5. SURFACE CHEMISTRY
Q. 1. What is activation energy ? How is the rate constant of a reaction related to its activation energy ?
Ans. The minimum extra amount of energy absorbed by reactant molecules so that their energy become equal to threshold value is called activation energy. In another words we can say that “Activation energy is the difference between the threshold energy and the average energy of the reactant molecule.
i.e. Activation energy = Total energy – Average energy of the reactant.
Q. 2. What is the difference between physisorption and chemisorption ?
Ans. Physisorption : When a gas is held (adsorbed) on the surface of a solid by van der Waal’s forces (which are weak intermolecular forces of attraction) without resulting into the formation of any chemical bond between the adsorbate and the adsorbent, it is called ‘physical adsorption’ or ‘van der Waal’s adsorption’ or ‘Physisorption’. It is characterised by (i) low heats of adsorption (20-40 kJ mol-1), (ii) the process is reversible, (iii) no activation energy is required, (iv) proceeds at low temperature and decreases with increases in temperature, (v) not specific in nature, (vi) forms multimolecular layers.
Chemisorption : Chemisorption on the other hand, is a phenomenon where the gas is held on to the surface of a solid by forces similar to those of a chemical bond. This adsorption results in the formation of what is called surface compound. Itis characterised by :
(i) High heats of adsorption (40-400 kJ mol-1), (ii) The process is irreversible, (iii) It requires activation energy, (iv) This type of adsorption first increases with increase in temperature, (v) It is specific in nature, (vi) It forms unimolecular layer.
Q. 3. Fluorine shows only one oxidation state, but rest of the halogens show different oxidation states.
Ans. Electronic configuration of fluorine is 2p2x , 2p2y,2p1z. It has only one orbital that cannot be paired highly electronegative. It shows only-1 oxidation state as it saturates its valence shell by accepting one electron.
In all other halogen s and p-orbital can be unpaired and the electrons are accommodated in d-orbitals i.e., showing + 3, + 5 and + 7 oxidation states.
Q. 4. What are the factors which influence the adsorption of a gas on a solid ?
Ans. The factors which influence the adsorption of a gas on a solid are :
(1) The nature of a gas adsorbate and nature of solid adsorbent, (2) Surface area of adsorbent, (3) Pressure of gas, (4) Temperature. (5) Activation of adsorbent.
Q. 5. Explain the terms with suitable examples (i) Alcosol (ii) Aerosol and (iii) Hydrosol.
Ans. (i) Alcosol : It is a colloidal sol in which the dispersion medium is alcohal. Example : Certain polymeric sol.
(ii) Aerosol : If the dispersion medium is gas the sol is called aerosol. Examples-smoke, Dusk, etc.
(iii) Hydrosol : When the dispersion medium is water, the sol is called hydrosol. Examples-Paints, cell fluids, etc.
Q. 6. (a) What is gel ? Give an example.
(b) How is gold sol prepared by reduction method ?
Ans. (a) Gel is a colloidal solution of liquid in a solid i.e., liquid is dispersed phase and solid is the dispersion medium. A gel is semi-solid which melts on heating. Examples of gels are : thin jellies, gum arobic, processed cheese, silicic acid.
(b) It is prepared by reduction of gold chloride (AuCl3) solution with stannous chloride
2AuCl3 + 3SnCl2 → 3SnCl4 + 2Au
(Violet gold sol.)
Q. 7. Copper can be extracted by hydro metallurgy but not zinc. Explain.
Ans. Hydro metallurgy method is based on the fact that more electropositive metals displace less electro positive metals from their salt solution. Copper is precipitated from their salt solution. Copper is precipitated from copper sulphate solution by adding iron.
CuSO4 + Fe → Cu↓
But Zinc is not precipitated from zinc sulphate solution by adding iron. Hence copper can be extracted by hydro metallurgy but no zinc.
Q. 8. What happens when freshly precipitated ferric hydroxide is treated with ferric chloride ?
Ans. Peptisation takes place and a positively charged colloidal sol of ferric hydroxide is obtained.
Q. 9. What is the range of particle size in colloidal sol in nm ?
Ans. 1- 100 nm.
Q. 10. What type of substances form lyophobic sols ?
Ans. Substances such as metals, their suphides etc. which do not mix directly with the dispersion medium to form a colloidal sol.
Q. 11. How will you prepare colloidal solution of gold ?
Ans. By Bredig’s arc process or by reduction of Aucl, with SnCl2.
2AuCl3 + 3SnCl2 → 2Au + 3SnCl4
Q. 12. What is Occlusion ?
Ans. The adsorption of gases on the surface of metals is called occlusion.
Q. 13. How does chemical adsorption of a gas on a solid vary with temperature ?
Ans. Chemical adsorption first increases and then decreases with increase of temperature.
Q. 14. Write down the heterogeneous catalyst involved in the polymerisation of ethylene.
Ans. TiCl4 + triaklyl aluminium (Zeigler-Natta Catalyst).
Q. 15. What happens when gelatin is added to gold sol ?
Ans. Gold sol which is lyophobic sol starts behaving like a lyophilic colloid when gelatin is added to it.
Q. 16. How will you obtain a colloidal sol of arsenious sulphide ?
Ans. By passing H2S through arsenious oxide solution.
Al2O3 + 3H2S → Ar2S3 + 3H2O
Q. 17. What happens when an electric field is applied to a colloidal dispersion ?
Ans. Colloidal particles move towards the oppositvely charged electrode, get neutralised and coagulated (electrophoresis takes place).
Q. 18. What is ZSM-5 ? What is its formula ?
Ans. ZSM-5 is a xeolite sieve of molecular porosity 5. Its formula is HX[(AlO2)X (SiO2)96-X]. 16H2O.
Q. 19. Which will absorb more gas, a lamp of charcol or its powder and why ?
Ans. Powdered charcoal will adsorb more gas because it has more surface area than a lamp of charcoal.
Q. 20. How can we remove moisture form glass apparatus ?
Ans. Anyhydrous CaCl2 when placed in the glass apparatus will absorb moisture from it.
Q. 21. What is the main cause of charge on a colloidal sol ?
Ans. It is due to the adsorption of common ions in the electrolyte on the surface of the colloidal particles, e.g., Fe3+ ions from FeCl3 on the surface of Fe(OH)3 particles.
Q. 22. Give an expression of Freundlich isotherms.
Ans. 
= Kp1/n or log x = log k + 
log P
where m = mass of adsorbent; x = mass of absorbate; p = gas pressure, k = constant and n is an integer.
Q. 23. What do x and m represent in the following expression ?
Ans. = kp1/n
x = mass of adsorbate, m = mass of adsorbent
Q. 24. Name two industrial processes in which heterogeneous catalysts are employed.
Ans. Haber’s process for the manufacture of ammonia (Fe-catalyst] and contact process for manufacture of H2SO4[V2O2 -Catalyst].
Q. 25. To which colloidal system does milk belong ?
Ans. Liquid in liquid or oil-in-water type (emulsion).
Q. 26. Why do colloidal solutions exihibit Tyndall effect ?
Ans. It is due to the size of colloidal particles (10° – 1000 A°) That they can scatter light.
Q. 27. Indicate a chemical reaction involving a homogeneous catalyst.
Ans. 2 SO2 (g) + O2 (g) 
2SO3 (g)
Q. 28. What happens when a beam of light is passed through As2S3 sol ?
Ans. The path of light becomes visible because of Tyndall effect.
Q. 29. How can we make dialysis fast ?
Ans. By applying electric field across the parchment bag.
Q. 30. Why are powdered substances more effective adsorbents than their crystalline forms ?
Ans. The extent of adsorption increases with the increase of surface area of the adsorbent. Thus finely divided metals and porous substances having large surface area are good adsorbents. The surface area of the crystalline substances is less than their powdered forms.
Q. 31. 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 electrolyte in it. On presistent dialysis, the electrolyte is completely removed. As a result, the colloidal sol becomes unstable and gets coagulated.
Q. 32. What is the difference in the nature of dilute soap solution and a concentrated soap solution ?
Ans. Dilute soap solution behaves like a true solution and a concentrated soap solution behaves like a colloidal sol.
Q. 33. Why are lyophilic sols more stable than Lyophobic colloidal sols ?
Ans. Lyophilic sols are highly hydrated due to adsorption of water molecules as they are solvent attracting. Lyophobic sols, on the other hand, hate the dispersion medium and are thus less stable.
Q. 34. How will you distinguish between an oil in-water type and water-in-oil type of emulsion ?
Ans. A small amount of an oil-soluble dye is added to the emulsion. If it is water-in-oil type of emulsion, it becomes deeply coloured, otherwise it remains colourless.
Q. 35. What do you mean by coagulation of flocculation value ?
Ans. The minimum amount of an electrolyte (millimoles) that must be added to one litre of a colloidal sol so as to bring about complete coagulation or flocculation is called the coagulation or flocculation value of an electrolyte.
Q. 36. Which will be adsorbed more readily on the surface of charcoal and why – NH3 or CO2 ?
Ans. NH3 has higher critical temperature than that of CO2, i.e., NH3 gas is more easily liquefiable than CO2 Hence NH3 has greater intermolecular forces of attraction and hence will be adsorbed more readily.
Q. 37. How is adsorption of a gas related to its critical temperature ?
Ans. Higher is the critical temperature of a gas, greater is the case of liquifaction, i.e., greater are the Van der Waal’s forces of attraction and hence greater is the adsorption.
Q. 38. In case of chemisorption, why adsorption first increases and then decreases ?
Ans. In the case of chemisorption the head supplied in the beginning acts as activation energy required for chemisorption. The decrease of adsorption afterwards is due to exothermic nature of adsorption equlibrium.
Q. 49. Why silica gel is used as a dehumidizer ?
Ans. Silica gel has strong adsorbing power for humidity, i.e., moisture present in air. Hence it is used as a dehumidizer.
Q. 49. What type of colloidal sols are formed in the following ? (i) Sulphur vapour 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 from a colloidal sol.
(ii) Macromolecular because protein molecules present in the white of the egg are macromolecules soluble in water.
(iii) Associated because RCOO- ions associate together to form micelles.
Q. 41. Which one of the following electrolytes is most effective for the coagulation of Fe (OH)3 sol and why ?
Ans. Fe(OH)3 is a positively charged sol. According to Hardy Suchlze rule, greater the charge on the oppositely charged ion of the elecrtolyte added, more effective it is in bringing about coagulation. Hence Na3PO4 (containing PO43- ions) is most effective.
Q. 42. How do size of the particles of the adsorbant pressure of gas and prevailing temperature influence the extent of adsorption of a gas on a solid ?
Ans. (i) Smaller the sixe of the particles of the adsorbent, greater is the surface area, greater is the adsorption.
(ii) At constant temperature, adsorption first increases with increase in pressure and then attains equilibrium at high pressure.
(iii) In physical adsorption, it decreases with increase of temperature, but in chemisorption, first it increases and then decreases.
Class 12th Chemistry Short Type Question English
