March 23, 2012

Surface Chemistry Three Marks

1. Give any three differences between physical adsorption and chemical adsorption.
Physical adsorption
Chemical adsorption
It is due to intermolecular Vander waal’s force.
It is due to chemical bond formation.
Depends on the nature of gas. Easily liquefiable gases are adsorbed readily.
More specific than the physical adsorption.
Heat of adsorption is small.
Heat of adsorption is large.
If occurs rapidly at low temperature and decreases with increase of temperature.
Increases with increase of temperature.
Increase of pressure increases adsorption.
Change of pressure has no effect.
Forms multimolecular layers on adsorbent surface.
Forms unimolecular layer.
2. What are emulsions?
Emulsions are liquid - liquid colloidal systems, i.e., the dispersion of finely divided droplets in another liquid.
Or If a mixture of two immiscible or partially miscible liquids is shaken, a coarse dispersion of one liquid in the other is obtained which is called emulsion.
Milk, Cream
3. What are lyophilic sols? Give example.
Colloidal solutions in which the dispersed phase has considerable affinity for the dispersion medium are called lyophilic (solvent loving) colloids.
Gelatin, protein and starch
4. What are promoters? Give an example.
A substance which, though itself not a catalyst, promotes the activity of a catalyst is called a promoter.
1. In the Haber’s process for the synthesis of ammonia, traces of molybdenum increase the activity of finely divided iron which acts as a catalyst.
N2 + 3H2  2NH3
                           + Mo

2. In the Bosch process for the manufacture of hydrogen, Chromic oxide acts as a promoter for the catalyst ferric oxide (Fe2O3).
5. What is Brownian movement? Give reason.
The continuous rapid zig-zag chaotic random and ceaseless movement executed by a colloidal particle in the dispersion medium is called brownian movement.
Unbalanced bombardment of the colloidal particles by the molecules of the dispersion medium.
6. What is Catalytic Poison? Give an example.
A substance which destroys the activity of the catalyst is called a poison.
The platinum catalyst used in the oxidation of SO2 in contact process is poisoned by arsenious oxide, As2O3.
SO2 + O2  2SO3
Poisoned by As2O3
7. What is electrodialysis?
In this process, dialysis is carried under the influence of electric field. Potential is applied between the metal screens supporting the membranes. This speeds up the migration of ions to the opposite electrode. Hence dialysis is greatly accelerated.
Evidently electrodialysis is not meant for non-electrolyte impurities like sugar and urea.
8. What is electrophoresis?
The movement of sol particles under an applied electric potential is called electrophoresis.
9. What is heterogeneous catalysis? Give an example
The catalytic process in which the reactants and the catalyst are in different phases is known as heterogeneous catalysis.
Oxidation of SO2 to SO3 in the presence of Pt metal or V2O5 as catalyst in the contact process for the manufacture of sulphuric acid.
2SO2(g) + O2(g)  2SO3
The reactants are in gaseous phase while the catalyst is in solid phase.
10. What is (meant by) Tyndall effect?
The scattering of light by the sol particles is called Tyndall effect.
                        Sol particles absorb light energy and then emit it in all directions.
11. What is peptisation? Give an example.
The dispersion of a precipitated material into colloidal solution by the action of an electrolyte in solution is termed as peptisation. The electrolyte used is called a peptizing agent.
Precipitate + Electrolyte (Peptizing agent) Colloidal solution (Sol)
1. Silver chloride can be converted into a sol by adding Hydrochloric acid

HCl H+ + Cl  ; AgCl + Cl AgCl | Cl  ( Vely charged Sol particle)

2. Ferric hydroxide yields a sol by adding Ferric chloride 
FeCl3 Fe3+ + 3Cl  ; Fe(OH)3 + Fe3+ Fe(OH)3 | Fe3+ (+ Vely charged Sol particle)
12. What is tanning?
Animal hides are colloidal in nature. When a hide, which has positively charged particles, is soaked in tannin, which contains negatively charged colloidal particles, mutual coagulation takes place, which results in the hardening of leather. The process is termed as tanning.
Chromium salts have been used in place of tannin.
13. Why colloidal system of gas in gas does not exist?
Colloidal system of gas in gas does not exist because gases are completely miscible and always form true solutions.
14. Write a note on auto-catalyst.
When one of the products formed during the reaction acts as a catalyst for that reaction. Such type of catalyst is called auto catalyst and the phenomenon is known as auto catalysis.
In the oxidation of oxalic acid by potassium permanganate, one of the products MnSO4 acts as a auto-catalyst because it increases the speed of the reaction.
5 |          + 2KMnO4 + 3H2SO4  2MnSO4 + K2SO4 + 10CO2 + 8H2O
15. Write any three general characteristics of catalytic reaction.
1. The catalyst remains unchanged in mass and in chemical composition at the end of the reaction.
2. Only a small quantity of catalyst is generally needed.
3. A catalyst cannot initiate a reaction. The function of a catalyst is only to alter the speed of the reaction which is already occurring at a particular rate.
4. A catalyst does not alter the position of equilibrium in a reversible reaction.
5. The catalyst is generally specific in its action. 
16. Define colloidal solution

The colloidal solutions are intermediate between true solutions and suspensions. When the diameter of the particles of a substance dispersed in a solvent ranges from about 10A° to 2000A°, the system is termed a colloidal solution.
17. How is Delta formed?
River water is colloidal solution of clay. Sea water contains a number of electrolytes. When river water meets the sea water, the electrolytes present in sea water coagulate the colloidal solution of clay which get deposited with the formation of delta.
18. What is catalysis? Give an example
A catalyst is a substance which alters the speed of a chemical reaction without itself undergoing any chemical change and the phenomenon is known as catalysis.
2KClO3 →2KCl + 3O2
In the above reaction, MnO2 acts as a catalyst
19. What are Active centres?
The catalytic surface has unbalanced chemical bonds on it. The reactant gaseous molecules are adsorbed on the surface by these free bonds. This accelerates the rate of the reaction. The distribution of free bonds on the catalytic surface is not uniform. These are crowded at the peaks, cracks and corners of the catalyst. The catalytic activity due to adsorption of reacting
molecules is maximum at these spots. These are, therefore, referred to as the active centres.
20. What are Lyophobic colloids? Give an example.
Colloidal solutions in which the dispersed phase has very little affinity for the dispersion medium are termed as lyophobic (solvent hating) colloids.
Colloidal solutions of metals, sulphur in water
21. Give the dispersed phase and dispersion medium for gel. Give an example for gel
Dispersed Phase - Liquid
Dispersion Medium – Solid
Example - Curd, cheese


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