Surface Chemistry Class 12 NCERT Solutions
Que 5.1. Distinguish between the meaning of the terms adsorption and absorption. Give one example of each.
Ans 5.1. Difference between adsorption and absorption
| S.No. | Adsorption | Absorption |
| 1 | The phenomenon of accumulation of the molecules of a substance on a solid or liquid surface | In absorption, the substance is uniformly distributed throughout the body of a solid or a liquid. |
| 2. | It is a surface phenomenon. | It is a bulk phenomenon. |
| 3. | Here, the concentration of the substance increases on the surface. | Here, the concentration of the substance increases in the bulk. |
| 4. | The rate varies throughout the process. | The rate remains the same. |
| 5. | For example. i). Water vapours are absorbed by anhydrous calcium chloride. ii). when we dip a chalk stick into an ink solution, only its surface becomes coloured. If we break the chalk stick, it will be found to be white from inside. |
For example: i). Water vapours are adsorbed by silica gel. ii). soaking up spilled milk with a paper towel. |
Que 5.2. What is the difference between physisorption and chemisorption?
Ans 5.2.
Que 5.3. Give the reason why a finely divided substance is more effective as an adsorbent.
Ans 5.3. As adsorption is a surface phenomenon, the extent of adsorption increases with an increase in surface area. A finely divided substance has a large surface area. Both physisorption and chemical adsorption increase with an increase in the surface area. Hence, a finely divided substance is more effective as an adsorbent.
Surface Chemistry Class 12 NCERT Solutions
Que 5.4. What are the factors which influence the adsorption of a gas on a solid?
Ans 5.4. The factors which influence the adsorption of a gas on a solid are:
Nature of the adsorbent: The same gas is adsorbed to different extents by different solids at the same temperature. Also, the greater the surface area of the adsorbent more is the gas adsorbed.
Nature of the adsorbate: Different gases are adsorbed to different extents by different solids at the same temperature. The higher the critical temperature of the gas, the greater is its amount adsorbed.
Temperature: Since adsorption is an exothermic process, applying Le Chatelier’s principle, we can find out that adsorption decreases with an increase in temperature.
Effect of pressure: At constant temperature, adsorption generally increases with an increase in pressure.
Specific area of the adsorbent: The surface area available for adsorption per gram of the adsorbent increases the extent of adsorption. The greater the surface area, the higher would be the adsorption, therefore porous or powdered adsorbents are used.
Que 5.5. What is an adsorption isotherm? Describe Freundlich adsorption isotherm.
Ans 5.5. Adsorption isotherm represents the variation of the mass of the gas adsorbed per gram of the adsorbent with pressure at a 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:
Surface Chemistry Class 12 NCERT Solutions
Que 5.6. What do you understand by activation of adsorbent? How is it achieved?
Ans 5.6. Activation of an adsorbent means, increase in the adsorbing power of the adsorbent.
This can be achieved by following ways:
i). By increasing the surface area of the adsorbent.
ii). Some specific treatments can also lead to the activation of the adsorbent. For
example, wood charcoal is activated by heating it between 650 K and 1330 K in a vacuum
or air. It expels all the gases absorbed or adsorbed and thus, creates a space for the adsorption of gases.
Que 5.7. What role does adsorption play in heterogeneous catalysis?
Ans 5.7. A catalytic process in which the catalyst and the reactants are present in different phases is known as heterogeneous catalysis.
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. The product molecules have no affinity for the solid catalyst and are de-adsorbed making the surface free for fresh adsorption.
Que 5.8. Why is adsorption always exothermic?
Ans 5.8. i). Adsorption leads to a decrease in the residual forces on the surface of the adsorbent. This causes a decrease in the surface energy of the adsorbent. Therefore, adsorption is always exothermic.
ii). When a gas is adsorbed on the surface of a solid, its entropy decreases, 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.
Que 5.9. How are the colloidal solutions classified on the basis of physical states of the dispersed phase and dispersion medium?
Ans 5.9. Colloids can be classified into eight types depending upon the physical state of the dispersed phase and the dispersion medium.
Que 5.10. Discuss the effect of pressure and temperature on the adsorption of gases on solids.
Ans 5.10. (i) Adsorption decreases with an increase in temperature since it is an exothermic process by applying Le Chatelier’s principle the reaction will proceed in the backward direction with an increase in temperature.
(ii) At a constant temperature, adsorption increases with an increase in pressure.
Surface Chemistry Class 12 NCERT Solutions
Que 5.11. What are lyophilic and lyophobic sols? Give one example of each type. Why are hydrophobic sols easily coagulated?
Ans 5.11. Lyophilic Sols: The colloidal solution in which the particles of the dispersed phase have a great affinity or love for the dispersion medium, are called lyophilic sols. Such solutions are easily formed by mixing dispersed phase with dispersion medium. These sols are reversible in nature i.e., if two constituents of the sol are separated by any means.
Lyophobic Sols: The colloidal solutions in which the particles of the dispersed phase have no affinity or love, rather have repulsion for the dispersion medium, are called lyophobic sols. These sols are irreversible in nature. For example sols of metals. For example, The solutions of metals like Ag and Au, hydroxides like Al(OH)3 and Fe(OH)3, and metal sulphides like As2S3.
If the charge of hydrophobic sols is removed (by the addition of electrolytes), then the particles present in them come closer and form aggregates, leading to precipitation.
Que 5.12. 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 5.12.
Que 5.13. What are enzymes? Write in brief the mechanism of enzyme catalysis.
Ans 5.13. Enzymes are complex nitrogenous organic compounds that act as biological catalysts and increase the rate of cellular processes. Enzymes are also called ‘biochemical catalysts’.
Mechanism of enzyme catalysis:
On the surface of the enzymes, various cavities are present with characteristic shapes. These cavities possess active groups such as −NH2, −COOH, etc. The reactant molecules having a complementary shape fit into the cavities just like a key fits into a lock. This leads to the formation of an activated complex. This complex then decomposes to give the product.
Hence,
Step 1: E + S → ES*
(Activated complex)
Step 2: ES* → E + P
Surface Chemistry Class 12 NCERT Solutions
Que 5.14. How are colloids classified on the basis of
(i) Physical states of components
(ii) Nature of dispersed phase and
(iii) Interaction between the dispersed phase and dispersion medium?
Ans 5.14. (i) On the basis of the physical state of the dispersed phase and dispersion medium colloids are classified as:
(ii) On the basis of the nature of dispersed phase and dispersion medium colloids are classified as:
(a) Aerosol: a liquid as dispersion phase in gas as dispersion medium is a colloidal solution. Example: fog
(b) Alcosol: a solid phase dispersed in alcohol as a dispersion medium is a colloidal solution.
(c) Hydrosol: a solid phase dispersed in water as a dispersion medium is a colloidal solution.
(d) Benzosol: a solid phase dispersed with benzene as dispersion medium is a colloidal solution.
(iii) On the basis of Interaction between the dispersed phase and dispersion medium colloids are classified as: On the basis of the nature of the interaction between the dispersed phase and dispersion medium, the colloids can be classified as lyophilic (solvent attracting) and lyophobic (solvent repelling).
Que 5.15. Explain what is observed
(i) When a beam of light is passed through a colloidal sol.
(ii) An electrolyte, NaCl is added to hydrated ferric oxide sol.
(iii) electric current is passed through a colloidal sol?
Ans 5.15. (i) When a beam of light is passed through colloidal particles, its path becomes clearly visible and is known as the Tyndall effect. It is due to the scattering of light by colloidal particles. The bright cone of the light is called the Tyndall cone.
(ii) When NaCl is added to ferric oxide sol, it 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) The colloidal particles move towards the oppositely charged electrode and this phenomenon is called electrophoresis.
Que 5.16. What are emulsions? What are their different types? Give examples of each type.
Ans 5.16. The colloidal solution in which both the dispersed phase and dispersion medium are liquids is called an emulsion. There are two types of emulsions:
(a) Oil in water type: In this type, oil is the dispersed phase while water is the dispersion medium. For example milk, vanishing cream, etc.
(b) Water in oil type: In this type, water is the dispersed phase while oil is the dispersion medium. For example cold cream, butter, etc.
Que 5.17. How do emulsifiers stabilise emulsion? Name two emulsifiers.
Ans 5.17. The substances thus added to stabilise the emulsions are called emulsifiers or emulsifying agents. The substances commonly used as emulsifying agents are soaps of various kinds, long-chain sulphonic acids.
Que 5.18. The action of soap is due to emulsification and micelle formation. Comment.
Ans 5.18. The cleansing action of soap is due to emulsification and micelle formation. Soaps are basically sodium and potassium salts of long-chain fatty acids, R-COO-Na+. The end of the molecule to which the sodium is attached is polar in nature, while the alkyl-end is non-polar. Thus, a soap molecule contains a hydrophilic (polar) and a hydrophobic (nonpolar) part.
When soap is added to water containing dirt, the soap molecules surround the dirt particles in such a manner that their hydrophobic parts get attached to the dirt molecule and the hydrophilic parts point away from the dirt molecule. This is known as micelle formation. Thus, we can say that the polar group dissolves in water while the non-polar group dissolves in the dirt particle. Now, as these micelles are negatively charged, they do not combine and a stable emulsion is formed.
Surface Chemistry Class 12 NCERT Solutions
