Important Questions from - Surface Chemistry | XII CBSE/ISC | Board Exam Preparation

Q1. What is the difference between physisorption and chemisorption?

Ans -

Q2. Give reason why a finely divided substance is more effective as an adsorbent?

Ans. Powdered substances have greater surface area as compared to their crystalline forms. Greater the surface area, greater is the adsorption.

Q3. What are the factors, which influence the adsorption of a gas on a solid?

Ans. The extent of adsorption of a gas on a solid surface depends on following factors: (i) The nature of gas (ii) Surface area of adsorbent (iii) Pressure (iv) Temperature (v) Activation of adsorbent.

Q4. Why is adsorption always exothermic?

Ans. As the adsorption progresses, the residual forces at the surface decreases resulting in the decrease of surface energy which appears as heat.

Q5. 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. Multimolecular colloids: In this type of colloids, colloidal particles are aggregates of atoms or molecules each having size less than 1 nm, e.g., sulphur sol, gold sol. Multimolecular colloids are generally lyophobic in nature.

Macromolecular colloids: In this type of colloids, colloidal particles are themselves large molecules of colloidal dimensions, e.g., starch, proteins, polythene, etc. Macromolecular colloids are generally lyophilic in nature.

Associated colloids: There are certain substances which at low concentrations behave as normal electrolyte, but at higher concentrations exhibit colloidal behaviour due to the formation of aggregates. Such colloids are known as associated colloids, e.g., soaps and detergents.

Q6. Explain what is observed when

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

(ii) An electrolyte, NaCl is added to hydrated ferric oxide sol.

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

Ans - (i) Scattering of light by the colloidal particles takes place and the path of light becomes visible (Tyndall effect).

(ii) The positively charged colloidal particles of Fe(OH)3 get coagulated by the oppositely charged Cl– ions provided by NaCl.

(iii) On passing direct current, colloidal particles move towards the oppositely charged electrode where they lose their charge and get coagulated.

Q7. What are micelles? Give an example of a miceller system.

Ans - The particles of colloidal size formed due to aggregation of several units of soap molecules (surfactants) in a dispersion medium are called ‘micelles’. A concentrated solution of soap in water is a miceller system. Such substances are also called associated colloids. In other words, miceller system (or associated colloid) behaves as a true solution in low concentration form and as a colloid in high concentration form. The micelles revert to individual ions on dilution.

Q8. Explain the following terms with suitable examples (i) Alcosol (ii) Aerosol and (iii) Hydrosol.

Ans - (i) Alcosol: It is a colloidal dispersion having alcohol as the dispersion medium, e.g., collodion.

(ii) Aerosol: It is a colloidal dispersion of a solid or liquid in a gas, e.g., smoke, fog.

(iii) Hydrosol: It is a colloidal sol of a solid in water as the dispersion medium e.g., starch sol.

Q9. What is ‘occlusion’?

Ans - The adsorption of gases on the surface of metals is called occlusion.

Q10. Define desorption.

Ans - The process of removal of an adsorbed substance from a surface on which it is adsorbed is called desorption.

Q11. What type of forces are responsible for the occurrence of physisorption?

Ans - van der Waals’ forces.

Q12. What is meant by chemical adsorption?

Ans - If the adsorbate is held on the surface of the adsorbent as a result of chemical reaction forming surface compounds, it is called chemical adsorption.

Q13. What is the effect of temperature on chemisorption?

Ans - Chemisorption initially increases then decreases with rise in temperature. The initial increase is due to the fact that heat supplied acts as activation energy. The decrease afterwards is due to the exothermic nature of adsorption equilibrium.

Q14. Write one similarity between physisorption and chemisorption.

Ans - Both increase with increase in surface area.

Q15. Adsorption of a gas on the surface of solid is generally accompanied by a decrease in entropy still it is a spontaneous process. Why?

Ans - According to the equation ΔG = ΔH – TΔS For a process to be spontaneous, ΔG should be negative. Even though ΔS is negative here, ΔG is negative because reaction is highly exothermic, i.e., ΔH is negative.

Q16. Which will be adsorbed more readily on the surface of charcoal and why: NH3 or CO2?

Ans - NH3 has higher critical temperature than CO2, i.e., NH3 is more liquefiable than CO2. Hence, NH3 has greater intermolecular forces of attraction and hence will be adsorbed more readily.

Q17. Define colloidal solution.

Ans - A colloidal solution is a state in which the particle size lies between 1 nm and 1000 nm. It appears to be homogeneous but actually it is heterogeneous.

Q18. What is collodion?

Ans - It is a 4% solution of nitrocellulose in a mixture of alcohol and ether.

Q19. Why are some medicines more effective in the colloidal form?

Ans - Medicines are more effective in the colloidal form because of large surface area and are easily assimilated in this form.

Q20. Write the dispersed phase and dispersion medium of butter.

Ans - Dispersed phase : Liquid, Dispersion medium : Solid

Q21. Give one example each of sol and gel

Ans - Sol: Paints, cell fluids

Gel: Butter, cheese

Q22. What is common in aquasols 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

Q23. What are lyophobic colloids? Give one example for them.

Ans - Lyophobic sols are those sols in which the particles of the dispersed phase have little affinity for the particles of the dispersion medium, e.g., sols of metal and their sulphides and hydroxides.

Q24. Give one example each of lyophobic sol and lyophilic sol.

Ans - Lyophobic sol — Gold sol, As2S3 sol

Lyophilic sol — Sol of starch, sol of gum

Q25. What is Kraft temperature?

Ans - Kraft temperature is the minimum temperature above which the formation of micelles takes place.

Q26. What is the principle of dialysis?

Ans - Dialysis is based on the principle that ions can pass through semipermeable membrane whereas colloidal particles cannot pass through it.

Q27. What happens when an electric field is applied to a colloidal dispersion?

Ans - The colloidal particles move towards the oppositely charged electrode and get neutralised and coagulated there.

Q28. What is electrodialysis?

Ans - It is a process by which colloidal solutions containing ionic impurities are purified. The colloidal solution containing ionic impurities is placed in a bag of parchment paper in distilled water in electric field. The ions come out through parchment paper and the sol is purified.

Q29. Define ultrafiltration.

Ans - In this process, colloidal solutions are purified by carrying out filtration through special types of graded filters called ultra-filters. Filter paper allows the passage of electrolyte but does not allow the passage of colloidal particles.

Q30. Why do colloidal solutions exhibit Tyndall effect?

Ans - Colloidal solutions exhibit Tyndall effect because the size of the colloidal particles (1 nm–1000 nm) is such that they can scatter light.

Q31. What causes Brownian movement in a colloidal solution?

Ans - Unbalanced bombardment of the particles of dispersed phase by molecules of dispersion medium causes Brownian motion. This stabilizes the sol.

Q32. What is the main cause of charge on a colloidal solution?

Ans - The charge on the colloidal particles is due to adsorption of common ions of the electrolyte on the surface of the colloidal particles, e.g., Fe3+ from FeCl3 on the surface of Fe(OH)3 particles.

Q33. Give one example of positively charged sol and one example of negatively charged sol.

Ans - Fe(OH)3 is a positively charged sol whereas As2S3 is a negatively charged colloid.

Q34. What causes electrophoresis?

Ans - Electrophoresis is due to charge on colloidal particles, the charged particles move towards one of the electrodes in electric field.

Q35. What is the type of charge on AgI colloidal sol formed when AgNO3 solution is added to KI solution?

Ans - Negatively charged sol, AgI/I– is formed when AgNO3 solution is added to KI solution.

Q36. Out of BaCl2 and KCl, which one is more effective in causing coagulation of a negatively charged colloidal sol? Give reason.

Ans - BaCl2, Ba2+ ion has greater coagulating power than K+ ion as Ba2+ ion has higher charge.

Q38. How can a lyophilic sol be coagulated?

Ans - This can be done (i) by adding an electrolyte, (ii) by adding a suitable solvent.

Q39. How will you distinguish between dispersed phase and dispersion medium in an emulsion?

Ans - On adding dispersion medium, emulsions can be diluted to any extent. The dispersed phase forms a separate layer if added in excess.

Q40. Write two differences between sols and emulsions.

Ans - (i) Sols are dispersions of solids in liquids while emulsions are dispersions of liquids in liquids.

(ii) Sols are quite stable whereas emulsions are less stable.

Q41. How do emulsifying agents stabilise the emulsion?

Ans - The emulsifying agent forms an interfacial layer between suspended particles and the dispersion medium thereby stabilising the emulsion.

Q42. A delta is formed at the meeting point of sea water and river water. Why?

Ans - River water is a colloidal solution of clay and sea water contains a number of electrolytes. When river water meets the sea water, the electrolytes present in the sea water coagulate the colloidal solution of clay resulting in its deposition with the formation of delta.

Q43. Explain the cleansing action of soap. Why do soaps not work in hard water?

Ans - The cleansing action of soap such as sodium stearate is due to the fact that soap molecules form micelle around the oil droplet in such a way that hydrophobic part of the stearate ions is in the oil droplet and hydrophilic part projects out of the grease droplet like the bristles. Since the polar groups can interact with water, the oil droplet surrounded by stearate ions is now pulled in water and removed from the dirty surface. Thus, soap helps in emulsification and washing away of oils and fats. Hard water contains calcium and magnesium salts. In hard water, soap gets precipitated as calcium and magnesium soap which being insoluble stick to the clothes as gummy mass. Therefore, soaps do not work in hard water.

Q44. Define the following terms:

(i) Brownian movement

(ii) Peptization

(iii) Multimolecular colloids

Ans - (i) Brownian movement: The motion of the colloidal particles in a zig-zag path due to unbalanced bombardment by the particles of dispersion medium is called Brownian movement.

(ii) Peptization: The process of converting a precipitate into colloidal sol by shaking it with dispersion medium in the presence of a small amount of suitable electrolyte is called peptization. During peptization, the precipitate absorbs one of the ions of the electrolyte on its surface. This causes development of positive or negative charge on precipitates, which ultimately break up into particles of colloidal dimension.

(iii) Multimolecular colloids: A large number of atoms or smaller molecules (diameter < 1 nm) of a substance on dissolution aggregate together to form species having size in the colloidal range. Such species are called multimolecular colloids. Examples: a sulphur sol consist of particles containing thousands of S8 sulphur molecules, a platinum or gold sol may have particles of various sizes having many atoms.

Q45. Define the following terms giving one suitable example for each:

(i) Electrophoresis (ii) Micelles (iii) Peptization

Ans - (i) The movement of colloidal particles towards oppositely charged electrodes in an electric field is called electrophoresis.

(ii) There are some substances such as soap which at low concentration behave as normal electrolytes, but at higher concentration exhibit colloidal behaviour due to the formation of aggregates. The aggregated particles thus formed are known as micelles or associated colloids.

(iii) The process of converting a precipitate into colloidal solution by shaking it with dispersion medium in the presence of small amount of electrolyte is called peptization.