CBSE Class 12 Important Questions Chapter Surface Chemistry

CBSE Class 12 Important Questions

1. What is Kraft temperature?
Ans 1. The specific temperature at which micelles formation takes place is called Kraft temperature.

2. Explain important characteristics of enzyme catalysis.
Ans 2. Important characteristics of enzyme catalysis are
(i) Highly specific in nature.
(ii) Highly active under optimum temperature.
(iii) In presence of co-enzymes activity gets increased.
(iv) Being colloidal in nature, efficiencies decrease in presence of electrolytes.
(v) Favourable range of pH is 5 to 7.

3. Explain critical micelles concentration.
Ans 3. The particular concentration above which true solution behaves as the colloidal solution is called critical micelles concentration or the concentration at which micelles formation takes place is called CMC.

4. Explain shape-selective catalysis by taking an example of zeolites.
Ans 4. The reactions which depend upon the size and shape of reactant and product molecules as well as upon the pores and cavities of catalysts are called shape-selective catalysis. Zeolites are shape-selective catalysts. These have honeycomb-like structures. The reaction catalysed by zeolites proceeds in a specific manner depending upon the size of the reactants and the molecules of the product compared to that of the pores of zeolites. Zeolites are used in petrochemical industries for the cracking of hydrocarbons and for isomerization. An important zeolite catalyst is ZSM-5 which converts alcohols directly into gasoline by dehydration.

5. What are lyophilic and lyophobic sols? Give one example of each type. Why are hydrophobic sols easily coagulated?
Ans 5. (i) Lyophilic sols: Colloidal sols that are formed by mixing substances such as gum, gelatin, starch, etc. with a suitable liquid (dispersion medium) are called lyophilic sols. These sols are reversible in nature i.e., if two constituents of the sol are separated by any means (such as evaporation), then the sol can be prepared again by simply mixing the dispersion medium with the dispersion phase and shaking the mixture.
(ii) Lyophobic sols: When substances such as metals and their sulphides etc. are mixed with the dispersion medium, they do not form colloidal sols. Their colloidal sols can be prepared only by special methods. Such sols are called lyophobic sols. These sols are irreversible in nature. For example sols of metals. Now, the stability of hydrophilic sols depends on two things- the presence of a charge and the salvation of colloidal particles. On the other hand, the stability of hydrophobic sols is only because of the presence of a charge. Therefore, the latter is much less stable than the former. 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.

6. 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 6. (i) In multi-molecular colloids, the colloidal particles are an aggregate of atoms or small molecules with a diameter of less than 1 nm. The molecules in the aggregate are held together by van der Waal’s forces of attraction. Examples of such colloids include gold sol and sulphur sol.
(ii) In macro-molecular colloids, the colloidal particles are large molecules having colloidal dimensions. These particles have a high molecular mass. When these particles are dissolved in a liquid, sol is obtained. For example starch, nylon, cellulose, etc.
(iii) Certain substances tend to behave like normal electrolytes at lower concentrations. However, at higher concentrations, these substances behave as colloidal solutions due to the formation of aggregated particles. Such colloids are called aggregated colloids.

7. The action of soap is due to emulsification and micelle formation. Comment.
Ans 7. The cleansing action of soap is due to emulsification and micelle formation. Soaps are basically sodium and potassium salts of long-chain fatty acids, RCOONa+. 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 (non-polar) 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 coalesce and a stable emulsion is formed.

8. What do you mean by activity and selectivity of catalysts?
Ans 8. (a) Activity of a catalyst: The activity of a catalyst is its ability to increase the rate of a particular reaction. Chemisorption is the main factor in deciding the activity of a catalyst. The adsorption of reactants on the catalyst surface should be neither too strong nor too weak. It should just be strong enough to make the catalyst active.
(b) Selectivity of the catalyst: The ability of the catalyst to direct a reaction to yield a particular product is referred to as the selectivity of the catalyst. For example, by using different catalysts, we can get different products for the reaction between H2 and CO.
CO(g) + H2 (g) —Ni→ CH4 + H2O

9. Why is adsorption always exothermic?
Ans 9. When a gas is adsorbed on the surface of the solid, its entropy decreases, i.e., is negative.
Now, ΔG = ΔH – TΔS and for the process to be spontaneous, free energy change, ΔG must be negative. As TΔS is negative, ΔG can be negative only if ΔH has a sufficiently high negative value. Hence, adsorption is always exothermic.

10. State Hardy Schulze rule. Give one example.
Ans 10. According to Hardy Schulze rule, the greater the valence of the flocculating ion added, the greater is its power to cause precipitation. For example, flocculating power of PO43– is greater than that of Cl.

CBSE Class 12 Important Questions

11. If in two beakers ‘A’ and ‘B’ each containing ferric hydroxide sol; Na3PO4 and KCl are added respectively what will you observe?
Ans 11. Ferric hydroxide sol is positively charged sol and hence, coagulated by adding negatively charged ions PO43– and Cl. According to Hardy-Schulze law coagulating action of PO43– is greater than Cl so in beaker ‘A’ coagulation will take place first.

12. Describe electrophoresis briefly.
Ans 12.  When an electric potential is applied across two platinum electrodes dipped in colloidal solution, the colloidal particles move towards one of the electrodes. The movement of colloidal particles under an electric field is called electrophoresis.

13. Give a reason for the formation of the delta.
Ans 13. River water is a colloidal solution of clay, Seawater contains a number of electrolytes. When river water meets seawater, the electrolytes. When river water meets seawater. The electrolytes present in seawater coagulate the colloidal solution of clay resulting in the deposition with the formation of deltas.

14. How do lyophilic colloids protect lyophobic colloids?
Ans 14. When a lyophilic sol is added to the lyophobic sol, the lyophilic particles form a layer around the lyophobic particles and thus lyophilic colloids protect lyophobic colloids from electrolytes and so from being coagulated.

15. What do you mean by coagulating ion? Mention its function.
Ans 15. The ion responsible for the neutralisation of charge on the colloidal particles is called coagulating ion. The function of coagulating ions is to coagulate colloids. Actually when an excess electrolyte is added the colloidal particles interact with ions of electrolyte i.e., coagulating ions. These coagulating ions carry opposite charge to that present on colloidal particles and this causes neutralisation and is followed by coagulation.

CBSE Class 12 Important Questions

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