Chapter 5 Surface Chemistry

Homogeneous catalysis is the process in which reactants, products and catalyst are in the same phase. So (iii) is the correct option.

In Heterogeneous catalysis, reactants, products, and catalyst are in a different phase. It is a process which occurs at the interface. So (ii) is wrong.

Crystallisation also occurs at the interface where solid substance gets crystallized at liquid interface.

Therefore, option (i) is wrong.

Conversion of metal into metal oxide at the interface of metal occurs during the corrosion process. So (iv) is also not correct.

At equilibrium, in adsorption process, the change in free energy ΔG= 0.

We know, ΔG= ΔH – TΔS

So for ΔG= 0, ΔH= TΔS

Therefore, (ii) is correct.

In options (iii) and (iv), ΔH is not equal to TΔS. So both are wrong options.

Adsorption process at equilibrium is not accompanied with an increase in enthalpy. So (i) is not correct.

Emulsion occurs at the liquid-liquid interface. So (i) is not correct.

Adsorption of a solid substance at a liquid interface is example of solid-liquid interface. So (ii) is wrong.

Adsorption of gas like O₂ gas at the interface of water is an example of a liquid-gas interface. Therefore (ii) is also wrong.

Sorption is a phenomenon in which one substance becomes attached to another surface. This can occur either by attracting substance into bulk (absorption)or the substance may get attached simply on the surface(adsorption).So (iii) is correct.

‘Absorption’ refers the enter of a substance into bulk phase only. So option (i) is wrong.

Similarly, in ‘Adsorption’ a substance gets adhered to a surface of the adsorbent. So option (ii) is also wrong.

The term ‘Desorption’ means the opposite process of sorption. Therefore, (iv) is an incorrect option.

Physisorption is exothermic by nature. So the rate of physisorption of a gas increases with decrease in temperature. So (ii) is correct.

With the increase in temperature gas adsorbed will escape from the surface of adsorbent. So the rate of physisorption decrease. Therefore, (i) is wrong.

The decrease in surface area of adsorbent will reduce the amount of adsorbed gas. So rate of physisorption decrease. So option (iii) is wrong.

Vander Waals forces help to adsorb a gas to the surface of the adsorbent. So with the decrease of strength of this force decreases physisorption. So (iv) is also wrong.

By increasing the amount of an adsorbate, the extent of interaction between adsorbate and adsorbent also increase. Therefore, option (i) is correct.

Decrease in surface area of adsorbent decrease the rate of adsorption.so (ii) is wrong.

If the temperature of solution increase, then gas may escape from the solution which results the decrease in rate of adsorption. So (ii) is also an incorrect option.

Decrease in amount of adsorbate in solution will decrease the extent of adsorption. So (iv) is wrong.

When a gas is adsorbed on solid surface, it follows the equation below:

ΔG = ΔH –TΔS

Here, change in entropy (ΔS) is negative for adsorption, since after adsorption, molecules of adsorbate get more ordered. So (iii) is not applicable.

Since adsorption is a spontaneous process, so it must be accompanied with decrease in free energy. So in adsorption, ΔG is also negative. Therefore, option (ii) is also not applicable.

Now since both ΔS is negative, so TΔS is positive. So for ΔG to be negative, ΔH must be negative. So option (iv) is also not applicable.

Only option (i) is not applied for the process of adsorption. So (i) is the correct answer here.

At constant temperature, the rate of adsorption increases with increase in pressure. So (i) is a favorable condition for adsorption.

Adsorption is an exothermic process, so ΔH is negative. Therefore, option (ii) is also favorable for adsorption.

Higher critical temperature of adsorbate increases the rate of adsorption. So (iii) is also true condition for adsorption.

At high temperature, the amount of gas adsorbed get decrease. So high temperature is not a favorable condition for adsorption. Therefore option (iv) is correct.

The basic difference between physical and chemical adsorption is that in the later case, a true chemical bond is formed between the adsorbent and the adsorbate. For this, chemisorption needs high temperature. But physisorption can occur at room temperature also.

So decreasing temperature cannot favor chemisorptions. So option (i) is wrong.

By increasing or decreasing the surface area of adsorbent it only changes the rate of physisorption. Therefore both (iii) and (iv) are wrong.

So, (ii) is the only correct option.

Physisorption is reversible in nature, because desorption of gas takes place either by increasing the temperature or decreasing the pressure. So option (iv) is wrong.

Physisorption does not depend upon the nature of gas, whether ideal or real. So (ii) is not true.

Since physisorption is an exothermic process, it has low enthalpy( 20-40 KJ/mol) in general. Therefore, option (iii) is not correct.

In physisorption, adsorbent and adsorbate get attached with the help of vander waals forces, which is irrespective of the nature of the gas. So (i) is the correct option.

Silica gel, finely divided nickel and metal surface all have a porous structure which makes them good adsorbent. Thus they facilitate the adsorption process.

Therefore, (i), (iii) and (iv) are wrong options.

Calcium chloride is a strong hygroscopic substance which absorbs moisture readily. Thus, water is easily absorbed by CaCl₂ making it good absorbent.

Therefore, (ii) is correct.

Critical temperature is the maximum temperature at which a gas can be liquefied by applying high pressure. Higher the critical temperature of a gas, more easily it can be liquefied and adsorbed.

Here, H₂ gas has least value of critical temperature. So it shows least adsorption. So (iv) is correct.

Other gases, CO₂, SO₂ and CH₄ have relatively higher values of critical temperature. So they show higher rate of adsorption. So options (i),(ii) and (iii) are wrong.

Heterogenous catalyst is the type of catalyst, whose physical state (solid, liquid, aqueous, gas) is not same as reactants and products.

For reaction (a), catalyst NO(g) is in the same physical state or phase with the reactants and products which are in gas phase. So (a) here heterogeneous catalysis is not involved. So (iii) is wrong.

For (d), the catalyst HCl(l) is in same phase with all the reactants and products which are in either liquid or aqueous phases. So this is also not heterogeneous catalysis.

Therefore, option (ii) and (iv) are also wrong.

In (b), catalyst Pt(s) is an heterogeneous catalyst, because it is not in same phase with the reactant SO₂(g) and the product SO₃(g).

Similarly in (c), catalyst Fe(s) is an heterogeneous catalyst, because it is not in same phase with the reactant N₂(g), H₂(g) and the product NH₃(g).

So both (b) and (c) are the examples of heterogeneous catalysed reactions.

Therefore option (i) is the correct.

At low concentration, soap solution behaves as a strong electrolyte. But at high concentration, soap molecules aggregate to form micelles. These are known as associated colloids. So option (ii) is correct.

Molecules having very high molecular masses form macromolecular colloid when dispersed in the suitable dispersion medium. Soap cannot form macromolecular colloid. So (iii) is wrong.

Associated colloids in the form pf micelles contain both lyophobic and lyophilic part. So (iv) is also wrong.

Molecular colloid are of two types – Macromolecular and Multimolecular. A large number of atoms or molecules aggregate to form multimolecular colloids. Example gold sol particles. So, option (i) is not correct.

At high concentration, soap solution behaves as associated colloid and form micelles. But micelles formation occurs only above Kraft temperature(T_K) and above critical micelle concentration(CMC). Colloidal particles can only show Tyndall effect. So (ii) is correct.

Option (i) is wrong because below critical micelle concentration soap solution cannot form micelle and thus can’t behave as colloids.so they don’t show Tyndall effect.

Aqueous solution of sodium chloride behaves as strong electrolyte and is dissociated into Na⁺ and Cl^- ions which don’t have colloidal particle size. So (iii) is not correct.

Aqueous solution of sugar is a non-electrolyte. Sugar molecules cannot aggregate to form colloidal sized particle. Therefore, option (iv) is also wrong.

Lyophobic sols are unstable and can be easily coagulated by adding electrolyte. This can be avoided by adding lyophilic sol which forms a thin film or protective layer.So option (iii) is correct.

Addition of oppositely charged sol also results in precipitation of lyophobic sol by the interaction between two oppositely charged ions. So (i) is wrong.

On boiling, collisions occur between lyophobic sol and dispersion medium particles. This results the reduction of a charge of sol particles and ultimately settle down as precipitate. So (iv) is also wrong.

Peptisation is the process which converts precipitate into colloidal sol when it is shaken with dispersion medium in the presence of a small amount of electrolyte.

Precipitate absorbs one of the ions of the electrolyte and ultimately break into colloidal particles.

So option (iv) is correct.

During coagulation, colloidal particles come together to form large size particles and form precipitate. So (i) is wrong.

Electrolysis brings chemical decomposition by passing electric current through electrolytes. So (ii) is wrong.

Diffusion is a method in which liquid or gas particles move from region of higher concentration to lower concentration. Therefore (iii) is also wrong.

According to Hardy-Schulze rule, greater the charge of the flocculating ion, greater is its ability to bring coagulation.

Here, in all of these, cation Na⁺ has equal charge = +1.

In (i), the anion is S^2-

In (ii), the anion is PO₄^3-.

In (iii), the anion is SO₄^2-

In (iv), the anion is Cl^-.

So highest charge of anion in option (ii). So (ii) is the correct option.

(i), (ii) and (iv) wrong according to Hardy-Schulze rule.

Based on physical states of dispersed phase and dispersion medium, colloidal system is classified into different categories.

According to this above table, we can see that options (i),(ii) and (iii) are wrong.

Only (iv) is correct.

The value of colligative property of a solution depends upon the number of particles present in solution. Value of colligative property increase with the increase in the number of particles. So it does not depends on other factors.

Therefore options (i),(ii) and (iii) are wrong.

Now, in true solution, the size of particle is very small compared to the colloidal particles (1-1000 nm). So for the same concentration of solution, the number of particles in colloidal particles is much less than that of true solution particles. So option (iv) is correct.

Here each diagram represents a meaningful process which occurs in modern adsorption theory.

(a) represents adsorption of adsorbate A and B on the surface

(c ) → (b)represents the interaction between A and B to form intermediate

(d) → (e) represents complete desorption

Therefore, option (ii) is only correct.

Emulsification occurs between two immiscible liquids. So (i) is not possible.

River water itself is a colloidal sol due to the presence of clay particles. But when river water meets with sea-water it doesn’t remain a colloidal solution, because sea-water contains electrolytes. So (ii) is not correct.

River water when comes in contact with the salty water of sea, the electrolytes present in sea-water brings coagulation of clay particles and thus delta is formed. Clay particles in river water are negatively charged and thus when combines with positively charged ions in sea-water coagulation take place. Therefore, option (iii) is correct.

Peptisation is a process of converting a precipitate into colloidal sol when shaken it with dispersion medium in the presence of small amount of electrolyte. So (iv) cannot be the correct option.

According to Freundlich isotherm, the amount of gas adsorbed(x) per unit mass of solid adsorbent (m) is related with pressure(P) at a particular temperature is given by:

x/m = k.P^1/n ( n > 1)

So, log(x/m) = 1/n.logP + logk ……..(equation-I)

So if we plot log(x/m) on y-axis and vs. logP in x-axis, it comes to be a straight line only.

In (i) and (ii) log(x/m) is being plotted against pressure P, which are not possible according to Freundlich adsorption isotherm(equation –I). So both (i) and (ii) are not possible.

According to equation – I, logk is the intercept when log(x/m) is plotted against logP. But in (iv) value of logk is zero. So option (iv) is also not possible.

Therefore, the correct option is (iii), which is in accordance with Freundlich isotherm.

The sol particles acquire electrical charge in the following ways:

a) due to dissociation of surface molecules into ions

b) due to selective adsorption of ions from the solution

c) by forming Helmholtz electrical double layer of positive and negative charges on molecule which occurs at the surface.

So all these three phenomenon occurs at the surface only, which occurs during adsorption process.

But in (iv), the process is absorption, which occurs at the bulk of the solution, which is not a surface phenomenon. So in (iv) electrical charge cannot be acquired. Therefore, option (iv) is the correct.

Here, absorption does not take place as it is a bulk phenomenon. In this figure, purification of raw sugar is done using the animal charcoal, which is a good adsorbent. So (i) is wrong.

Coagulation occurs when two oppositely charged ions get neutralized in presence of electrolyte. Here it is not applicable. So (iii) is also not correct.

Emulsification occurs between two immiscible liquids, which is not the case according to the figure. So (iv) is wrong.

In this figure, impurities present in raw sugar get adsorbed by animal charcoal. So this clearly indicates an adsorption process. So option (ii) is correct.

Micelle formation by soap in aqueous solution is not possible at all temperatures. Micelles formation occurs only above a particular temperature known as Kraft temperature (T_K). So (i) is wrong.

Micelles formation occurs only above critical micelle concentration (CMC). So (ii) is correct.

On dilution, the intermolecular force of attraction between the soap particles decreases and soap solution micelles get converted to their respective ions. So (iii) is also correct.

Soap solution at low concentration behaves like normal electrolytic solution. But at high concentration soap solution behaves as associated colloid and form micelles. So option (iv) is wrong.

Catalyst is a substance which is required in very less amount to catalyse any reaction. So (iii) is wrong.

Chemisorption involves the formation of chemical bond through which an adsorbate gets adsorbed on adsorbent. So if the interaction between the reactant molecules is large enough, formation of activated complex becomes easier using a catalyst. So (iv) is wrong.

When the same reactants may give different product by using different catalysts this is known the selectivity of a catalyst.

For example, during the catalytic hydrogenation of alkyne, if we use Palladium as a catalyst, the product will be alkane. But if hydrogenation is carried out using Lindlar catalyst, we get alkene as a product.

So option (i) is correct.

Using a catalyst, it does not interfere with the amount of heat absorbed or released for a chemical reaction. So the change in enthalpy is independent of catalyst. Therefore, (ii) is also correct.

According to Freundlich isotherm, the amount of gas adsorbed(x) per unit mass of solid adsorbent (m) is related with pressure(P) at a particular temperature is given by:

x/m = k.P^1/n ( n > 1)

So, log(x/m) = 1/n.logP + logk ……..(equation-I)

So if we plot log(x/m) on y-axis and vs. logP in x-axis, it comes to be a straight line only.

If 1/n = 0, log(x/m) = logk, so adsorption becomes independent of pressure. So (i) is correct but (ii) is wrong.

When n = 0, then x/m becomes a constant and the Freundlich isotherm is not followed. So x/m vs p graph is a line parallel to x-axis, then (iii) is correct.

If n = 0, then plot of x/m vs. P will not be curve since it will be a straight line. So (iv) is wrong.

Die to the presence of only two electrons, H₂ molecule has lesser intermolecular forces of attraction. So H₂ gas posses weaker van der waals forces. Therefore, (i) is wrong and (ii) is correct.

Higher the value of critical temperature, more easily a gas will be liquefiable and will be adsorbed readily. Since H₂ gas has lower value of critical temperature, it cannot be easily adsorbed on activated charcoal.

Therefore, (iii) is correct but (iv) is wrong.

When two oppositely charged sols are mixed, coagulation takes place due to neutralization. So (i) is wrong.

If there are similar charges on colloidal particles, neutralization does not take place and the colloid becomes stable.so (ii) is correct.

When dispersed phase is added in excess, it forms a separate layer and thus the emulsion gets destabilized. So (iii) is also wrong.

Brownian movement helps to counter the force of gravity acting on colloidal particles and thus don’t allow to settle them down. Therefore, option (iv) is also correct.

Emulsions are liquid-liquid colloidal system.

They cannot be broken by adding more amount of dispersion medium and emulsifying agents as on adding dispersion medium they become dilute and on adding emulsifying agent they become stable.

The negative charge can be precipitated by positive charge only an electrolyte like KCl and NaCl produces ions on dissolution.

Urea and Glucose are non-electrolyte thus, they produce no ions on dissolution.

The lyophilic sol also known as reversible sol are water-loving sol.

Thus, these solutions can’t be coagulated easily. The stability is due to the charge on colloidal particles and solvation of colloidal particles.

Lyophobic solutions are unstable in nature when a lyophilic sol is added to it, becomes stable due to a protective film of lyophilic sol present over the lyophobic sol.

Thus, option (i) and (iii) are correct.

The movement of the colloidal particles in the presence of an electric field is called electrophoresis.

When this movement is prevented by some suitable means it is observed that whole dispersion medium begins to move in an electric field. This is known as electroosmosis.

In a reaction, a catalyst changes physically and qualitatively as it is unaltered during the reaction.

Thus, it remains quantitatively intact after the completion of reaction and chemically does not change.

When chalk is dipped in an ink solution both adsorption and absorption takes place.

Adsorption is a surface phenomenon and absorption is a bulk phenomenon.

In adsorption process, surface of adsorbent adsorbs the desired gases. If the surface is already occupied by some other gases, it becomes saturated and thus cannot adsorb desired gases further. So having clean surface in surface chemistry is very much important.

In chemisorption, a true chemical bond formation occurs between reactant and adsorbent molecules. We know, high activation energy is required for chemical bond formation, which requires high temperature. Therefore, chemisorption is known as activated adsorption.

Soap solution at low concentration behaves like normal electrolytic solution.

But at high concentration soap solution behaves as associated colloid and form micelles. Micelles formation occurs only above Kraft temperature(T_K) and above critical micelle concentration(CMC).

Gold sol is lyophobic colloid by nature. When gelatin is added to it, gelatin forms a protective layer on gold sol, thus preventing coagulation. So gold sol becomes stabilized by adding gelatin which is a lyophilic sol.

Clouds are colloidal in nature and carry charge. For rainfall to occur, two oppositely charged clouds must combine. Silver iodide AgI being an electrolyte, helps in coagulation process between colloidal particles of water in cloud. Thus it forms bigger raindrops causing artificial rain. AgI is usually sprayed from aeroplane or remote controlled rockets.

Ice-cream is an emulsion. Here, gelatin acts as an emulsifying agent which is a hydrophilic colloid.Gelation performs the following important roles:

i) This absorbs free water in the ice-cream and thus prevents the formation of large crystals.

ii) Gelatin acts as a good source of protein.

iii) it gives soft texture and shiny appearance to ice-cream.

Collodion is 4% solution of nitro cellulose prepared in a mixture of alcohol and ether. it is the flammable and syrupy solution.

Chemical formula of alum is KAl(SO₄)₂ known as Potassium aluminium sulphate.In raw water, bicarbonate salts are present as impurities. Alum, when added react with alkaline bicarbonates and forms a gelatinous precipitate. This also attracts other fine and suspended particles and settled down at the bottom of the container. In this way we can get purified water.

When an electric field is applied to a colloidal solution, colloidal particles move towards different electrode depending on their respective charge.

If they are positively charged, they move towards cathode and if negatively charged they move towards anode. This phenomenon is also called electrophoresis.

Brownian motion of colloidal particles occur due to the collision between colloidal particles with each other and also with the particles of dispersion medium. This results in zigzag motion of colloidal particles.

FeCl₃ when added into hot water, it forms hydrated ferric oxide, which is a positively charged sol due to absorption of Fe³⁺ ions. In NaCl, there are negatively charged chloride ions which coagulate the positively charged hydrated ferric oxide. This results the coagulation of sol.

Emulsifying agents act as surfactants which decreases the surface tension between dispersed phase and dispersion medium. Emulsifying agents stabilize the emulsion by forming an interfacial film. For example, agar is an emulsifying agent used in food.

Colloidal solution has the larger surface area than a true solution. The surface area is directly proportional to the rate of assimilation in our body. So most of the medicines act more effectively in colloidal form.

The skin of an animal is colloidal in nature and carries positive charge. Tannin is a negatively charged colloidal solution. When leather is soaked in tannin, coagulation takes place due to the interaction of positively charged animal skin and negatively charged tannin. Thus leather gets hardened.

Cottrell precipitator is a filtration device that removes the fine particles like dust and smoke from a flowing gas. Cottrell precipitator neutralizes the charge on carbon particles. So smoke coming out from chimney gets freed from charged particles.

An emulsion is a dispersion of one liquid in another immiscible liquid. Dispersed phase is always present in less amount than dispersion medium.

By increasing the concentration of either dispersed phase or dispersion medium we can distinguish between them.

When dispersed phase is added in excess, it forms a separate layer. But by adding excess dispersion medium an emulsion gets diluted to any extent.

According to Hardy-Schulze rule the coagulation property of an electrolyte depend upon the valency of the coagulation ion. Higher the charge on flocculating ion smaller is the amount of electrolyte required for precipitation. Phosphate ion has -3 charge while chloride ion carries only -1 charge. So coagulating power of phosphate is higher than chloride.

Blood is a colloidal sol. This contains charged protein molecules. Moist alum has highly charged Al³⁺ and SO₄^2- ions which neutralize the charged protein molecules present in blood. This results coagulation of different blood proteins and bleeding stops.

When FeCl₃ is added to hot water, hydrated ferric oxide is formed. This hydrated ferric oxide preferably adsorbs Fe³⁺ ions resulting positively-charged colloid.

In physisorption, adsorbent and adsorbate get attached through weak vander waals forces. With increase in temperature, this bond becomes weaker and the amount of adsorbate decrease.

But in chemisorption, chemical bond formation occurs between adsorbent and adsorbate. This requires certain amount of activation energy which is achieved by increasing the temperature. Thus the rate of chemisorptions becomes faster with the increase in temperature.

During dialysis process, blood from the patient body runs through the tubes made of a semi-porous membrane for purification. But if the dialysis continues for a long time, the traces of electrolyte present in blood also get completely removed. Since the electrolytes are responsible for the stabilization of colloidal blood, in absence of electrolytes blood coagulation occurs.

The surface of silver halide acts as a good adsorbent. It can adsorb the pigments of eosin dye, which is coloured. Thus silver halide appears coloured.

Activated charcoal is porous in nature, thus acts as a good adsorbent. It can easily volume large volume of poisonous gases, but cannot adsorb oxygen gas effectively. So in coal mines, activated charcoal in gas mask provides fresh air for inhaling.

River water contains clay particles which are colloidal particles. When it comes in contact with salty water of sea, the electrolytes present in sea water brings coagulation of clay particles and thus delta is formed.

Adsorption of hydrogen gas on finely divided nickel surface takes place by physisorption through weak vander waals forces. With increase in temperature, hydrogen gas is dissociated into hydrogen atoms which makes chemical bonds with the adsorbent. Thus chemisorption occurs.

In the adsorption process, the reaction between adsorbent and adsorbate takes place to form product. After the formation of product, it is important to release the product from the surface of adsorbent to create the free surface again. This process is called desorption. Henceforth, more reacting molecules can be adsorbed.

So desorption is important for a substance which increase the rate of the adsorption process by releasing the product faster.

In heterogeneous catalysis, gas is adsorbed on the surface of a solid substance by adsorption. In the same way, the product formed leaves the surface by diffusion which occurs during desorption.

When the gaseous molecules get adsorbed on solid catalyst, their concentration increases. Greater the concentration and closeness of the gaseous molecules, higher will be the rate of the reaction. Also the rate of reaction increases by releasing heat, as the process of adsorption is an exothermic process.

Enzymes show their maximum activity at a particular temperature range, known as optimum temperature. The optimum temperature range for enzymatic activity is 298-310K.Normal human body temperature is 310K. So during fever, at more than 310K, enzyme activity gets adversely affected.

(i) – (b)

(ii) – (c)

(iii) – (d)

(iv) – (a)

EXPLANATION:

(i) Multimolecular colloids are those where the solution is formed as a result of the aggregation of a large number of atoms or small molecules (having diameters of less than 1nm) of the dispersed media. The dispersed particles are held together by Van der Val forces.

Example: Sulphur sol can be prepared by passing vapours of sulphur through cold water.

(ii) Associated colloids or micelles are those colloids which act like strong electrolytes at lower concentrations but at higher concentrations, they exhibit colloidal properties.

At a particular concentration, the molecules of dispersed phase align in such a way as to form micellar structures. This particular concentration is known as critical micellar concentration. Thus those colloids that form micelles are known as associated colloids.

So, when soaps are mixed with water, above critical micelle concentration, it leads the formation of associated colloids.

(iii) Macromolecular colloids are those which have very high molecular masses that form large molecules called macromolecules.

When these macromolecules are dispersed in a suitable dispersion medium, the resulting colloidal solutions are known as macromolecular colloids.

We know, proteins have high molecular mass and egg white contains proteins. So, when egg white is whipped with water, it becomes an example of macromolecular colloid.

(iv) Certain colloids behave as strong electrolytes at lower concentrations. So, when soaps are mixed with water below the critical micelle concentration, they form normal electrolytic solution.

(i) → (c)

(ii) → (d)

(iii) → (b)

(iv) → (a)

EXPLANATION:

(i) Electroosmosis is the motion of liquid induced by an applied potential across a porous material, membrane, etc. The dispersion liquid moves according to the electric field. So the solvent molecules pass through semi permeable membrane towards solvent side.

(ii) Reverse osmosis is the process of forcing a solvent from a region of high solute concentration to a region of low-solute concentration through a semipermeable membrane by applying a pressure in excess of the osmotic pressure.

(iii) Electrophoresis is a separations technique that is based on the mobility of ions in an electric field. Positively charged ions migrate towards a negative electrode and negatively charged ions migrate toward a positive electrode.

So, it is basically the movement of charged colloidal particles under the influence of applied electric potential towards oppositely charged electrodes.

(iv) Osmosis is the process where solvent molecules move through a semipermeable membrane from a dilute solution into a more concentrated solution. So, the solvent molecules pass to the solution side under a semi permeable membrane.

(i) → (b)

(ii) → (c)

(iii) → (d)

(iv) → (a)

EXPLANATION:

(i) ‘Philic’ means loving. Lyophilic colloids are therefore, colloidal solutions in which dispersed phase has great affinity for dispersion medium. These solutions are quite stable and are reversible in nature. Eg. sols of gum, gelatine, starch, proteins and certain polymers in organic solvents.

They have a property of protecting the lyophobic colloids by forming a layer around them when lyophilic sol is added to lyophobic sol. Thus they protect the colloid.

(ii) An emulsion is a special type of mixture made by combining two liquids that normally don't mix (immiscible).It is a colloid of two or more immiscible liquids where one liquid contains a dispersion of the other liquids. This process of turning a liquid mixture into an emulsion is called emulsification.

(iii) Charge on the colloidal particles is due to the preferential adsorption of either positive or negative ions on their surface. When FeCl₃ is added to water it will prepare a Fe(OH)₃ sol where Fe³⁺ ions are present in excess in dispersion medium and are common to lattice of sol so it get preferentially adsorbed on sol give the sol overall positive charge.

Now, when FeCl₃ is added in NaOH, the dispersion medium is excess with OH^- ion hence it gets preferentially adsorbed to the sol giving overall negative charge due to adsorption of Fe³⁺ charge.

(iv) Charge on the colloidal particles is due to the preferential adsorption of either positive or negative ions on their surface. Due to the prefferentIal adsorption of OH^-ions, a negatively charged sol is obtained when FeCl₃ is treated with NaOH.

(i) → (b)

(ii) → (c)

(iii) → (d)

(iv) → (a)

Exp:

(i) A sol is a colloid made out of very small solid particles in a continuous liquid medium. Here the dispersion medium is liquid and the dispersed phase is liquid.

(ii) A gel is a solid jelly-like material that can have properties ranging from soft and weak to hard and tough. Here, in gels, dispersion of molecules of a liquid within a solid in which liquid particles are dispersed in the solid medium.

(iii) An emulsion is a special type of mixture made by combining two liquids that normally don't mix (immiscible).It is a colloid of two or more immiscible liquids where one liquid contains a dispersion of the other liquids. This process of turning a liquid mixture into an emulsion is called emulsification.

(iv) Foam is a colloidal system where the particles are gas bubbles and the medium is a liquid. Here the dispersion medium is liquid and the dispersed phase is gas. They are made by trapping air or gas bubbles inside a solid or liquid.

(i) → (d)

(ii) → (c)

(iii) → (a)

(iv) → (b)

EXPLANATION:

(i) Dialysis is the separation of suspended colloidal particles from dissolved ions or molecules of small dimensions (crystalloids) by means of their unequal rates of diffusion when they pass through a semipermeable membrane. So, this process is used in the purification of colloids.

(ii) Peptisation is defined as a process of converting a precipitate into colloidal sol by shaking it with dispersion medium in the presence of small amount of electrolyte.

It is the process used in the formation of stable dispersion of colloidal particles in dispersion medium. It is also called deflocculation.

(iii) An emulsion is a special type of mixture made by combining two liquids that normally don't mix (immiscible). It is a colloid of two or more immiscible liquids where one liquid contains a dispersion of the other liquids. This process of turning a liquid mixture into an emulsion is called emulsification. The process of cleaning and washing clothes is done by emulsification by removing oily or greasy dirt.

(iv) Electrophoresis is a separations technique that is based on the mobility of ions in an electric field. Positively charged ions migrate towards a negative electrode and negatively charged ions migrate toward a positive electrode.

So, it is basically the movement of charged colloidal particles under the influence of applied electric potential towards oppositely charged electrodes. They get discharged and precipitated. This process of setting of colloidal particles is called coagulation.

(i) → (d)

(ii) → (c)

(iii) → (a)

(iv) → (b)

EXPLANATION:

(i) Butter is an example of dispersion of liquid in solid as the dispersed phase is solid and dispersion medium is liquid.

(ii) Pumice stone is an example of dispersion of gas in solid as the dispersed phase is gas and dispersion medium is solid. Here, dispersion of gas takes place and the gas bubbles are pierced within the solid particles.

(iii) Milk is an example of dispersion of liquid in liquid as the dispersed phase is liquid and dispersion medium is liquid.

(iv) Paints are an example of dispersion of solid in liquid as the dispersed phase is solid and dispersion medium is liquid.

Colloidal particles can pass through the filter papers as their pores are too large as compared to the size of the colloidal particles. The pores of filter paper are reduced when it is impregnated with the collodion solution due to which the flow of the colloidal particles is stopped. The pore size in this case is less than the colloidal particles due to which they stop.

Colloidal particles are large in size.

Colloidal solutions show colligative properties as the colloidal particles have large size. As a result, colloidal particles have small value of colligative properties because number of particles is small in comparison to normal solution due to their large size.

The phenomenon of Brownian motion was first observed by Robert Brown in the form of random zig-zag motion of pollen grains suspended in water.

Colloidal particles show Brownian motion and it is responsible for the stability of the particle. Colloidal particles in a sol are continuously bombarded by the molecules of the dispersion medium on all sides due to which some impacts are created. These impacts are not equal in every direction. As a result, the sol particles show random or zig-zag movements. This random or zig-zag motion of the colloidal particles in a sol is called Brownian motion or Brownian motion.

The Hardy Schulze rule tells that:-

• Coagulating power of an electrolyte is directly proportional to the valency of the active ions.

• The ions which are carrying the charge opposite to that of sol particles are effective in causing coagulation of the sol.

So, we get to know that greater the valency of the active ion or flocculating ion, greater will be its coagulating power. The coagulation capacity depends upon the valency of the active ion as well and is called flocculating ion.

As the valency of Al is 3 which is more than Na whose valency is 1, the coagulating power of Al³⁺ is more than Na⁺.

CMC means critical micelle concentration. The CMC is an important characteristic of a surfactant.

All the concentration of surfactants above this CMC favours the formation of micelles and all additional surfactants added to the system go to micelles.

Cleansing action of detergents involves the formation of

micelles. These are formed when the concentration of detergents becomes equal to CMC. Lesser the CMC, better and more economical is the detergent as micelle formation is needed for the cleaning of oil and dirt from our clothes.

When catalysts exist in different phase than that of the reactants, it is said to be heterogeneous catalyst. These catalysts performing catalysis, is termed as heterogeneous catalysis.

Heterogeneous catalysis is also called surface catalysis. This is because generally, the catalyst is solid while the reactants are gases, due to which the reaction starts at the surface of the solid catalyst.

Eg.

• In the Contact process reaction (oxidation reaction of SO₂ to SO₃ using O₂), solid V₂O₅ is used as a catalyst, whereas the reactants, SO₂ and SO₃ are gases.

• In Haber’s process, manufacture of ammonia gas is done by mixing nitrogen gas and hydrogen gas in presence of finely divided iron (which is a solid) as a catalyst at 400-450°C, and at 200 atm pressure.

• Hydrogenation of oils to form vegetable ghee using Ni as catalyst.

The mechanism and role of heterogeneous catalysis in increasing the rate of reaction is explained on the basis of process of adsorption :

(a) Old Adsorption Theory:

It stated that reactants in the gaseous state or in the solution state get adsorbed on the surface of solid catalysts. The rate of the reaction was increased with the concentration of the reactants on the surface of the catalysts.

It also told that adsorption is an exothermic process and heat of the adsorption is processed up in enhancing the rate of the reaction.

(b) Modern Adsorption Theory:

This theory is a combination of the old adsorption theory and the compound formation theory.

It tells that the catalytic activity is localised on the surface of the catalysts itself. The surfaces of the catalysts have free valencies which provide the main region of chemical forces of attraction.

The mechanism of catalysis involves these steps:

(I) Diffusion of reactant molecule to the surface of the catalyst.

(II) Adsorption of reactant molecules to the surface of the catalyst.

(III) Formation of an intermediate by the reaction of a chemical reaction on the surface of catalyst (due to the free valencies present on them).

(IV) Desorption of the reaction products occur from the surface of the catalyst. Now, this catalyst is available for more such reacting molecules.

(V) The reaction products diffuse away from the surface of the catalyst.

The applications of adsorption include-

• Gas masks – It is a device which consists of activated charcoal or some mixture of adsorbents. It is worn in region like coal mines where it adsorbs the poisonous gases like CO, CH₄, etc.

• Control of humidity – Silica and aluminium gels are used as adsorbents for removing moisture and in controlling the humidity.

• Removal of colouring matter from solutions – Animal charcoal is used as decolouriser in the sugar industry.

• Heterogeneous catalysis – Adsorption of reactants on the solid surface of the catalysts increase the rate of a reaction.

• Separation of inert gases – A mixture of noble gases can be separated by adsorption on coconut charcoal at different temperatures. This is because these gases are adsorbed to different extent by the charcoal.

• Deionisation of water – Water is deionised with the help of different anionic and cationic exchange resins. These resins adsorb different ions present in water and thus making it free from ions.

Sulphide ores are preferentially wetted by oils while the gangue is preferentially wetted by water .The ore particles get adsorbed on oil droplets and come to the surface where they can be collected as froth gangue is wetted by water and gets settled down. Eg. It is done for PbS ores.

Here, in this froth floatation process, low grade sulphide ores are concentrated by separating them from earthly and silicious (containing silicon) matter. Here, pine oil is used as the frothing agent. The sulphide ore is shaken with pine oil and water, the ore particles are adsorbed on froth that floats and the gangue particles settle down as discussed earlier.

Thus, role of adsorption in froth floatation process can be understood as following processes.

(i) Adsorption of pine oil on sulphide ore particles.

(ii) Formation of emulsion takes place.

(iii) Froth is formed along with ore particles.

(iv) As sulphide are extracted using froth floatation method therefore, only sulphide ore particles will show these type of adsorbing tendency. It is showing shape selective catalysis.

The catalytic action that depends on the pore structure of the catalyst and the size of the reactant and product molecules is known as shape selective catalysis.

Zeolites exhibit shape selective catalysis due to the honeycomb-like structure, having a ‘truncated octahedra’ as their building blocks. Zeolites are microporous aluminosilicates having a general formula of M_x/n [(AlO₂)_x(SiO₂)_y].mH₂O where n is the valency of the metal cation, Mⁿ⁺. Some of the silicon atoms are replaced by aluminium atoms, thereby giving a Al-O-Si framework.

Zeolites to be used as a catalyst are heated in molecules so that the molecules of hydration are pushed out. This causes formation of cavities in the network structure and it becomes porous. The pores in zeolites range from 260-740 pm.

Thus, molecules having a pore size more than this cannot enter the zeolite and undergo the reaction and small sized molecules are absorbed in the pores and cavities of zeolites.

Zeolites are widely used as catalysts in petrochemical industries for isomerism and cracking of hydrocarbons.

One prominent example of zeolite used in petroleum industry is ZSM-5(Zeolite Sieve of Molecular porosity-5). It is used in the direct conversion of alcohols to gasoline (petrol).

Structure of zeolite.