{H} _{2}$ over $1 \mathrm{~g}$ charcoal

ii) radius of adsorbed hydrogen molecule

iii) specific surface area of charcoal.

PRACTICE QUESTIQNS

Question 1- Potassium stearate is obtained by saponification of an oil or fat. It has the formula $\mathrm{CH} _{3}$ $\left(\mathrm{CH} _{2}\right) _{6}-\mathrm{COO}^{-} \mathrm{K}^{+}$. The molecule has a lyophobic end $\left(\mathrm{CH} _{3}\right)$ and a lyophilic end $\left(\mathrm{COO}^{-} \mathrm{K}^{+}\right)$. Potassium stearate is an example of

(a) Iyophilic colloid

(b) multimolecular colloid

(c) macromolecular colloid

(d) associated colloid or micelle

Question 2- Consider the following statements about micelles:

i) At critical micelle concentration, several properties of the solutions of surfactants such as molar conductivity, surface tension and osmotic pressure exhibit a dramatic change

ii) No micelles from ionic surfactants are formed below a particular temperature

iii) The enthalpy of micelle formation in aqueous solution is slightly negative Which of these are correct?

(a) i) only

(b) i) and ii)

(c) i), ii) and iii)

(d) i) and iii)

Question 3- Sulphur sol contains

(a) Discrete sulphur atoms

(b) Discrete sulphur molecules

(c) Large aggregates of sulphur molecules

(d) Water dispersed in solid sulphur.

Question 4- What is the equation form of Langmuir isotherm under high pressure?

(a) $\dfrac{x}{m}=\dfrac{a}{b}$

(b) $\dfrac{x}{m}=a P$

(c) $\dfrac{x}{m}=\dfrac{1}{a P}$

(d) $\dfrac{x}{m}=\dfrac{b}{a}$

Question 5- The colloidal dispersion of solid in a gas is called

(a) Foam

(b) Aerosol

(c) Gel

(d) Sol

Question 6- Butter is an example

(a) Gel

(b) Emulsion

(c) Sol

(d) Foam

Question 7- Fog is a colloid consisting of

(a) Gas dispersed in liquid

(b) Liquid dispersed in a gas

(c) Gas dispersed in gas

(d) Solid dispersed in gas

Question 8- According to Langmuir adsorption isotherm, the amount of the gas adsorbed at very high pressure

(a) reaches a constant limiting value

(b) goes on increasing with pressure

(c) goes on decreasing with pressure

(d) first increases and then decreases with pressure

Question 9- A particle of suspension of radius $1 \mathrm{~mm}$ is broken to form colloidal particles of radius $1000 \mathrm{~A}$. Their total surface area will times the surface area of particle of suspension.

(a) $10^{3}$

(b) $10^{4}$

(c) $10^{5}$

(d) $10^{6}$

Question 10- The physical states of dispersing phase and dispersion medium in colloid like pesticide spray respectively are

(a) Solid, gas

(b) gas, liquid

(c) liquid, gas

(d) liquid, solid

PRACTICE QUESTIONS

Question 1- Which one of the following statements about the zeolites is false?

(a) The are used as cation exchangers

(b) They have open structure which enables them to take up small molecules

(c) Zeolites are aluminosilicates having three-demensional network

(d) Some of the $\mathrm{SiO} _{4}{ }^{4-}$ units are replaced by $\mathrm{AlO} _{4}{ }^{5-}$ and $\mathrm{AlO} _{6}{ } _{6}$ - ions in zeolites.

Question 2- Identify the correct statement regarding enzymes

(a) Enzymes are specific biological catalysts that can normally function at very low temperature (T-100 K)

(b) Enzymes are normally heterogeneous catalysts that are very specific in action

(c) Enzyms are specific biological catalysts that cannot be poisoned

(d) Enzymes are specific biological catalysts that possess well defined active sites.

Question 3- Ferric chloride is applied to stop bleeding because

(a) $\mathrm{Fe}^{3+}$ ions coagulate negatively charged blood solution

(b) $\mathrm{Fe}^{3+}$ ions coagulate positively charged blood solution

(c) Cl ions coagulate positively charged blood solution

(d) $\mathrm{Cl}^{-}$ions coagulate negatively charged blood solution

Question 4- Which one of the following statement is wrong?

(a) Greater the gold number of a protective colloid, less is its protective power

(b) Hardy-Schulze rule is applicable only to the coagulation of Iyophilic sols

(c) Greater flocculation value of an electrolyte means its poor coagulating power

(d) For coagulation of a positive sol, positive ions of the electrolyte have no significance.

Question 5- In petrochemical industry, alcohols are directly converted to gasoline by passing over heated

(a) Platinum

(b) ZSM-5

(c) Iron

(d) Nickel

Question 6- Select the non-elastic gel out of the following

(a) Starch

(b) Agar agar

(c) Silicic acid

(d) Gelatin

Question 7- Elastic gels when placed in contact with water, they absorb water and swell. This property is called

(a) Thixotropy

(b) Weeping

(c) Syneresis

(d) Imbibition

Question 8- Which one of the following emulsifying agents helps in the formation of w/o emulsion?

(a) Proteins

(b) Gums

(c) Insoluble soaps

(d) Soluble soaps

Question 9- Which of the following electrolyte will have maximum flocculation value for $\mathrm{Fe}(\mathrm{OH}) _{3} \mathrm{sol}$ ?

(a) $\mathrm{NaCl}$

(b) $\mathrm{Na} _{2} \mathrm{~S}$

(c) $\left(\mathrm{NH} _{4}\right) _{3} \mathrm{PO} _{4}$

(d) $\mathrm{K} _{2} \mathrm{SO} _{4}$

Question 10- Zeta potential (or electrokinetic potential) is the

(a) potential required to bring about coagulation of a colloidal sol

(b) potential required to give the particles a speed of $1 \mathrm{~cm} / \mathrm{s}$ in the sol

(c) potential difference between fixed charged layer and the diffused layer having opposite charge

(d) potential energy of the colloidal particles.

Question 11- A colloidal solution is subjected to an electrical field. The particles move towards anode. The coagulation of same sol is studied using $\mathrm{NaCl}, \mathrm{BaCl} _{2}$ and $\mathrm{AlCl} _{3}$ solutions. Their coagulating power should be

(a) $\mathrm{NaCl}>\mathrm{BaCl} _{2}>\mathrm{AlCl} _{3}$

(b) $\mathrm{BaCl} _{2}>\mathrm{AlCl} _{3}>\mathrm{NaCl}$

(c) $\mathrm{AlCl} _{3}>\mathrm{BaCl} _{2}>\mathrm{NaCl}$

(d) $\mathrm{BaCl} _{2}>\mathrm{NaCl}>\mathrm{AlCl}$,

Question 12- Colloid of which one of the following can be prepared by electrical dispersion method as well as reduction method?

(a) Sulphur

(b) Ferric hydroxide

(c) Arsenious sulphide

(d) Gold

Question 13- The method which does not result in sol destruction is

(a) Electrophoresis

(b) Addition of electrolyte

(c) Diffusion through animal membrane

(d) Mixing two oppositely charged sols

Question 14- Collodion is a $4 \%$ solution of which on of the following in alcohol-ether mixture?

(a) nitroglycerine

(b) cellulose acetate

(c) glycol dinitrate

(d) nitrocellulose

Question 15- Bredig arc method cannot be used to prepare colloidal solution of which of the following?

(a) $\mathrm{Pt}$

(b) $\mathrm{Fe}$

(c) $\mathrm{Ag}$

(d) $\mathrm{Au}$

Question 16- Purification of blood in the body is based upon the phenomenon of

(a) Electrophoresis

(b) Electro-osmosis

(c) Dialysis

(d) Coagulation

CONCEPT WEB ON SURFACE CHEMISTRY