NCERT Solutions for Class 12 Chemistry Chapter 5 - Surface Chemistry

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1. Distinguish between the meaning of the terms adsorption and absorption. Give one example of each.

Ans. Adsorption: It is a surface phenomenon as the adsorbed matter is concentrated only at the surface and does not penetrate through the surface to the bulk of adsorbent.

Example: Adsorption of acetic acid by activated charcol.

Absorption: It is a bulk phenomenon as the absorbed matter is uniformly distributed inside as well as at the surface of the bulk of a substance.

Example: Absorption of ink by blotting paper.

2. What is the difference between physisorption and chemisorption?

Ans.

Physisorption	Chemisorption
1. It occurs due to weak Van der Waal’s forces.	It is due to formation of chemical bonds.
2. It has low adsorption enthalpy of 20-40 kJ/mol.	It has high adsorption enthalpy of 80-240 kJ/mol.
3. It usually occurs at low temperature.	It usually occurs at high temperature.
4. It is reversible in nature.	It is irreversible in nature.
5. It is not specific in nature, i.e., all gases may be adsorbed on the surface of a solid.	It is highly specific in nature and occurs or there is bond formation between adsorbate molecules.
7. The extent of adsorption is directly related with the ease if liquefaction of the gas.	There is no correlation between extent of a and the ease of liquefaction of the gas.
8. Activation energy is less in physisorption.	Activation energy is high in chemisorption.

3. Give reason why a finely divided substance is more effective as an adsorbent?

Ans. A finely divided substance is more effective as an adsorbent due to:

(i) It has more surface area so more adsorption occurs.

(ii) The number of active sites becomes more and the extent of adsorption increases.

4. What are the factors which influence the adsorption of a gas on a solid?

Ans. The adsorption of a gas on a solid surface is an example of physical adsorption. It is influenced by the following factors:

(i) Nature of the adsorbate

(ii) Nature of the adsorbent

(iii) Surface area of the adsorbent

(iv) Activation of the adsorbent

(v) Effect of pressure

(vi) Effect of temperature.

Factors affecting adsorption of a gas on solids are:

(i) Nature of the adsorbate: The same gas is adsorbed to different extents by different solids at the same temperature. Also, greater the surface area of the adsorbent, more is the gas adsorbed.

(ii) Nature of the adsorbent: Different gases are adsorbed to different extents by different solids at the same temperature. Higher the critical temperature of the gas, greater is its amount adsorbed

(iii) Surface area of the adsorbent: Surface area available for adsorption per gram of the adsorbent increases the extent of adsorption. Greater the surface area, higher would be the adsorption therefore, porous or powdered adsorbents are used.

(iv) Activation of adsorbent: It means increasing the adsorbing power of an adsorbent by increasing its surface area. It is done by:

(a) Making the adsorbent’s surface rough

(b) Removing gases already adsorbed

(v) Pressure: At constant temperature, the adsorption of gas increases with pressure.

(vi) Temperature: Since adsorption is an exothermic process, applying Le Chatelier’s principle, we can find out that adsorption decreases with an increase in temperature.

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

Ans. Adsorption isotherm represents the variation of the mass of the gas adsorbed per gram of the adsorbent with pressure at constant temperature.

Freundlich Adsorption isotherm:

Freundlich, in 1909, gave an empirical relationship between the quantity, of gas adsorbed by unit mass of solid adsorbent and pressure at a particular temperature. The relationship can be expressed by the following equation:

$$\frac{x}{m}=k\text{P}^{\frac{1}{n}}(n>1)\qquad...\text{(i)}$$

where x is the mass of the gas adsorbed by mass ‘m’ of the adsorbent at pressure P, k and n are constants which depend on the nature of the adsorbent and the gas at a particular temperature. The relationship is generally represented in the form of a curve where mass of the gas adsorbed per gram by the adsorbent is plotted against pressure. These curves indicate that at a fixed pressure, there is a decrease in physical adsorption with increase in temperature. These curves always seem to approach saturation at high pressure.

Taking log of equation (i), we get

$$\text{log}\frac{x}{m}=\text{log}\space K+\frac{1}{n}\text{log}\space\text{P}\\\text{When log}\space\frac{x}{m}\space\text{is plotted on y-axis and P on x-axis,}$$

we get a straight line. It indicates the validity of Freundlich isotherm.

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

Ans. Activation of adsorbent implies increasing its adsorbing power. This is achieved by increasing the surface area of the adsorbent and also the number of pores and vacant sites (active centres) per unit area. The activation can be achieved in a number of ways.

(i) By increasing the surface area of the adsorbent which is done by breaking the adsorbent into small pieces.

(ii) By making the surface of adsorbent rough.

(iii) By some specific treatments. for example, wood charcoal is activated by heating it at a temperature between 650 K-1330 K in air or vaccum.

7. What role does adsorption play in heterogeneous catalysis?

Ans. In heterogeneous catalysis, generally the reactants are gaseous whereas catalyst is a solid. The reactant molecules are adsorbed on the surface of the solid catalyst by physical adsorption or chemisorption. As a result, the concentration of the reactant molecules on the surface increases further leading to increase in rate of reaction. Alternatively, one of the reactant molecules undergoes fragmentation on the surface of the solid catalyst producing active species which react faster. The product molecules in either case have no affinity for the solid catalyst and are deadsorbed making the surface free for fresh adsorption.

8. Why is adsorption always exothermic?

Ans. When a gas is adsorbed on the surface of a solid, its movement is limited resulting in a decrease in the gas’s entropy, i.e., ΔS is negative.

Now , ΔG = ΔH–TΔS.

For a process to be spontaneous, ΔG must be negative.

As here, ΔS is negative, therefore, TΔS is positive, ΔG can be negative only if ΔH is negative. Hence, adsorption is always exothermic.

9. How are the colloidal solutions classified on the basis of physical states of the dispersed phase and dispersion medium?

Ans. Colloids can be classified into eight types depending upon the physical state of the dispersed phase and the dispersion medium.

10. Discuss the effect of pressure and temperature on the adsorption of gases on solids.

Ans. Effect of pressure on adsorption: At constant temperature the adsorption increases with increase in pressure.

Effect of temperature on adsorption: Adsorption is generally temperature dependent. Being exothermic, adsorption decreases with increase in temperature. However, for an endothermic process, adsorption increases with increase in temperature.

11. What are lyophilic and lyophobic sols ? Give one example of each type. Why are hydrophobic sols easily coagulated?

Ans. Lyophilic colloids: The colloidal solution in which the particles of the dispersed phase have a great affinity (or love) for the dispersion medium, are called lyophilic colloids. Such solutions are easily formed the moment the dispersed phase and the dispersion medium come in direct contact. e.g., sols of gum, gelatin, starch, etc.

Lyophobic colloids: The colloidal solutions in which the particles of the dispersed phase have no affinity or love, rather have hatred for the dispersion medium, are called lyophobic colloids. The solutions of metals like Ag and Au, hydroxides like Al(OH)₃ and Fe(OH)₃ and metal sulphides like As₂S₃ are examples of lyophobic colloids.

Reasons for coagulation of hydrophobic sols: These sols are precipitated due to the removal or neutralization of charge on their particles, i.e., they are come nearer, form aggregate and settle down. This is done by addition of a small amount of electrolyte.

Dispersed phase	Dispersion medium	Types of colloid	Example
Solid	Solid	Solid sol	Coloured glasses, gem stones
Solid	Liquid	Sol	Paints, cell fluid
Solid	Gas	Smoke	Particulated, dust
Liquid	Solid	Gel	Cheese, butter, jellies
Liquid	Liquid	Emulsion	Milk, hair cream
Liquid	Gas	Aerosol	Fog, mist, clouds
Gas	Solid	Solid foam	Pumic stone, foam rubber
Gas	Liquid	foam	Froth, whipped cream

12. What is the difference between multimolecular and macromolecular colloids? Give one example of each.

How are associated colloids different from these two types of colloids?

Ans. Comparison of some important characteristic of multimolecular, macromolecular and associated colloids.

	Multimolecular colloids	Macromolecular colloids	Associated Colloids
(1)	They are formed by the aggregation of a large number of atoms or molecules which generally have diameters less than 1 nm.	They are molecules of large size.	They are formed by the aggregation of a large number of ions in concentrated solution.
(2)	Examples: sols of gold and sulphur.	Examples: polymers like rubber, nylon, starch.	Example: soap sol.
(3)	Their molecular masses are not very high.	They have high molecular masses.	Their molecular masses are generally high.
(4)	Their atoms or molecules are held together by weak van der Waal’s force.	Due to long chain, the van der Waal’s force holding them are comparatively stronger.	Higher is the concentration greater are the van der Waal’s forces.

13. What are enzymes? Write in brief the mechanism of enzyme catalysis.

Ans. Enzyme: Enzyme are biocatalyst that increases the rate of a chemical reaction by lowering the energy required for activation.

There are two parts of enzyme a protein part and a cofactor.

Mechanism of enzyme catalysis: It includes following steps:

Step 1: Enzyme (E) binds to substrate (S) to form enzyme substrate complex (ES)

E + S ⇌ ES

Step 2: Enzyme substrate complex (ES) converts to enzyme product complex (EP)

$$\text{ES}\xrightarrow{}\text{EP}$$

Step 3: Enzyme product complex (ES) dissociates into enzyme (E) and product (P)

$$\text{EP}\xrightarrow{}\text{E}+\text{P}$$

14. How are colloids classified on the basis of:

(i) Physical states of components

(ii) Nature of dispersed phase and

(iii) Interaction between dispersed phase and dispersion medium?

Ans. (i) Colloids can be classified into eight types depending upon the physical state of the dispersed phase and the dispersion medium.

Dispersed phase	Dispersion medium	Types of colloid	Example
Solid	Solid	Solid sol	Coloured glasses, gem stones
Solid	Liquid	Sol	Paints, cell fluid
Solid	Gas	Smoke	Particulated, dust
Liquid	Solid	Gel	Cheese, butter, jellies
Liquid	Liquid	Emulsion	Milk, hair cream
Liquid	Gas	Aerosol	Fog, mist, clouds
Gas	Solid	Solid foam	Pumic stone, foam rubber
Gas	Liquid	foam	Froth, whipped cream

(ii) Depending upon the type of the particles of the dispersed phase, colloids are classified as : Multimolecular, macro- molecular and associated colloids:

(a) Multimolecular colloids: The colloids in which the colloidal particles consist of aggregates of atoms or small molecules are called multimolecular colloids .
For Example: gold sol, sulphur sol etc.

(b) Macromolecular colloids: The colloids in which large particles of colloidal range having high molecular masses are dissolved in a suitable liquid are called macromolecular colloids.

Example: proteins, starch and cellulose form macromolecular colloids.

(c) Associated colloids (Micelles): Those colloids which behave as normal strong electrolyte at low concentration but show colloidal properties at high concentration due to the formation of aggregated particles of colloidal dimensions. Such substances are also referred to as associated colloids.

(iii) On the basis of interaction between disperse phase and dispersion medium colloids classified into:

(a) Lyophillic colloids (solvent loving) are those substances that directly pass into the colloidal state when brought in contact with the solvent, e.g., proteins, starch, rubber, etc.

These sols are quite stable because of the strong attractive forces between the particles of dispersed phase and dispersion medium.

(b) Lyophobic colloids (solvent hating) are those substances that do not form the colloidal sol readily when mixed with the dispersion medium.

These sols are less stable than the lyophilic sols.

15. Explain what is observed:

(i) When a beam of light is passed through colloidal sol.

(ii) An electrolyte NaCl is added to ferric hydroxide sol.

(iii) Electric current is passed through a colloidal sol?

Ans. (i) When a beam of light is passed through a colloidal solution, the scattering of light is observed and the path of light become visible (Tyndall effect).

(ii) When an electrolyte, NaCl is added to hydrated ferric oxide solution, the NaCl dissociates to give Na⁺ and Cl^– ions. Particles of ferric oxide sol are positively charged. Thus, they get coagulated in the presence of negatively charged Cl^– ions.

(iii) When an electric current is passed through a colloidal sol, the colloidal particles move towards the oppositely charged electrode and the phenomenon is called electrophoresis.

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

Ans. Emulsions: It is a colloidal system in which both the dispersed phase and the dispersion medium are liquids, e.g., milk consists of small drops of liquid fat dispersed in water.

Types of emulsions:

(a) Oil-in-water type in which small droplets of an oil are dispersed in water, e.g., milk, cod-liver oil, etc.

(b) Water-in-oil type in which water droplets are dispersed in an oil medium, e.g., butter.

17. How do emulsifiers stabilise emulsion? Name two emulsifiers.

Ans. Emulsifiers stabilize an emulsion by reducing the interfacial tension between two liquids forming the emulsions.

Casein and soap are the example of emulsifier

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

Ans. Soap is sodium salt of higher fatty acids (RCOONa). The polar head of soap dissolve in water and non-polar tail of water dissolve in oil. Thus each oil droplet is surrounded by soap aggregates and emulsion of oil in water is formed, is called micelle. These oil droplets then get detached from the cloth surface by rubbing and can be washed with water.

19. Give four examples of heterogeneous catalysis.

Ans. Examples of heterogeneous catalysis are given as:

(i) Ammonia is manufactured from hydrogen and nitrogen in Haber’s process.

$$\text{N}_2\text{(g)} + 3\text{H}_2(g)\xrightarrow{\text{Fe(s)}}2\text{NH}_3\text{(g)}$$

(ii) Oxidation of sulphur dioxide into sulphur trioxide in the presence of Pt.

$$2\text{SO}_2\text{(g)}\xrightarrow{\text{Pt(s)}} 2\text{SO}_3\text{(g)}$$

(iii) Nickel used as catalyst in dehydrogenation of ethanol.

$$\text{CH}_3\text{CH}_2\text{OH}\xrightarrow{\text{Ni}}\text{CH}_3\text{CHO}+\text{H}_2$$

(iv) Hydrogenation of vegetable oils to from saturated fats in the presence of Ni as catalyst.

$$\text{Vegetable oil(l) + H}_2\text{(g)}\xrightarrow{\text{Ni}}\text{vegetable ghee(s)}$$

20. What do you mean by activity and selectivity of catalysts?

Ans. Important features of solid catalyst:

(i) Activity: The activity of a catalyst is its ability to accelerate chemical reactions. It depends upon the strength of chemisorption to a large extent.

The catalytic activity of a metal for hydrogenation increases as we move from Group 5 metals to Group 11. The maximum activity is shown by metals of Groups 7, 8 and 9.

(ii) Selectivity: The selectivity of a catalyst is its ability to direct a reaction to yield a particular product. It mean a substance which acts as a catalyst in one reaction may not act as a catalyst in other reaction e.g., we get different products when we use different catalysts in the reaction between H₂ and CO.

$$\text{CO(g) + 3H}_2\text{(g)}\xrightarrow{\text{Pt}}\text{CH}_4\text{(g)}+\text{H}_2\text{O(g)}\\\text{CO(g) + 2H}_2\text{(g)}\xrightarrow{\text{Cu/ZnO-Cr}_2\text{O}_3}\text{CH}_3\text{OH(g)}\\\text{CO(g)+H}_2\text{(g)}\xrightarrow{\text{Cu}}\text{HCHO(g)}$$

21. Describe some features of catalysis by zeolites.

Ans. (i) Zeolites are hydrated aluminosilicates which have a three dimensional network structure containing water molecules in their pores.

(ii) The pores are made vacant by heating before catalysis.

(iii) The reactions taking place in zeolites depend upon the size and shape of reactant and product molecules and also on the pores and cavities in them, e.g., ZSM-5 converts alcohols to hydrocarbons by dehydrating them.

$$\text{Alcohols}\xrightarrow{\text{ZSM} 5}\text{Hydrocarbons}$$

22. What is shape selective catalysis?

Ans. The catalytic reaction that depends upon the pore structure of the catalyst and the size of the reactant and product molecules is called shape selective catalysis. Zeolites are good shape selective catalysts because of their honeycomb like structures. They are microporous aluminosilicates with three dimensional network of silicates in which some silicon atoms are replaced by aluminium atoms giving Al-O-Si framework. The reactions taking place in zeolites depend upon the size and shape of reactant and product molecules as well as upon the pores and cavities of the zeolites. They are found in nature as well as synthesised for catalytic selectivity.

23. Explain the following terms :

(i) Electrophoresis

(ii) Coagulation

(iii) Dialysis

(iv) Tyndall effect

Ans. (i) Electrophoresis: When an electric field is applied to an colloid solution, the ions moves towards the oppositively charged electrodes. The phenomenon is called electrophoresis.

(ii) Coagulation: The formation of aggregates or precipitates of colloidal particles by addition of a suitable electrolyte is called coagulation.

(iii) Dialysis: The process of removing a dissolved substance from the colloidal solution by means of diffusion or through membrane is called dialysis.

(iv) Tyndall effect: When a beam of light is passed through a colloidal solution and the solution is viewed at right angles to the passage of light, they also a mild to strong opalescence i.e., the path of light is illuminated. This phenomenon is calle Tyndall effect.

24. Give four uses of emulsions.

Ans. (i) Some of the medicines are effective as emulsions.

(ii) Paints are emulsions which are used in our daily life.

(iii) Soaps and detergents act as cleansing agents, action of which is based on emulsification.

(iv) Photographic films are coated with emulsion of AgBr on its surface.

25. What are micelles? Give an example of a micellers system.

Ans. Micelles are substance that behave as normal strong electrolytes at low concentration but at high concentrations behave as colloids due to formation of aggregates. They are also called associated colloids, e.g., soaps and detergents. They can form ions and may contain 100 or more molecules to form a micelle.
The example of a micellers system is soap and detergents.

26. Explain the terms with suitable examples :

(i) Alcosol

(ii) Aerosol

(iii) Hydrosol

Ans. (i) Alcosol: The sol in which alcohol is used as dispersion medium is called alcosol e.g., sol of cellulose nitrate in ethyl alcohol.

(ii) Aerosol: The sol in which dispersion medium is gas and dispersed phase is either solid or liquid, the colloidal system is called aerosol e.g., fog, insecticides, sprays, etc.

(iii) Hydrosol: The sol in which dispersion medium is water is called hydrosol e.g., starch sol.

27. Comment on the statement that “colloid is not a substance but a state of substance”.

Ans. This statement is true because the same substance in one solvent may behave as a colloid while in the other as crystalloid. For example, sodium chloride behaves as crystalloid in water but as colloid in benzene solvent. This means that the colloid is not a substance. It depends upon the particle size of the dispersed phase which varies from 10³ to 10⁶ pm.

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