Surface chemistry chapter class 12th solved problems


[I.] Adsorption . |

Q 1. What is the difference when a concentrated solution of KCI is shaken with blood charcoal in one case and a dilute solution of KCI in the second case ?

Ans- With concentrated solution, there is positive adsorption but with dilute solution, there is negative adsorption. 

Q 2. In which of the following does adsorption take place and why ? 
1. Silica gel placed in the atmosphere saturated with water.  
2. Anhydrous CaCl, placed in the atmosphere saturated with water. 

Ans. silica gel is a good adsorbent and hence water vapour are adsorbed on its surface. Anhydrous CaCl, undergoes absorption because it combines with water molecules to form hydrated calcium chloride,  CaCl, . 2 H,0.  

 Q 3.How is adsorption of a gas related to its critical temperature ? 

Ans.  Higher is the critical temperature of a gas, greater is the ease of liquefaction, i.e., greater are the van der waals force of attraction and hence greater is the adsorption.

Q 4. Adsor ption of a gas on the surface of solid is generally accompanied by decrease in entropy. Still it  is a spontaneous process. Explain. 

Ans.- Adsorption is an exothermic process, 1.e., energy factor favours the process. AS AG =AH-—TAS, in ,adsorption, though AS is —ve but AH is also—ve and AH>TAS in magnitude so that AG is —ve. Hence, the process is spontaneous.

5. Which will be adsorbed more readily on the surface of charcoal and why —NH, or CO, ?

Ans.- NH; has higher critical temperature than that of CO,, i.e., NH; is more easily liquefiable than CO). Hence, NH; has greater intermolecular forces of attraction and hence will be adsorbed more readily.

6. How do size of particles of adsorbent, pressure of gas and prevailing temperature influence the extent of adsorption of a gas on a solid ?

Ans. i) Smaller the size of the particles of the adsorbent, greater is the surface area, greater is the adsorption.

(ii) At constant temperature, adsorption first increases with increase of pressure and then attains equilibrium at high pressures.

(iii) In physical adsorption, it decreases with increase of temperature but in chemisorption, first it increases and then decreases.

7. How can the constants k and n of the Freundlich adsorption equation be calculated ?

Ans. According to Freundlich equation, “=k P!". Therefore, log — =log k + log P. Thus, on plotting log ~ vessus log P, we will get a straight line whose slope = - and intercept on y-axis = log k. Hence, measuring the slope and the intercept, k and n can be calculated.

8 What form Freundlich adsorption isotherm equation will take at high pressures ?

Ans. x\m=k, ie., it becomes independent of pressure at constant temperature.

9. In case of chemisorption, why adsorption first increases and then decreases ?

Ans. The initial increase is due to the fact that heat supplied acts as a activation energy. The decrease afterwards is due to the exothermic nature of adsorption equilibrium.

10. Why silica gel is used as a dehumidizer ?  

‘Ans. Silica gel has stronger adsorption for humidity, 7.e., moisture of the air. Hence, it is used as a dehumidizer.

11. ‘What is the difference in the nature of a dilute soap concentrated soap solution  and a concentration soap solution ?

Ans.- Dilute soap solution behaves like a true solution whereas concentrated soap solution behaves like a collidal solution.


12. What is the difference between a colloidal  sol, gel and emulsion ?

Ans. In a colloidal sol, the dispersed phase is a solid and the dispersion medium is liquids . in agel it is opposite . In an emulsion , both the dispersed phase and dispersed medium are liquids. 

13. Why lyophilic colloidal sols are more stable than lyophobic collidal sols ?

Ans. This is because lyophilic sols are highly hydrated in the solution .

14. What type of colloidal sols are formed in the following -

 (i) Sulphur vapours are passed through cooled water.

(ii)White of an egg is mixed with water.

(iii)Soap solution. 

Ans.- (i) Multimolecular because sulphur molecules associate together to form colloidal sol. 

(ii) Macromolecular because protein molecules present in the white of the egg are macrommolecules soluble in water. 

(iii) Associated because RCOO™ ions associate together to form micelles. 

15. What is common in aqua sols and solid aerosols ? How do they differ ? | :

Ans. Aquasol and solid aerosol both have solid as the dispersed phase. They differ in dispersion medium. Aquasols have water as the dispersion medium while aerosols have gas as the dispersion medium.

16. What happens when persistent dialysis of a colloidal solution is carried out ?

Ans. The stability of a colloidal sol is due to the presence of a small amount of the electrolyte. On persistent  dialysis, the electrolyte is completely removed. As a result, the colloidal sol becomes unstable and gets coagulated.

17. Frequently, preparation of a colloid such as a protein can be made more stable if the colloid is dialyzed. Why is this so ? 

Ans. This is because dialysis helps in removing undesirable ions from a colloidal preparation which tend todestabilize the colloid.

18. What causes Brownian movement in a colloidal solution ? (CBSE 2008)

Ans. Brownian movement, Le, Zig-Zag movement of the colloidal particles is due to hitting of these particles by the molecules of the dispersion medium with different forces from different directions.

19. Colloidal solutions show Tyndall effect. Give two reasons. (Raj. Board 2012)

  • diameter of the dispersed particles is not much smaller as compared to the wavelength of light used.
  • There 1s a large difference in the refractive indices of the dispersed phase and the dispersion medium.

20. How can a colloidal solution and true solution of the same colour be distinguished from each other ?

Ans. If the solutions are taken in glass broken and light is focussed on them, the collidal solution will scatter light due to tyndall effect and the path of light becomes visible but no scattering of light occurs in true solution and hence the path is not visible. 

21. Why is a colloidal sol stable ?  

Ans. All the particles in a colloidal sol carry the same charge and hence keep on repelling each other and cannot aggregate together to form bigger particles. 

22. What happens when a colloidal sol of Fe(OH), is mixed with that of As3S3?  

Ans. Fe(OH); sol being positively charged whereas AsS3 being negatively charged, their charges are neutralized on mixing. Hence, their mutual precipitation (coagulation) occurs.

23. A collidal solution of Agl is prepared by two different methods as shown in fig  
(i) what is the charge on Agl colloidal particles in the two  tubes (A) and (B).
(ii) Give reason for the origin of charge ?

Ans. (i) In tube A, as AgNO, is added in excess, th Agti G are adsorbed on Agl particles. Hence charge on the  collidal particles is positive. In tube B, as KT is added in excess, common  I ions are adsorbed on the colloidal particles and charge is negative. 

(ii) Reason for origin of charge is the preferential  adsorption of common ions of the electrolyte present in excess on the colloidal  particles.  

24. Which one of the following electrolytes is most effective for the coagulation of Fe(OH), sol and why ?

NaCl, Na,SO4, Na;P0,

Ans. Fe(OH)3 is  a positively charged sol. According to Hardy-Schulze rule, greater the charge on the oppositely charged ion of the electrolyte added, more effective it is in bringing about coagulation. Hence Na3PO4 (containing PO” ions) is most effective.

25. What happens to a gold sol if gelatin is added to it ? (CBSE 2007)

Ans. Gold sol which is lyophobic starts behaving like a lyophilic sol because gelatin acts as a protective colloid.

26. What is the significance of reciprocal of ‘gold number’ ?

Ans. Smaller the gold number, greater is its protective power. Hence, reciprocal of gold number is a direct measure of the protective power of the colloid. 

27. Give specific term to show the effect of the following processes :
(a) Ferric hydroxide is mixed with arsenic sulphide sol
(b) Ferric chloride solution is mixed with freshly prepared precipitate of ferric hydroxide
(c) H,S gas is passed through arsenic oxide solution
(d) A beam of light is passed through arsenic oxide solution. (Raj. Board 2012)

Ans. (a) Coagulation (b) Peptization (c) Double decomposition (d) Tyndall effect

28. Match the pairs (choose the correct answer from Section B for Section A) :

          Section A                                                     Section B  

(a) Gold sol                                                      (i) Hardy-Schulze rule
(b) Gold No.                                                    (ii) van der Waal force
(c) Coagulation power                                  (iii) Electrochemical phenomenon
(d) Physical adsorption                                 (iv) Lyophilic colloid
(e) Corrosion                                                  (v) Lyophobic colloid
                                                                        (vi) Tyndall effect  

Ans. (a) - (v), (b) — (iv), (c) - (i), d- (ii); (e) — (iii).

29. The conductance of an emulsion increases on adding common salt. What type of emulsion is this ?

Ans. Oil-in-water type. 

30. Gelatin is generally added to ice creams. Why ?

Ans. Ice cream is an emulsion of milk or cream in water, i.e., oil-in-water type. Gelatin is added to act as an emulsifier, i.e., it helps to stabilize the emulsion.

31. Why artificial rain can be caused by throwing common salt on the clouds ?

Ans. Clouds are colloidal dispersion of water particles in a The corneal water particles carry charge. On throwing common salt on the clouds, coagulation of water particles takes place as common salt is an electrolyte.

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