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XII -05 SURFACE CHEMISTRY -11I
Adsorption form solution phase:- Solids can adsorb solutes from solutions also When a solution of acetic acid in water is shaken with charcoal, a part of the ac adsorbed by the charcoal and the concentration of the acid decreases in the solution The following observations have been made in the case of adsorption from solution phase: (i) The extent of adsorption decreases with the increase of temperature (ii) The extent of adsorption increases with the increase of surface area of the adsorbent (ili) The extent of adsorption depends on the concentration of the solute in solution (iv) The extent of adsorption depends upon the nature of the adsorbent and the adsorbate The extent of adsorption xkC1/ On taking log on both sides: log = logk + log c m n
Adsorption Isobars and isostere A graph between degree of adsorption (n/m) and temperature '' at a constant pressure of adsorbate gas is known as adsorption isobar. P constart P constart E 2
Adsorption Isostere: Degree of adsorption depends on temperature as well as an pressure. When temperature increases, the extent of adsorptio decreases. >The plot of temperature versus pressure for a give amount of adsorption is called adsorption isostere.
Application of Adsorption: 1. Production of high vaccunm 2. Gas masks: 3. Humidity control: 4. Removal of colouring matter from solution 5. Heterogenous catalysis: 6. Separation of inert gases: Apart from the above applications adsorption is also used in the following: (i) In curing diseases (ii) Cleaning agents (ii) Froth floatation process (iv) Chromatographic analysis, (v) kinetics
Catalysis The rates of a number of reactions can be altered by adding a foreign substances. The systematic study of the effect of various foreign substance on the rates of reaction has studied by Berzilius. Such substances are called catalysts. They are defined as substances which changes the reaction rates withou affecting the overall energies of the reaction. > The phenomenon of altering the rate of a reaction by catalysts are called catalysis.
Types of Catalysis: 1. Homogenous Catalysis: When the reactants, products and catalysts are all in the same phase, the catalysis is said to b homogenous For example: NO(g) 2S02(g) + 02(g) 2S03 (g) CH3C0OCH3(1) + H20() CH3COOH) + CH30H) CH30H) HCI(I) 2. Heterogenous catalysis: The catalytic process in which the reactant, catalysts and products are in the differ heterogeneous catalysis: For. eg: Pt(s) 2502(g) + O2(g)--2S03(g) N2O + 3H2(g) 2NH3(g) 4NH3 (g) + SO2(g)- 4N0(g) + 6H2O(g) Pt(s)
Types of catalysis:- 1. Positive Catlalysis: When the rate of reaction is accelerated by the foreign substance is said to be a positive catalyst and the phenomenon as positive cafalysis Example Pt 2H202 2H20 + 02 2KC Og 2KCI + 302 2S02 + 02 2S03 Positive catalyst increase the rate by lowering activation energy of reaction. 2700C V205 Catalyst changes the mechanism by changing the intermediate i.e on intermediate of low energy is formed.
2. Negative Catalysis: There are certain substances which when added to the reaction mixture retard the reaction rate instead of increasing it. These are called negative catalysts or inhibitors and the phenomenon is called negative catalyst Examples: (i) The oxidation of Na2SO3 by air is retarded by alcohol. (ii) The oxidation of benzaldehyde is retarded if some diphenyl amine is added. Alchol 2c,HsCHO+2ipheny asCOOH
3. Autocatalysis: In certain reactions, one of the product act as catalyst. Initially the reaction is slow but as soon as the products are formed, the reaction rate increases due to the action of one of the produet as catalyst. This phenomenon is called autocatalysis. Examples of Autocatalysis: > The rate of oxidation of oxalic acid by acidified potassium permanganat increases on the reaction progresses. This is due to presence of Mn2+ ions act as auto catalyst 5H2 C204 2KMnO4 +3H2SO42MnSO4 + K2S04 + 10C02 + 8H20 When nitric acid is poured on copper, the reaction is very slow in the beginning gradually the reaction becomes faster due to formation of ions during the reaction which acts as an auto catalyst