Theories Of Catalysis
Category : JEE Main & Advanced
Catalytic reactions can be broadly divided into the following types,
(1) Homogeneous catalysis : When the reactants and the catalyst are in the same phase (i.e. solid, liquid or gas). The catalysis is said to be homogeneous. The following are some of the examples of homogeneous catalysis.
(i) In the lead chamber process
\[2S{{O}_{2}}(g)+{{O}_{2}}(g)\xrightarrow{NO(g)}2S{{O}_{3}}(g)\]
(ii) In the hydrolysis of ester
\[C{{H}_{3}}COOC{{H}_{3}}(l)+{{H}_{2}}O(l)\xrightarrow{HCl(l)}\]
\[C{{H}_{3}}COOH(l)+C{{H}_{3}}OH(l)\]
(iii) In the hydrolysis of sugar
\[\underset{\text{(Sucrose solution)}}{\mathop{{{C}_{12}}{{H}_{22}}{{O}_{11}}(l)}}\,+{{H}_{2}}O(l)\xrightarrow{{{H}_{2}}S{{O}_{4}}(l)}\]
\[\underset{\text{(Glucose solution)}}{\mathop{{{C}_{6}}{{H}_{12}}{{O}_{6}}(l)}}\,+\underset{\text{(Fructose solution)}}{\mathop{{{C}_{6}}{{H}_{12}}{{O}_{6}}(l)}}\,\]
(2) Heterogeneous catalysis : The catalytic process in which the reactants and the catalyst are in different phases is known as heterogeneous catalysis. Some of the examples of heterogeneous catalysis are given below.
(i) In contact process for \[{{H}_{2}}S{{O}_{4}}\]
\[2S{{O}_{2}}(g)+{{O}_{2}}(g)\underset{Br{{V}_{2}}{{O}_{5}}}{\mathop{\xrightarrow{Pt(s)}}}\,2S{{O}_{3}}(g)\]
(ii) In Haber’s process for \[N{{H}_{3}}\]
\[{{N}_{2}}(g)+3{{H}_{2}}(g)\xrightarrow{Fe(s)}2N{{H}_{3}}(g)\]
(iii) In Ostwald’s process for \[HN{{O}_{3}}\]
\[4N{{H}_{3}}(g)+5{{O}_{2}}(g)\xrightarrow{Pt(s)}4NO(g)+6{{H}_{2}}O(g)\]
(3) Positive catalysis : When the rate of the reaction is accelerated by the foreign substance, it is said to be a positive catalyst and phenomenon as positive catalysis. Some examples of positive catalysis are given below.
(i) Decomposition of \[KCl{{O}_{3}}\]
\[2KCl{{O}_{3}}(s)\underset{{{270}^{o}}C}{\mathop{\xrightarrow{Mn{{O}_{2}}(s)}}}\,2KCl(s)+3{{O}_{2}}(g)\]
(ii) Oxidation of \[S{{O}_{2}}\]
\[2S{{O}_{2}}(g)+{{O}_{2}}(g)\underset{or\,Pt(s)}{\mathop{\xrightarrow{{{V}_{2}}{{O}_{5}}(s)}}}\,2S{{O}_{3}}(g)\]
(iii) Decon’s process
\[4HCl(g)+{{O}_{2}}(g)\underset{{{450}^{o}}C}{\mathop{\xrightarrow{CuC{{l}_{2}}(s)}}}\,2C{{l}_{2}}(g)+2{{H}_{2}}O(g)\]
(4) Negative catalysis : There are certain, substance which, when added to the reaction mixture, retard the reaction rate instead of increasing it. These are called negative catalyst or inhibitors and the phenomenon is known as negative catalysis. Some examples are as follows.
(i) Oxidation of sodium sulphite
\[2N{{a}_{2}}S{{O}_{3}}(s)+{{O}_{2}}(g)\xrightarrow{Alcohol(l)}2N{{a}_{2}}S{{O}_{4}}(s)\]
(ii) Oxidation of benzaldehyde
\[2{{C}_{6}}{{H}_{5}}CHO(l)+{{O}_{2}}(g)\underset{amine(l)}{\mathop{\xrightarrow{Diphenyl}}}\,2{{C}_{6}}{{H}_{5}}COOH(l)\]
(iii) Tetra ethyl lead (TEL) is added to petrol to retard the ignition of petrol vapours on compression in an internal combustion engine and thus minimise the knocking effect.
(5) Auto-catalysis : In certain reactions, one of the product acts as a catalyst. In the initial stages the reaction is slow but as soon as the products come into existences the reaction rate increases. This type of phenomenon is known as auto-catalysis. Some examples are as follows,
(i) The rate of oxidation of oxalic acid by acidified potassium permanganate increases as the reaction progresses. This acceleration is due to the presence of \[M{{n}^{2+}}\] ions which are formed during reaction. Thus \[M{{n}^{2+}}\]ions act as auto-catalyst.
\[5{{H}_{2}}{{C}_{2}}{{O}_{4}}+2KMn{{O}_{4}}+3{{H}_{2}}S{{O}_{4}}\to 2MnS{{O}_{4}}+{{K}_{2}}S{{O}_{4}}\]
\[+10C{{O}_{2}}+8{{H}_{2}}O\]
(ii) When nitric acid is poured on copper, the reaction is very slow in the beginning, gradually the reaction becomes faster due to the formation of nitrous acid during the reaction which acts as an auto-catalyst.
(6) Induced catalysis : When one reaction influences the rate of other reaction, which does not occur under ordinary conditions, the phenomenon is known as induced catalysis. Some examples are as follows,
(i) Sodium arsenite solution is not oxidised by air. If, however, air is passed through a mixture of the solution of sodium arsenite and sodium sulphite, both of them undergo simultaneous oxidation. The oxidation of sodium sulphite, thus, induces the oxidation of sodium arsenite.
(ii) The reduction of mercuric chloride \[(HgC{{l}_{2}})\]with oxalic acid is very slow, but potassium permanganate is reduced readily with oxalic acid. If, however, oxalic acid is added to a mixture of potassium permanganate and \[HgC{{l}_{2}}\]both are reduced simultaneously. The reduction of potassium permanganate, thus, induces the reduction of mercuric chloride.
(7) Acid-base catalysis : According to the Arrhenius and Ostwald H+ or H– ion act as a catalyst.
(i) For example, Hydrolysis of an ester,
\[C{{H}_{3}}COO{{C}_{2}}{{H}_{5}}(l)+{{H}_{2}}O(l)\underset{O{{H}^{-}}}{\mathop{\xrightarrow{{{H}^{+}}\,or}}}\,C{{H}_{3}}COOH(l)+{{C}_{2}}{{H}_{5}}OH(l)\]
(ii) Inversion of cane sugar,
\[\underset{\text{Sugar}}{\mathop{{{C}_{12}}{{H}_{22}}{{O}_{11}}(l)+{{H}_{2}}O}}\,\xrightarrow{{{H}^{+}}}\underset{\text{Fructose}}{\mathop{{{C}_{6}}{{H}_{12}}{{O}_{6}}(l)}}\,+\underset{\text{Glucose}}{\mathop{{{C}_{6}}{{H}_{12}}{{O}_{6}}(l)}}\,\]
(iii) Conversion of acetone into diacetone alcohol,
\[C{{H}_{3}}COC{{H}_{3}}(l)+C{{H}_{3}}COC{{H}_{3}}(l)\xrightarrow{O{{H}^{-}}}\]
\[C{{H}_{3}}COC{{H}_{2}}.C{{(C{{H}_{3}})}_{2}}OH(l)\]
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