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Surface chemistry III (b)
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Types of catalysis

Shaillee Kaushal is teaching live on Unacademy Plus

Shaillee Kaushal
Faculty in Chemistry with a teaching experience of 14 years. Specialised in teaching for boards and competitions.

Unacademy user
  1. 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. 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. 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

  4. 4. Induced catalysis When one reaction influences the rate of other reaction, which does not occur under ordinary conditions, the phenomenon is called induced catalysis. Examp es -> Sodium arsenite solution is not oxidized by air. However, if air is passed through a mixture of the solution of sodium arsenite and sodium sulphate both of them undergo simultaneous oxidation. The oxidation of sodiu sulphate thus induces the oxidation of sodium arsenite. Na2S03 0Na2SO4