CBSE Class 12 Chemistry Surface Chemistry Worksheet

1.Bleeding caused by a nick from a razor during shaving can be stopped by rubbing with alum. Why?

Ans: Blood is a negatively charged colloidal solution. When alum is rubbed the positively charged Al³⁺ ions from alum neutralizes the charge on the particles and makes it coagulate to form a clot.

2. In which of the following does adsorption take place and why?

(i) Silica gets placed in the atmosphere saturated with water.

(ii) Anhydrous CaCl₂ placed in the atmosphere saturated with water.

Ans: Water vapour gets adsorbed on the surface of silica since it is a good adsorbent.

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

Ans: Soap molecules adsorb on to the dirt surface and try to emulsify it by forming micelles. Soap micelles thus make the insoluble dirt(greasy materials) into colloidal form such that it gets washed away by water.

4. What is the use of ZSM-5?

Ans: It converts alcohols directly into gasoline (petrol) by dehydrating them to give a mixture of hydrocarbons.

5. Give one example for each (i) sol (ii) gel.

Ans: (i) paints (ii)Butter or any other correct options.

6. What is collodion?

Ans: It is a 4% solution of nitrocellulose in a mixture of alcohol and ether.

7. Can we form a colloid with two gaseous components?Comment.

Ans: No. a gas mixed with another gas forms a homogeneous mixture which does not come under a colloidal system.

8. What are the physical states of the dispersion medium and dispersed phase in froth?

Ans: In froth, the dispersed phase is a gas while dispersion medium is a liquid.

Important Questions for NCERT Class 12 Chemistry Surface Chemistry

Question. Preparation of Lyophobic sols by chemical method involves
(a) double decomposition
(b) oxidation & reduction
(c) hydrolysis
(d) all of these

Question. A colloidal solution is subjected to an electrical field. The particles move towards anode. The coagulation of same sol is studied using NaCl, BaCl₂ and AlCl₃ solutions. Their coagulating power should be
(a) NaCl > BaCl₂ > AlCl₃
(b) BaCl₂ > AlCl₃ > NaCl
(c) AlCl₃ > BaCl₂ > NaCl
(d) BaCl₂ > NaCl > AlCl₃

Question. Under the influence of an electric field, the particles in a sol migrate towards cathode. The coagulation of the same sol is studied using NaCl, Na₂SO₄ and Na₃PO₄ solutions. Their coagulating values will be in the order 
(a) NaCl > Na₂SO₄ > Na₃PO₄
(b) Na₂SO₄ > Na₃PO₄ > NaCl
(c) Na3PO₄ > Na₂SO₄ > NaCl
(d) Na₂SO₄ > NaCl > Na₃PO₄

Question. Gold numbers of protective colloids A, B, C and D are 0.50, 0.01, 0.10 and 0.005, respectively. The correct order of their protective powers is 
(a) D < A < C < B
(b) C < B < D < A
(c) A < C < B < D
(d) B < D < A < C

Question. The disease kala azar is caused by 
(a) colloidal antimony
(b) milk of magnesia
(c) argyrols
(d) colloidal gold

Question. Which one of the following impurities present in colloidal solution cannot be removed by electrodialysis? 
(a) Sodium chloride
(b) Potassium sulphate
(c) Urea
(d) Calcium chlorid

Question. The density of gold is 19 g/cm3. If 1.9 × 10^–4 g of gold is dispersed in one litre of water to give a sol having spherical gold particles of radius 10 nm, then the number of gold particles per mm³ of the sol will be 
(a) 1.9 × 10 ¹²
(b) 6.3 × 10 ¹⁴
(c) 6.3 × 10¹⁰
(d) 2.4 × 10⁶

Question. Which of the following electrolyte will have maximum flocculation value for Fe(OH)₃ sol?
(a) NaCl
(b) Na₂S 
(c) (NH₄)₃PO₄
(d) K₂SO₄

ONE MARK QUESTIONS

1. What is the basic difference between adsorption and absorption?

2. Why are finely divided substances more effective as an adsorbent? 

3. Why is the adsorption phenomenon always exothermic? 

4. What is meant by shape selective catalysis?

5. Out of AlCl3 and NaCl, which is more effective in causing coagulation of a negative sol and why? 

TWO MARK QUESTIONS

1. What is the difference between a colloidal solution and emulsion? What is the role of emulsifier in forming emulsion? 

2. What are the characteristics of a solid catalyst?

3. What is activation of an adsorbent? How can it be achieved?

THREE MARK QUESTIONS

1.Differentiate between giving examples.

a) Homogeneous and heterogeneous catalysis

b) Physical and chemical adsorption

c) Lyophobic and lyophilic colloids

2. What is the difference between multimolecular and macromolecular colloids? Give one example of each type. How associated colloids are different from these two types of colloids. (2010)

3. Explain how the phenomenon of adsorption finds application in each of the following processes:

a) Production of vacuum

b) Heterogeneous catalysis

c) Froth floatation process

4. Explain the following terms:
a) Tyndall effect
b) Electrophoresis
c) Dialysis 

5. What happens when 
a) Electric current is passed through a colloidal solution.
b) Solution of NaCl is added to a colloidal solution of Fe (OH)₃.
c) An emulsion is subjected to centrifugation.

6. Define adsorption with an example. Why is adsorption exothermic in nature? Write the types of adsorption based on the nature of forces between adsorbate and adsorbent. (2015

Important Questions for NCERT Class 12 Chemistry Surface Chemistry

Question. The enzyme which hydrolyses triglycerides to fatty acids and glycerol is called
(a) maltase
(b) lipase
(c) zymase
(d) pepsin.

Question. Measuring zeta potential is useful in determining which property of colloidal solution?
(a) Viscosity
(b) Solubility
(c) Stability of the colloidal particles
(d) Size of the colloidal particles 

Question. Which mixture of the solutions will lead to the formation of negatively charged colloidal [AgI]I– sol ?
(a) 50 mL of 0.1 M AgNO₃ + 50 mL of 0.1 M KI
(b) 50 mL of 1 M AgNO₃ + 50 mL of 1.5 M KI
(c) 50 mL of 1 M AgNO₃ + 50 mL of 2 M KI
(d) 50 mL of 2 M AgNO₃ + 50 mL of 1.5 M KI

Question. On which of the following properties does the coagulating power of an ion depend?
(a) The magnitude of the charge on the ion alone
(b) Size of the ion alone
(c) Both magnitude and sign of the charge on the ion
(d) The sign of charge on the ion alone

Question. The coagulation values in millimoles per litre of the electrolytes used for the coagulation of As₂S₃ are given below :
I. (NaCl) = 52, II. (BaCl₂) = 0.69,
III. (MgSO₄) = 0.22
The correct order of their coagulating power is
(a) I > II > III
(b) II > I > III
(c) III > II > I
(d) III > I > II

Question. Which among the following is mainly a surface phenomenon? (Adsorption, Absorption)
Answer : adsorption

Question. Which gas will be adsorbed more ? (CO₂ , H₂)
Answer : CO₂

Question. High pressure is more favourable in case of ? (Physisorption, Chemisorption, Both)
Answer : Both

Question. In which case extent of adsorption is more? (Charcoal Block, Powdered charcoal)
Answer : Powdered charcoal

Question. Which type of adsorption decreases with increase of temperature ? (Physisorption, Chemisorption)
Answer : Physisorption

Question. Adsorption is always (Endothermic, exothermic)
Answer : exothermic

Fill in the Blanks

Question. log x/m=log k+1/n ----
Answer : log p

Question. Froth floatation makes use of-------- process
Answer : Adsorption

Question. Enthalpy of adsorption is more in case of -----------------
Answer : Chemisorption

Question. The dispersion medium in the case of smoke is ------------
Answer : Air 

Question. ------------ Process is the reverse of Adsorption
Answer : Desorption 

Match the following:-

Answer : 1-c, 2- a, 3-e, 4-h , 5-b , 6-g ,7-f, 8- d

Name it.

Question. Give an example of shape selective catalyst.
Answer : ZSM-5

Question. Substance which increases the activity of the enzymes is called
Answer : Co enzyme

Question. The concentration above which the micelle formation takes place
Answer : CMC

Question. The temperature above which the micelle formation takes place
Answer : Kraft temperature

Question. Name an example of macromolecular colloid.
Answer : polystyrene

Give reasons:-

Question. Why does the sky appear blue ?
Answer : Due to scattering of blue light by dust particles along with water suspended in air

Question. Why FeCl₃ is better for coagulation of blood than KCl?
Answer : According to Hardy schulze rule ,Fe³⁺ has more flocculating power than K⁺

Question. Why does the smoke get precipitated by Cottrell precipitator?
Answer : The charge of colloidal smoke particles gets neutralized by oppositely charged plates in chimney.

Question. Why is an animal hide soaked in tannin before use in leather industry ?
Answer : Due to mutual coagulation of positively charged animal hide particles and negatively charged colloidal particles of tannin.

Question. How are deltas formed?
Answer : Colloidal clay particles carried by river water are coagulated by electrolytes In sea water.

Important Questions for NCERT Class 12 Chemistry Surface Chemistry

Question. Which property of colloidal solution is independent of charge on the colloidal particles?
(a) Electroosmosis
(b) Tyndall effect
(c) Coagulation
(d) Electrophoresis

Question. The protecting power of lyophilic colloidal sol is expressed in terms of
(a) coagulation value
(b) gold number
(c) critical micelle concentration
(d) oxidation number. 

Question. Position of non-polar and polar part in micelle
(a) polar at outer surface but non-polar at inner surface
(b) polar at inner surface non-polar at outer surface
(c) distributed over all the surface
(d) are present in the surface only. 

Question. Which one of the following method is commonly used method for destruction of colloid?
(a) Dialysis
(b) Condensation
(c) Filteration by animal membrane
(d) By adding electrolyte 

Question. At the critical micelle concentration (CMC) the surfactant molecules
(a) associate
(b) dissociate
(c) decompose
(d) become completely soluble.

Question. The ability of anion, to bring about coagulation of a given colloid, depends upon
(a) magnitude of the charge
(b) both magnitude and charge
(c) its charge only
(d) sign of the charge alone.

Question. When a few typical solutes are separated by a particular selective membrane such as protein particles, blood corpuscles, this process is called
(a) transpiration
(b) endosmosis
(c) dialysis
(d) diffusion

1 MARKS

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

Question. What is the cause of Brownian movement among colloidal particles?
Answer : Due to collision between particles

Question. Arrange the solutions: True solution, colloidal solution, suspension in decreasing order of their particles size?
Answer : Suspension > colloidal > true solution

Question. Give an example of micelles system?
Answer : Sodium stearate (C₁₇ H₃₅ COO^- Na⁺)

Question. Why is it necessary to remove CO when ammonia is obtained by Haber’s process?
Answer : CO acts as poison catalyst for Haber’s process therefore it will lower the activity of solution therefore it is necessary to remove when NH3 obtained by Haber’s process.

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

Question. What is meant by Shape Selective Catalyst?
Answer : On the Shape Selective Catalyst, the rate depends upon pore size of the catalyst and the shape & size of the reactant and products molecules

Question. Of the physiorption&chemisorptions, which type of adsorption has higher enthalpy of adsorption?
Answer : chemisorptions.

Question. Write down the Example of Positive Sol?
Answer : Ferric hydro-oxide sol

Question. Write down the Example of Negative Sol?
Answer : Arsenic sulphide.

2 marks question.

Question. Define hardy-Schulze rule?
Answer : It is the capacity of an electrolyte to cause coagulation of a sol depends upon the number of charges on the ion having charge opposite to that possessed by the sol particles, but is independent of the charges on the ion having similar charge.

Question. Define flocculation value?
Answer : The number of milli moles of an electrolyte required to bring about the coagulation of one litre of a colloidal solution is called its flocculation value.

Question. What is the difference between a sol and a gel?
Answer : In a sol, dispersed medium is liquid and dispersed phase is solid. On the other hand, in a gel, dispersion medium is solid and dispersed phase is liquid.

Question. What are macromolecular and multimolecular colloids? How are they different from associated colloids?
Answer : Macromolecular colloids:-
i) They are molecules of large size.
ii) They have lyophobic property.
Multimolecular colloids:-
i) They are formed by the aggregation of large number of atoms or molecules which have diameter less than 1nm.
ii) They have lyophilic property.Associated colloids:-i) they are formed by the aggregation of large number of ions in concentrated solutionii) They contain both lyophilic and lyophobic groups

Question. Give any two reasons for the origin of electrical charge on the colloidal particles.
Answer : The two reasons are:
i) Due to electron capture by sol particles during electro dispersion of metals, due to preferential adsorption of ions from solution
ii) Dissociation of colloidal sols.

Question. Differentiate between electrophoresis and electro-osmosis?
Answer : Electrophoresis is the movement of colloidal particles under the influence of an electrical field.Electroosmosis is the movement of dispersion medium molecules under the influence of electric field when colloidal particles are not allowed to move

Question. Why lyophilic colloids are called reversible sols while lyophobic sols are called irreversible sols? Give on example of each.
Answer : In the lyophilic colloids if the dispersed medium is separated from the dispersion medium the sol can beprepared again by simply remixing with the dispersion medium. So they are called reversible sols. Example: Starch.
In lyophobic sols if small amount of electrolyte is added, the sols are readily precipitated and do not give back the colloid by simple addition of the dispersion medium. So they are called irreversible sols. Example: metal sulphides.

Question. Explain cleansing action of soap.
Answer : Action of soap is due to emulsification and micelle formation. Soaps are sodium salt of higher fatty acids like sodium stearate, C₁₇H₃₅COO^-Na⁺

The anionic head of stearate ion (COO^-) is hydrophobic in nature and has great affinity for water, while the hydrocarbon part (C₁₇H₃₅) is hydrophilic in nature and great affinity for oil,greaseetc.When soap is used in water, the anions (C₁₇H₃₅COO^-) form micelle and encapsulate oil or grease inside. These micelle are removed by rinsing with water; while free dirt (from oil or grease) either settle down or are washed away by water.
Thus the main function of a soap is to entrap oil or grease with the micelles through emulsification, thereby freeing dirt from grease and oil.

Question. Differentiate between chemisorption and physisorption.
Answer : Physisorotion.
a) The forces operating are weak van der Waal’s forces.
b) The heat of adsorption is low.
c) Does not require any activation energy.
d) Formsmultimoleculerlayer.
Chemisorption:
a) Forces acting are similar to those of chemical bonds.
b)The heat of adsorption is high.
c) Requires activation energy.
d) Forms unimolecular layer.

Question. Why is adsorption always exothermic?
Answer : Adsorption occurs because of attraction between adsorbate and adsorbent molecules and therefore, energy is always released during adsorption. Hence adsorption is an exothermic process.

Important Questions for NCERT Class 12 Chemistry Surface Chemistry

Question. Lyophilic colloids are stable due to:
(a) Small size of the particle
(b) Large size of the particle
(c) Layer of dispersion medium on the particle
(d) Charge on the particle
Answer : C

Question. Purple of cassius is colloidal solution of :
(a) Silver
(b) Lead 
(c) Gold
(d) Mercury
Answer : C

Question. Milk is colloid in which : 
(a) Liquid is dispersed in liquid
(b) Gas is dispersed in liquid
(c) Sugar is dispersed in water
(d) Solid is dispersed in liquid
Answer : A

Question. The colloid is : 
(a) urea
(b) blood
(c) cane sugar
(d) NaCl
Answer : B

Question. The movement of colloidal particles, under applied electric current is known as : 
(a) electrodialysis
(b) dialysis
(c) electrophoresis
(d) none of the above
Answer : C

Question. The size of colloidal particle is 
(a) 10^–3 to 10^–9 m
(b) 10^–3 to 10^–12 m
(c) 10^–6 to 10^–9 m
(d) 10^–12 to 10^–19 m
Answer : C

Question. A catalyst 
(A) changes the equilibrium constant
(B) lowers the activation energy
(C) increases the forward and backward reactions at different speeds.
(D) follows same mechanism for the reaction.
Answer : B

Question. Which of the following is a lyophobic colloidal solution ? 
(A) Aqueous starch solution
(B) Aqueous protein solution
(C) Gold solution
(D) Polymer solvent in some organic solvents
Answer : C

Question. Which of the following ions will have the minimum coagulating value for the sol obtained by adding FeCl₃ solution to slight excess of NaOH 
(A) SO^2-₄
(B) [Fe(CN)₆ ]^3-
(C) Ba²⁺
(D) Al³⁺
Answer : D

Q1. Anhydrous CaCl₂ is put in a vessel containing water vapours. Name the phenomena that will occur.

Q2. Why is absorption always exothermic? Justify on basis of thermodynamic criteria.

Q3. In an absorption process 20 kJ mol^-1 heat is liberated. What type of absorption will this be? Give any two characteristics of this absorption.

Q4. Give reasons for the following:-

(a) Sky appears blue in colour.

(b) Gelatin is generally added to ice creams.

(c) Silica gel is used as a dehumidizer.

(d) If ferric chloride is added to water, -ve sol is formed whereas addition of FeCl₃ to sodium hydroxide solution gives +sol.

(e) Colloidal sol of Ag I prepared by mixing KI & AgNo3 can bear +ve or -ve charge depending upon stoichlometry of solutions.

(f) Bleeding from a cut can be stopped by rubbing alums.

(g) Rubber can be obtained by adding acetic and to latex.

(h) Alums are used to purify muddy water.

(i) Deltas are formed when river water meets sea water.

(j) Most of the medicines used are in the colloidal form.

Q5. NaCl added to an emulsion enhances its conductivity. What type of emulsion will this be?

Q6. How does catalyst affect the following;-
(a) Rate constant
(b) Equilibrium constant.
(c) Enthalpy of Reaction
(d) Free energy of Reaction.

Q7. What type of solution NaCl forms in benzene?

Q8. What is the difference between dilute soap solution and concentrated soap solution?

Q9. What will happen if Fe (OH)₃ sol is mixed with equimolar amount of As₂S₃ sol ?

Q10. Define Zeta-potential.

Q11. Process of tanning is carried out for hardening of animal hides. Explain it on basis of surface chemistry.

Q12. A group between log x/m and 10 gp is a straight line at angle of 45⁰ with intercept on the y-axis (logK0 equal to .3010. Calculate the amount of the gas absorbed per gram of the absorbent under a pressure of 0.4 atmosphere.
Ans = 0.8

Q13. The coagulation of 200 ml of a positive sol took place when 0.73g HCl was added it without changing the volume much. The flocculation value of HCl for colloid will be?
Ans = 100

1 Mark Questions:-

Question. Bleeding caused by a nick from a razor during shaving can be stopped by rubbing with alum. Why?
Answer: Blood is a negatively charged colloidal solution. When alum is rubbed, the positively charged \( \text{Al}^{3+} \) ions from alum neutralize the charge on the colloidal particles, causing them to coagulate and form a clot.

Question. In which of the following does adsorption take place and why?
(i) Silica gets placed in the atmosphere saturated with water.
(ii) Anhydrous \( \text{CaCl}_2 \) placed in the atmosphere saturated with water.
Answer: Water vapour gets adsorbed on the surface of silica since it is a good adsorbent. On the other hand, anhydrous \( \text{CaCl}_2 \) undergoes absorption as it combines with water vapour to form a hydrate.

Question. Action of soap is due to emulsification and micelle formation. Comment.
Answer: Soap molecules adsorb onto the dirt surface and emulsify it by forming micelles. These soap micelles enclose the insoluble greasy dirt, pulling it into a colloidal form that is easily washed away by water.

Question. What is the use of ZSM-5?
Answer: It converts alcohols directly into gasoline (petrol) by dehydrating them to produce a mixture of hydrocarbons.

Question. Give one example for each (i) sol (ii) gel.
Answer: (i) Sol: paints (ii) Gel: butter or cheese.

Question. What is collodion?
Answer: It is a 4% solution of nitrocellulose in a mixture of alcohol and ether.

Question. Can we form a colloid with two gaseous components? Comment.
Answer: No. When two gases are mixed, they form a completely homogeneous mixture rather than a heterogeneous colloidal system.

Question. What are the physical states of the dispersion medium and dispersed phase in froth?
Answer: In froth, the dispersed phase is a gas while the dispersion medium is a liquid.

Question. Write the equation for the preparation of colloidal sulphur?
Answer: \( \text{SO}_2 + 2\text{H}_2\text{S} \rightarrow 3\text{S(sol)} + 2\text{H}_2\text{O} \)

Question. Why does physisorption decrease with increase of temperature?
Answer: Adsorption is an exothermic phenomenon. According to Le Chatelier's principle, an increase in temperature shifts the equilibrium in the reverse direction, favouring desorption. Therefore, physisorption decreases as the temperature rises.

Question. What is CMC (Critical Micelle Concentration)?
Answer: It is the minimum concentration above which surfactant particles aggregate to form micelles or associated colloids.

Question. Why are powdered substances more effective adsorbents as compared to their crystalline forms?
Answer: The powdered form provides a significantly larger surface area compared to the crystalline form, which increases the extent of adsorption.

Question. Why is it necessary to remove CO when ammonia is obtained by Haber's process?
Answer: In Haber's process, iron is used as a catalyst. If carbon monoxide (\( \text{CO} \)) is present in the system, it acts as a catalytic poison and deactivates the catalyst.

Question. What is electrophoresis due to?
Answer: It is due to the presence of an electrical charge (either positive or negative) on the surface of the colloidal particles, which causes them to migrate under an applied electric field.

Question. Mention two ways by which lyophilic colloids can be coagulated. 
Answer: 1) By adding an electrolyte.
2) By adding a suitable solvent that can dehydrate the dispersed phase.

Question. What is Kraft temperature?
Answer: It is the minimum temperature above which the formation of micelles takes place.

Question. What is Brownian movement due to? 
Answer: It is due to the unbalanced bombardment of the colloidal particles by the molecules of the dispersion medium.

Question. Name two industrial processes in which heterogeneous catalysts are employed. 
Answer: (i) Manufacture of ammonia by Haber's process.
(ii) Manufacture of sulphuric acid by the Contact process.

Question. What is an emulsion? 
Answer: An emulsion is a colloidal system composed of two immiscible liquids where one liquid acts as the dispersion medium and the other acts as the dispersed phase.

Question. What is Zeta potential?
Answer: It is the electrical potential difference between the fixed layer and the diffused double layer of opposite charges surrounding a colloidal particle.

2 Marks Questions

Question. Explain why lyophilic sols are more stable than lyophobic sols?
Answer: Lyophilic colloids have a strong affinity for the dispersion medium, meaning their dispersed phase particles are highly solvated. This protective solvent layer prevents aggregation, making lyophilic sols significantly more stable than lyophobic sols.

Question. State two features of chemical adsorption which are not found with physical adsorption.
Answer: (i) Chemical adsorption has a very high enthalpy of adsorption.
(ii) It is highly specific in nature as it involves chemical bond formation.

Question. Describe the following types of colloids giving one example for each?
(i) Multi molecular colloids
(ii) Macro molecular colloids (C.B.S.E 2007)
Answer: (i) Multi molecular colloids: These are formed by the aggregation of a large number of atoms or small molecules with diameters less than \( 1 \text{ nm} \). The aggregated particles are held together by weak van der Waals forces, e.g., gold sol or sulphur sol (\( \text{S}_8 \)).
(ii) Macromolecular colloids: These consist of large polymeric molecules (macromolecules) whose size lies naturally in the colloidal range (\( 1 - 1000 \text{ nm} \)) when dissolved in a suitable solvent, e.g., starch, proteins, or nylon.

Question. Explain the process of Ultra filtration.
Answer: Ultra filtration is a process used to separate colloidal particles from the solvent and dissolved electrolytes. It utilizes specially prepared filter papers that are impregnated with a collodion solution and hardened with formaldehyde, reducing the pore size so that only solvent molecules and ions can pass through, while the larger colloidal particles are retained.

Question. What is shape selective catalysis?
Answer: Shape selective catalysis refers to a catalytic reaction where the catalytic activity depends on the pore structure of the catalyst and the molecular dimensions of the reactant and product molecules. For example, zeolites like ZSM-5 are shape-selective catalysts used to convert alcohols into gasoline.

Question. (a) Adsorption of a gas on the surface of solid is generally accompanied by a decrease in entropy. Still it is a spontaneous process. Explain.
(b) How does an increase in temperature affect both physical as well as chemical adsorption?
Answer: (a) Adsorption is an exothermic process, which means the enthalpy change (\( \Delta H \)) is negative. Even though the molecules lose freedom of movement leading to a negative entropy change (\( \Delta S < 0 \)), the overall Gibbs free energy change (\( \Delta G = \Delta H - T\Delta S \)) remains negative because the high exothermic heat offsets the entropy loss, making the process spontaneous.
(b) The extent of physical adsorption (physisorption) continuously decreases as the temperature increases. However, the extent of chemical adsorption (chemisorption) first increases with rise in temperature (due to the requirement of activation energy) and then decreases at higher temperatures.

Question. What is an adsorption isotherm? Describe Freundlich adsorption isotherm.
Answer: An adsorption isotherm is a graph plotting the mass of gas adsorbed per unit mass of adsorbent (\( x/m \)) against the equilibrium pressure of the adsorbate gas (\( p \)) at a constant temperature. Once the saturation pressure is reached, no further adsorption occurs.

The Freundlich adsorption isotherm mathematically represents this relationship as:
\( x/m = kp^{1/n} \quad (\text{where } n > 1) \)

Taking logarithms on both sides yields a linear equation:
\( \log(x/m) = \log(k) + \frac{1}{n} \log(p) \)

Question. What is the difference between dialysis and osmosis?
Answer: Both processes involve diffusion through a semi-permeable membrane. However, in osmosis, only the solvent molecules can pass through the membrane, whereas in dialysis, both solvent molecules and smaller ions or electrolyte particles can pass through, leaving the larger colloidal particles behind.

Question. Describe Bredig's Arc method for preparation of gold sol?
Answer: In Bredig's Arc method, an electric arc is struck between gold electrodes immersed in water, which is kept cooled by an ice bath. The intense heat of the arc vapourises some of the gold metal, which then immediately condenses in the cold water to form colloidal-sized gold particles.

Question. Name four different ways by which coagulation of lyophobic sols can be carried out?
Answer: Coagulation of lyophobic sols can be achieved by: (i) Electrophoresis, (ii) Boiling the sol, (iii) Mixing two oppositely charged sols, or (iv) Adding an electrolyte to the sol.

3 Marks Questions

Question. What are emulsions? What are their different types? Give an example of each type.
Answer: Emulsions are colloidal systems where both the dispersed phase and the dispersion medium are immiscible liquids. Since these systems are thermodynamically unstable, they require a third component called an emulsifying agent (emulsifier), such as soap or proteins, to stabilize them.

Emulsions are divided into two main categories:
(i) Oil-in-water (O/W) emulsions: Liquid oil is dispersed in water. Example: milk (where liquid fat droplets are dispersed in water, stabilized by casein).
(ii) Water-in-oil (W/O) emulsions: Water is dispersed in a liquid oil medium. Example: butter, cold cream.

Question. Explain what is observed when:
(i) a beam of light is passed through a colloidal sol
(ii) an electrolyte, say \( \text{NaCl} \), is added to hydrated ferric oxide sol
(iii) electric current is passed through a colloidal sol?
Answer: (i) When a light beam passes through a colloidal sol in a darkened room, the path of the beam becomes illuminated because the light is scattered in all directions by the colloidal particles. This is the Tyndall effect.
(ii) Hydrated ferric oxide sol is positively charged. Adding an electrolyte like \( \text{NaCl} \) introduces negatively charged chloride ions (\( \text{Cl}^- \)), which neutralize the positive charge on the sol particles, causing them to aggregate and coagulate.
(iii) Passing an electric current through the sol causes the charged colloidal particles to migrate toward the oppositely charged electrode (electrophoresis). Upon reaching the electrode, they discharge and coagulate.

Question. Write a short note on (i) Activity (ii) Selectivity and give examples?
Answer: (i) Activity: The catalytic activity refers to the capability of a catalyst to accelerate a reaction, which heavily relies on the strength of chemisorption. The reactants must adsorb strongly onto the catalyst's active sites to become activated, but not so strongly that they become immobile.
Example: \( 2\text{H}_2(g) + \text{O}_2(g) \xrightarrow{\text{Pd}} 2\text{H}_2\text{O}(l) \)

(ii) Selectivity: This is the ability of a catalyst to direct a reaction involving the same starting materials toward a specific, desired product by using different catalytic agents.
Examples:
(i) \( \text{CO}(g) + 3\text{H}_2(g) \xrightarrow{\text{Ni}} \text{CH}_4(g) + \text{H}_2\text{O}(g) \)
(ii) \( \text{CO}(g) + 2\text{H}_2(g) \xrightarrow{\text{Cu/ZnO-Cr}_2\text{O}_3} \text{CH}_3\text{OH}(g) \)

Question. Name & explain three methods of purification of colloids.
Answer: 1) Dialysis: It is the process of removing dissolved impurities and electrolytes from a colloidal solution by allowing them to diffuse out through a semi-permeable membrane while retaining the larger colloidal particles.
2) Electro-dialysis: This is an accelerated dialysis process where an applied electric field forces the ionic impurities to migrate more quickly through the membrane toward electrodes of opposite charge.
3) Ultra-filtration: This method uses specialized filters with exceptionally small pores that prevent colloidal particles from passing through, effectively separating them from the solvent and smaller solutes.

Question. What is Hardy Schulze rule? Explain with examples?
Answer: The Hardy-Schulze rule states that the coagulating power of an active ion is directly proportional to its valency; the greater the valency of the coagulating ion, the greater is its power to cause precipitation.

Examples:
- In the coagulation of a negatively charged sol (e.g., \( \text{As}_2\text{S}_3 \) sol), the coagulating power of cations follows the order: \( \text{Al}^{3+} > \text{Ba}^{2+} > \text{Na}^+ \).
- In the coagulation of a positively charged sol (e.g., \( \text{Fe(OH)}_3 \) sol), the coagulating power of anions follows the order: \( [\text{Fe(CN)}_6]^{4-} > \text{PO}_4^{3-} > \text{Cl}^- \).

Question. What is Tyndall effect, under what conditions is it observed?
Answer: The scattering of light by colloidal particles as it passes through a colloidal dispersion is known as the Tyndall effect.

It is observed only when the following two conditions are met:
(i) The diameter of the dispersed particles is not much smaller than the wavelength of the light used.
(ii) The refractive indices of the dispersed phase and the dispersion medium differ significantly in magnitude.

Question. Explain the mechanism of an enzyme-catalyzed reaction?
Answer: Enzyme-catalyzed reactions are highly specific and typically proceed in a lock-and-key fashion through two main steps:

Step 1: The binding of the enzyme (\( \text{E} \)) to the substrate (\( \text{S} \)) to form an activated enzyme-substrate complex (\( \text{ES}^* \)).
\( \text{E} + \text{S} \rightleftharpoons \text{ES}^* \)

Step 2: The decomposition of the activated complex to yield the final product (\( \text{P} \)) and regenerate the free enzyme.
\( \text{ES}^* \rightarrow \text{E} + \text{P} \)

Question. Differentiate between homogeneous and heterogeneous catalysis. Give one example of each.
Answer: Homogeneous catalysis: A catalytic system in which the reactants, products, and catalyst are all present in the same physical phase.
Example: The oxidation of sulphur dioxide to sulphur trioxide in the presence of gaseous nitric oxide catalyst: \( 2\text{SO}_2(g) + \text{O}_2(g) \xrightarrow{\text{NO}(g)} 2\text{SO}_3(g) \).

Heterogeneous catalysis: A catalytic system where the reactants and the catalyst exist in different physical phases.
Example: The oxidation of sulphur dioxide on a solid vanadium pentoxide catalyst: \( 2\text{SO}_2(g) + \text{O}_2(g) \xrightarrow{\text{V}_2\text{O}_5(s)} 2\text{SO}_3(g) \).

Question. Write short notes on (a) multimolecular, (b) macromolecular, and (c) associated colloids.
Answer: (a) Multimolecular colloids: Formed when a large number of individual atoms or small molecules (with diameters \( < 1 \text{ nm} \)) aggregate together in a dispersion medium to form particles within the colloidal range, e.g., sulphur sol.
(b) Macromolecular colloids: Formed when natural or synthetic polymers (like starch, proteins, or polythene) having large molecular sizes are dissolved in a suitable solvent, forming solutions where the solute molecules are already within the colloidal range, e.g., cellulose sol.
(c) Associated colloids: Substances that behave as normal strong electrolytes at low concentrations but aggregate to form colloidal-sized micelles above a certain concentration (CMC) and temperature, e.g., soap solutions.

Question. Account for the following:
(i) Delta is formed when river meets sea water
(ii) For water purification alum is added
(iii) Cottrell smoke precipitator is used to reduce pollution.
Answer: (i) River water contains colloidal particles of clay and silt. When it meets sea water, the high concentration of electrolytes in the sea water neutralizes the charge on these clay particles, causing them to coagulate and settle at the mouth of the river, forming a delta.
(ii) Alum yields positively charged \( \text{Al}^{3+} \) ions in water. These ions neutralize the negative charges on suspended colloidal clay and dirt particles, causing them to coagulate and precipitate so they can be easily separated.
(iii) Smoke is a colloidal system of solid carbon and dust particles in air. In the Cottrell precipitator, smoke is passed through a chamber with highly charged metal plates, which neutralize the charges on the smoke particles, causing them to coagulate and settle before the clean air is released.