Characteristics Of Catalysis
Category : JEE Main & Advanced
The following are the characteristics which are common to must of catalytic reactions.
(1) A catalyst remains unchanged in mass and chemical composition at the end of the reaction.
(2) A small quantity of the catalyst is generally sufficient to catalyses almost unlimited reactions
(i) For example, in the decomposition of hydrogen peroxide, one gram of colloidal platinum can catalyses \[{{10}^{8}}\]litres of hydrogen peroxide.
(ii) In Friedel craft’s reaction, anhydrous aluminium chloride is required in relatively large amount to the extent of 30% of the mass of benzene,
\[{{C}_{6}}{{H}_{6}}+{{C}_{2}}{{H}_{5}}Cl\xrightarrow{AlC{{l}_{3}}}{{C}_{6}}{{H}_{5}}{{C}_{2}}{{H}_{5}}+HCl\]
(3) The catalyst can not initiate the reaction: The function of the catalyst is to alter the speed of the reaction rather than to start it.
(4) The catalyst is generally specific in nature: A substance, which acts as a catalyst for a particular reaction, fails to catalyse the other reaction , different catalysts for the same reactant may for different products.
Examples : 
(5) The catalyst can not change the position of equilibrium : The catalyst catalyse both forward and backward reactions to the same extent in a reversible reaction and thus have no effect on the equilibrium constant.
(6) Catalytic promoters : Substances which themselves are not catalysts, but when mixed in small quantities with the catalysts increase their efficiency are called as promoters or activators.
(i) 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.
(ii) In the manufacture of methyl alcohol from water gas \[(CO+{{H}_{2}})\], chromic oxide \[(C{{r}_{2}}{{O}_{3}})\] is used as a promoter with the catalyst zinc oxide \[(ZnO)\].
(7) Catalytic poisons : Substances which destroy the activity of the catalyst by their presence are known as catalytic poisons.
(i) For example, the presence of traces of arsenious oxide \[(A{{s}_{2}}{{O}_{3}})\] in the reacting gases reduces the activity of platinized asbestos which is used as catalyst in contact process for the manufacture of sulphuric acid.
(ii) The activity of iron catalyst is destroyed by the presence of \[{{H}_{2}}S\] or \[CO\] in the synthesis of ammonia by Haber’s process.
(iii) The platinum catalyst used in the oxidation of hydrogen is poisoned by \[CO\].
(8) Change of temperature alters the rate of catalytic reaction as it does for the same reaction in absence of catalyst : By increasing the temperature, there is an increase in the catalytic power of a catalyst but after a certain temperature its power begins to decrease. A catalyst has thus, a particular temperature at which its catalytic activity is maximum. This temperature is termed as optimum temperature.
(9) A positive catalyst lowers the activation energy
(i) According to the collision theory, a reaction occurs on account of effective collisions between the reacting molecules.
(ii) For effective collision, it is necessary that the molecules must possess a minimum amount of energy known as activation energy (Ea).
(iii) After the collision molecules form an activated complex which dissociate to yield the product molecules.
(iv) The catalyst provides a new pathway involving lower amount of activation energy. Thus,
larger number of effective collisions occur in the presence of a catalyst in comparison to effective collisions at the same temperature in absence of a catalyst. Hence the presence of a catalyst makes the reaction to go faster.
(v) Figure shows that activation energy \[{{E}_{a}}\], in absence of a catalyst is higher than the activation energy Ea, in presence of a catalyst.
(vi) \[{{E}_{R}}\] and \[{{E}_{p}}\] represent the average energies of reactants and products. The difference gives the value of \[\Delta G\], i.e., \[\Delta G={{E}_{R}}-{{E}_{P}}\]
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