Catalyst

Chemistry

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Catalyst

A substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change. Hydrogen peroxide will decompose into water and oxygen gas. Two molecules of hydrogen peroxide will produce two molecules of water and one molecule of oxygen. A catalyst of potassium permanganate can be used to speed up this process.

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Types of catalysis

Two types of catalysis:
1) Homogeneous catalysis: When the reactant and catalyst are in the same phase.
2) Heterogeneous catalysis: When the reactant and the catalyst are in different phase.

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Catalytic promotors and Catalytic poisons

Substances which themselves are not catalysts, but when mixed in small quantities with the catalysts increase their efficiency are called as promoters or activators.For example, in Haber's process for the synthesis of ammonia, traces of molybdenum increases the activity of finely divided iron which acts as a catalyst. Catalytic poisons : Substances which destroy the activity of the catalyst by their presence are known as catalytic poisons. For example, the presence of traces of arsenious oxide  in the reacting gases reduces the activity of platinized asbestos which is used as catalyst in contact process for the manufacture of sulphuric acid.

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Types of catalysis

Homogeneous catalysis  are catalytic reactions where the catalyst is in the same phase as the reactants, so homogeneous catalysis applies to reactions in the gas phase and even in a solid. Heterogeneous catalysis refers to the form of catalysis where the phase of the catalyst differs from that of the reactants. Phase here refers not only to solid, liquid or gas, but also immiscible liquids, e.g. oil and water.

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Homogeneous catalysis

Homogeneous catalysis is a type of catalysis in which the catalyst  and the reactant operates in the same phase.
Example:  

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Heterogeneous and surface catalysis

This involves the use of a catalyst in a different phase from the reactants. Typical examples involve a solid catalyst with the reactants as either liquids or gases.

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Important features of solid catalyst

ActivityCatalyst which are used to increase the speed of the reaction.
Example : 
Selectivity : Catalyst which are used to form a particular product in a reaction.
Example :

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Chracteristics of a catalyst and a catalysed reaction

 The catalyst remains unchanged (in mass and chemical composition ) in the reaction (Activity of catalyst).  A small quantity of the catalyst is required. e.g. One mole of colloid Pt catalysis. The catalyst does not change the equilibrium constant. But the equilibrium approaches earlier.

Characteristic of catalytic reaction : Catalysis reactions are catalyst dependent.