CBSE Class 12 Chemistry Surface Chemistry Questions Answers

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Class 12 Chemistry: Chapter 5 Surface Chemistry, Questions and Answers

FAQ: Chapter 5, Surface Chemistry

1 Mark Questions

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 Al³⁺ ions from the alum neutralise the charge on the particles and make them coagulate to form a clot.

In which of the following does adsorption take place and why? (i) Silica gel placed in an atmosphere saturated with water. (ii) Anhydrous CaCl₂ placed in an atmosphere saturated with water.

Answer: In (i), because water vapour gets adsorbed on the surface of silica gel, which is a good adsorbent. (In (ii) water is absorbed, not adsorbed, as hydrated CaCl₂ is formed.)

The action of soap is due to emulsification and micelle formation. Comment.

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

What is the use of ZSM-5?

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

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

Answer: (i) Paints (ii) Butter, or any other correct example.

What is collodion?

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

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

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

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

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

Write the equation for the preparation of colloidal sulphur.

Answer: \(\text{SO}_2 + 2\text{H}_2\text{S} \longrightarrow 3\text{S (sol)} + 2\text{H}_2\text{O}\)

Why does physisorption decrease with an increase of temperature?

Answer: Adsorption is an exothermic phenomenon. An increase in temperature favours the reverse process, i.e. desorption, according to Le Chatelier's principle. Therefore physisorption decreases with an increase in temperature.

What is CMC (Critical Micelle Concentration)?

Answer: It is the particular concentration above which the particles aggregate to form micelles or associated colloids.

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

Answer: The powdered form has more surface area than the crystalline form.

Why is it necessary to remove CO when ammonia is obtained by Haber's process?

Answer: In Haber's process, iron is used as the catalyst. If CO is present, it acts as a catalytic poison.

What is electrophoresis due to?

Answer: It is due to the existence of positive or negative charge on the colloidal particles.

Mention two ways by which lyophilic colloids can be coagulated. (C.B.S.E. 2008)

Answer: (1) By adding an electrolyte. (2) By adding a suitable solvent.

What is the Kraft temperature? (C.B.S.E. Foreign 2004)

Answer: It is the temperature above which the formation of micelles takes place.

What is Brownian movement due to? (C.B.S.E. 1999)

Answer: It is due to the unbalanced bombardment of the colloidal particles by the molecules of the dispersion medium.

Name two industrial processes in which heterogeneous catalysts are employed. (C.B.S.E. 2008)

Answer: (i) Manufacture of ammonia by Haber's process. (ii) Manufacture of sulphuric acid by the contact process.

What is an emulsion? (C.B.S.E. Foreign 2009)

Answer: An emulsion is a colloidal solution of two immiscible liquids, of which one is the dispersion medium and the other is the dispersed phase.

What is zeta potential?

Answer: It is the potential difference between the fixed layer and the diffused layer of opposite charges around the colloidal particles.

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

Answer: The dispersed phase is a gas and the dispersion medium is a liquid.

What is the cause of Brownian movement among colloidal particles?

Answer: It is due to the collisions of the molecules of the dispersion medium with the colloidal particles.

Arrange the following in decreasing order of their particle size: true solution, colloidal solution, suspension.

Answer: Suspension > colloidal solution > true solution.

Give an example of a micelle system.

Answer: Sodium stearate (C₁₇H₃₅COO⁻Na⁺).

Why is it necessary to remove CO when ammonia is obtained by Haber's process?

Answer: CO acts as a poison for the catalyst of Haber's process, and therefore lowers its activity. So it is necessary to remove it when NH₃ is obtained by Haber's process.

How is the adsorption of a gas related to its critical temperature?

Answer: The higher the critical temperature of a gas, the greater is the ease of its liquefaction. Greater van der Waals forces of attraction operate, and hence a larger adsorption occurs.

What is meant by a shape-selective catalyst?

Answer: For a shape-selective catalyst, the rate of reaction depends upon the pore size of the catalyst and on the shape and size of the molecules of the reactants and products.

Of physisorption and chemisorption, which type of adsorption has the higher enthalpy of adsorption?

Answer: Chemisorption.

Give an example of a positive sol.

Answer: Ferric hydroxide sol.

Give an example of a negative sol.

Answer: Arsenic sulphide sol.

2 Marks Questions

Explain why lyophilic sols are more stable than lyophobic sols.

Answer: Lyophilic colloids have a great affinity for the dispersion medium, i.e. the dispersed-phase particles are solvated to a greater extent. Hence lyophilic sols are relatively more stable than lyophobic sols.

State two features of chemical adsorption which are not found in physical adsorption.

Answer: (i) Chemical adsorption has a high enthalpy of adsorption. (ii) It is highly specific.

Describe the following types of colloids, giving one example of each: (i) multimolecular colloids, (ii) macromolecular colloids. (C.B.S.E. 2007)

Answer: (i) Multimolecular colloids consist of aggregates of atoms or small molecules with a diameter of less than 1 nm. The colloidal particles are held together by weak van der Waals forces, e.g. sols of S₈. (ii) Macromolecular colloids are those in which the molecules of the dispersed phase are themselves of colloidal dimensions (1–1000 nm). These have a very high molecular mass, e.g. sols of proteins.

Explain the process of ultrafiltration.

Answer: Ultrafiltration is the process of separating the colloidal particles from the solvent and soluble solutes present in a colloidal solution by specially prepared filters, which are permeable to all substances except the colloidal particles. These filters are prepared by impregnating ordinary filter paper with collodion solution and hardening it with formaldehyde.

What is shape-selective catalysis? (C.B.S.E. 2003, Foreign 2004)

Answer: A catalyst whose catalytic action depends upon its pore structure and on the molecular sizes of the reactants as well as the products is known as a shape-selective catalyst, and the catalytic action is called shape-selective catalysis. For example, zeolites act as shape-selective catalysts. Recently ZSM-5 has been used in producing gasoline from alcohol.

(a) Adsorption of a gas on the surface of a 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 and chemical adsorption?

Answer: (a) Adsorption is exothermic, i.e. \(\Delta H\) is negative. During adsorption the freedom of movement of the molecules becomes restricted, so \(\Delta S\) is negative. But \(\Delta G = \Delta H - T\Delta S\) becomes negative because the magnitude of \(\Delta H\) is larger than that of \(T\Delta S\). Hence the process is spontaneous. (b) The extent of physical adsorption decreases with an increase in temperature, whereas the extent of chemical adsorption first increases and then decreases as the temperature increases.

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

Answer: The graph drawn between the amount of gas adsorbed per unit mass of adsorbent, \(x/m\), and the pressure of the gas at a constant temperature is known as an adsorption isotherm. After the saturation pressure there is no more adsorption on the surface. The Freundlich adsorption isotherm can be expressed as \(\dfrac{x}{m} = kp^{1/n}\) (\(n > 1\)), or \(\log\dfrac{x}{m} = \log k + \dfrac1n\log p\).

What is the difference between dialysis and osmosis?

Answer: The two processes appear to be the same, since both involve diffusion through a semipermeable membrane. However, in osmosis only the solvent, and not the solute particles, can pass through the membrane, but in dialysis even small ions of an electrolyte can pass through the membrane, while the colloidal particles, which are comparatively big in size, cannot.

Describe Bredig's arc method for the preparation of a gold sol.

Answer: An electric arc is struck between electrodes made of gold immersed in the dispersion medium, water. The intense heat produced vaporises the metal, which then condenses to form particles of colloidal gold.

Name four different ways by which coagulation of lyophobic sols can be carried out.

Answer: (i) By electrophoresis. (ii) By boiling. (iii) By mixing two oppositely charged sols. (iv) By the addition of electrolytes.

State the Hardy-Schulze rule.

Answer: The capacity of an electrolyte to cause coagulation of a sol depends upon the number of charges on the ion having a charge opposite to that possessed by the sol particles, but is independent of the charge on the ion having a similar charge.

Define flocculation value.

Answer: The minimum number of millimoles of an electrolyte required to bring about the coagulation of one litre of a colloidal solution is called its flocculation value.

What is the difference between a sol and a gel?

Answer: In a sol, the dispersion medium is a liquid and the dispersed phase is a solid. In a gel, the dispersion medium is a solid and the dispersed phase is a liquid.

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 a large number of atoms or molecules with a diameter of less than 1 nm. (ii) They have lyophilic property. Associated colloids: (i) They are formed by the aggregation of a large number of ions in concentrated solution. (ii) They contain both lyophilic and lyophobic groups.

Give any two reasons for the origin of the electrical charge on colloidal particles.

Answer: (i) Electron capture by sol particles during electro-dispersion of metals. (ii) Preferential adsorption of ions from the solution. (The ionisation of surface groups of the colloidal particles can also give them a charge.)

Differentiate between electrophoresis and electro-osmosis.

Answer: Electrophoresis is the movement of colloidal particles under the influence of an electric field. Electro-osmosis is the movement of the molecules of the dispersion medium under the influence of an electric field when the colloidal particles are not allowed to move.

Why are lyophilic colloids called reversible sols while lyophobic sols are called irreversible sols? Give one example of each.

Answer: If the dispersed phase of a lyophilic colloid is separated from the dispersion medium, the sol can be prepared again by simply remixing it with the dispersion medium. So they are called reversible sols. Example: starch. In lyophobic sols, if a small amount of electrolyte is added, the sols are readily precipitated and do not give back the colloid by the simple addition of the dispersion medium. So they are called irreversible sols. Example: metal sulphides.

Explain the cleansing action of soap.

Answer: The action of soap is due to emulsification and micelle formation. Soaps are sodium salts of higher fatty acids, like sodium stearate, C₁₇H₃₅COO⁻Na⁺, which ionises as \(\text{C}_{17}\text{H}_{35}\text{COO}^-\text{Na}^+ \rightleftharpoons \text{C}_{17}\text{H}_{35}\text{COO}^- + \text{Na}^+\). The ionic head of the stearate ion (COO⁻) is hydrophilic in nature and has a great affinity for water, while the hydrocarbon part (C₁₇H₃₅) is hydrophobic in nature and has a great affinity for oil, grease, etc. When soap is used in water, the anions (C₁₇H₃₅COO⁻) form micelles and encapsulate the oil or grease inside. These micelles are removed by rinsing with water, while the free dirt (from the oil or grease) either settles down or is washed away by water. Thus the main function of a soap is to entrap oil or grease in the micelles through emulsification, thereby freeing the dirt from the grease and oil.

Differentiate between chemisorption and physisorption.

Answer: The differences are given in the table above.

Why is adsorption always exothermic?

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

3 Marks Questions

What are emulsions? What are their different types? Give an example of each type.

Answer: Emulsions are colloidal solutions of two immiscible liquids in which the dispersed phase as well as the dispersion medium are liquids. Since the two do not mix well, the emulsion is generally unstable and is stabilised by adding a suitable reagent called an emulsifier, e.g. gum, soap, etc. Types of emulsions: (i) Oil in water emulsions, in which oil acts as the dispersed phase while water acts as the dispersion medium. For example, milk is an emulsion of fats in water, and here casein acts as the emulsifier. (ii) Water in oil emulsions. In this case, water acts as the dispersed phase while oil acts as the dispersion medium. For example, butter and cold cream.

Explain what is observed when (i) a beam of light is passed through a colloidal sol, (ii) an electrolyte, say NaCl, is added to a hydrated ferric oxide sol, (iii) an electric current is passed through a colloidal sol.

Answer: (i) When a beam of light is passed through a colloidal sol placed in a dark room, the beam of light is scattered by the colloidal particles and the path of the beam becomes visible. The phenomenon is called the Tyndall effect. (ii) When NaCl is added to a hydrated ferric oxide sol, the Cl⁻ ions of NaCl neutralise the positive charge on the ferric hydroxide sol particles, and coagulation of the sol occurs. (iii) When an electric current is passed through a colloidal sol, the dispersed phase moves towards the oppositely charged electrode. On reaching the electrode, the particles lose their charge and get coagulated.

Write a short note on (i) activity and (ii) selectivity of catalysts, and give examples.

Answer: (i) Activity: The activity of a catalyst depends upon the strength of chemisorption to a large extent. The reactants must get adsorbed strongly on the surface of the catalyst to become active. Example: \(2\text{H}_2(g) + \text{O}_2(g) \xrightarrow{\text{Pt/Pd}} 2\text{H}_2\text{O}(l)\). (ii) Selectivity: It is the ability of a catalyst to direct a reaction to yield a particular product. For example, starting with H₂ and CO and using different catalysts, we get different products: (a) \(\text{CO}(g) + 3\text{H}_2(g) \xrightarrow{\text{Ni}} \text{CH}_4(g) + \text{H}_2\text{O}(g)\); (b) \(\text{CO}(g) + 2\text{H}_2(g) \xrightarrow{\text{Cu/ZnO-Cr}_2\text{O}_3} \text{CH}_3\text{OH}(g)\).

Name and explain three methods of purification of colloids.

Answer: Dialysis: Separation of electrolytes from a colloidal solution using a semipermeable membrane. Electrodialysis: When dialysis is carried out with an electric field around the membrane, the purification process is enhanced. Ultrafiltration: Use of special filters which are permeable to all substances except the colloidal particles.

What is the Hardy-Schulze rule? Explain with examples.

Answer: “The greater the valence of the flocculating ion, the greater is its power to cause coagulation.” Example: in the coagulation of a negative sol, the flocculating power is in the order Al³⁺ > Ba²⁺ > Na⁺. In the case of a positive sol, the flocculating power is in the order [Fe(CN)₆]⁴⁻ > PO₄³⁻ > Cl⁻.

What is the Tyndall effect? Under what conditions is it observed?

Answer: The scattering of light by colloidal particles is called the Tyndall effect. The conditions required to observe this effect are: (i) the diameter of the dispersed particles is not much smaller than the wavelength of the light used, and (ii) the refractive indices of the dispersed phase and the dispersion medium differ greatly in magnitude.

Explain the mechanism of an enzyme-catalysed reaction.

Answer: \(\text{E} + \text{S} \rightleftharpoons [\text{ES}^*] \longrightarrow \text{E} + \text{P}\). Thus enzyme-catalysed reactions may be considered to proceed in two steps. Step 1: binding of the enzyme to the substrate to form an activated complex, \(\text{E} + \text{S} \rightleftharpoons \text{ES}^*\). Step 2: decomposition of the activated complex to form the product, \(\text{ES}^* \longrightarrow \text{E} + \text{P}\).

Differentiate between homogeneous and heterogeneous catalysis. Give one example of each.

Answer: Homogeneous catalysis: the catalyst and the reactants are in the same phase (liquid or gas). Example: oxidation of sulphur dioxide into sulphur trioxide with NO(g) as catalyst; all are in the gaseous state. Heterogeneous catalysis: the reactants and the catalyst are in different phases. Example: \(2\text{SO}_2(g) + \text{O}_2(g) \xrightarrow{\text{V}_2\text{O}_5(s)} 2\text{SO}_3(g)\).

Write short notes on (a) multimolecular, (b) macromolecular and (c) associated colloids.

Answer: (a) Multimolecular colloids have many atoms or molecules aggregated together. Example: sulphur sol. (b) Macromolecular colloids consist of macromolecules in a suitable solvent. Example: dispersion of cellulose in a suitable solvent. (c) Associated colloids are substances which at low concentrations behave as normal electrolytes, while at high concentrations they behave as colloids. Example: soap solution.

Account for the following: (i) A delta is formed when a river meets sea water. (ii) For water purification, alum is added. (iii) The Cottrell smoke precipitator is used to reduce pollution.

Answer: (i) The electrolytes of sea water coagulate the colloidal clay particles of river water. (ii) Alum coagulates the suspended colloidal impurities. (iii) In the Cottrell smoke precipitator, the colloidal smoke particles lose their charge as they pass through it and coagulate.

9. Write the equation for the preparation of colloidalsulphur ?

Ans:- SO₂ + 2H₂S →3S(sol) + 2H₂O

Ans:– Suspension > colloidal > true solution

24. Give an example of micelles system?