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

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

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 reaction$NO^-_2$ 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.

$Na_2SO_3 \overset{Air}{\rightarrow} Na_2SO_4$

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

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

Related posts:

1. Catalysis worksheet The rate of change of chemical reaction due to the...
2. Catalysis Many reactions proceed quite slowly when the reactants are mixed...
3. Zero Order Reaction Zero Order Reaction If the rate of reaction is independent...
4. Rate constant or Specific Reaction rate According to collision theory, the rate of the reaction is...
5. Factors Influencing Rate of Reaction Some important factors which influence the rate of reaction are...