Types of catalysts





Types of catalysts are listed below:

 

 

(i)  Positive catalysts:

A catalyst which increases the rate of reaction is called positive catalyst. Such catalyst decreases activation energy by accepting a smaller path, so rate of reaction is increased.

Positive Catalyst

Positive Catalyst


E.g.

2SO_2(g) + O_2 (g) \overset{Pt(s)}{\rightarrow} 2SO_3(g)

 

2KClO_3 (s) \overset{MnO_2(s)}{\rightarrow} 2KCl (s) + 3O_2(g)

 

N_2(s) + 3H_2(g) \overset{Fe(s)}{\rightarrow} 2NH_3(g)

 

CH_2 = CH_2(g) + H_2(g) \overset{Ni(s)}{\rightarrow} CH_3- CH_3(g)

 

(ii) Negative catalysts (Inhibitors):

A catalyst which decreases or retards the rate of reaction is called negative catalysts.

Negative Catalysts

Negative Catalysts


It is because a -ve catalyst increase activation energy by taking a longer alternative path.

E.g. (1) 2H_2O_2 \overset{H_3PO_4(s)}{\underset{or Glycerine} \rightarrow} 2H_2O + O_2

(2) T.E.L (Tetra Ethyl Lead) an ant knocking substance is added to petrol to decrease the ignition of petrol vapours.

(3) Na_2 SO_3 \overset{C_2 H_5 OH}{\underset{Atmospheric} \rightarrow} Na_2SO_4 Here C_2 H_5 OH acts as -ve catalyst to slow the oxidation .

(4) Acetanilide also acts as —ve catalyst in decomposition of H_2O_2

 

(iii) auto-catalysts:

When one of the products formed in the reaction acts as a catalyst is known as auto-catalyst.

CH_3COOC_2 H_5 + H_2O \to CH_3COOH (auto-catalyst)+ C_2H_2OH

 

2KMnO_4 + 5H_2C_2O_4 + 3H_2SO_4 \overset{80^0 c}{\rightarrow} K_2SO_4 + 2MnSO_4(\text{autocatalyst}) + 10CO_2 +8H_2O

 

2AsH_3 \to 2As + 3H_2 (\text{autocatalst})

 

3Cu + 8HNO_3(dil.) \to 3Cu(NO_3)_2 + 4H_2O + 2NO

 

In this reactionNO^-_2 acts as auto catalyst which is formed as a side reaction :

2NO+ H_2O + O \to 2H^+ + 2NO^-_2

 

Auto catalysis reaction starts slowly in the beginning but as auto catalyst is formed rate of reaction starts increasing.

 

(iv) Induced catalyst:

The substance which influences the speed of other reaction, which is not possible under ordinary conditions, is known as induced catalyst.

E. g.

Sodium sulphite solution readily oxidises in air, but sodium arsenite solution does not oxidise by passing air in the solution. When both these solutions are mixed and air is passed then both the substances get oxidised.

Na_2SO_3 \overset{Air}{\rightarrow} Na_2SO_4

 

Na_2 AsO_3 \overset{Air}{\rightarrow} \text{No reaction}

 

Na_3AsO_3 + Na_2SO_3(\text{Induced catalyst}) \to Na_3AsO_4(\text{Sodium Arsenate}) + Na_2SO_4

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