# Characteristics of Catalysts

Characteristics of Catalysts are as follows:

1. The catalyst remains unchanged (in mass and chemical composition ) in the reaction (Activity of catalyst.)

2. A small quantity of the catalyst is required. e.g. One mole of colloid Pt catalyses $10^8L H_2O_2$

3. The catalyst does not change the equilibrium constant. But the equilibrium approaches earlier.

4. Specificity of Catalyst:

The catalyst is specific in nature. It means by the change of catalyst, nature of the products changes or specific catalyst for a specific reaction.

Or $CO + 3H_2 \overset{Ni}{\rightarrow} CO + H_2O$

$CO + 2H_2 \overset{ZnO}{\underset{Cr_2O_3} \rightarrow} CH_3COOH$

$CO + H_2 \overset{Cu}{\rightarrow} HCHO$

5. The catalyst can not make impossible reaction to occur and does not intiate a reaction.

Catalyst Poison: There are certain substances which decrease or destroy the activity of the catalyst. Such substances are known as catalytic poisons. E.g. arsenic destroys the catalytic activity of the platinum catalyst in the manufacture of sulphuric acid.

Catalyst Promoter: There are certain substances which increase the activity of the catalyst. Such substances are known as catalyst promoters e.g. Mo acts as a promoter in the manufacture of ammonia by I-laber’s process.

Ex. $N_2 + H_2 \overset{Fe(catalyst)}{\underset{Mo(promoter)} \rightarrow} 2NH_3$

Ex. 2. In Bosch process of preparation of $H_2 , Cr_2O_3$ acts as a promoter for catalyst $Fe_2 O_3$. Catalyst Poison or Promoter does not act like a catalyst.

6. The catalyst exhibits maximum activity at a particular temperature which is known as optimum temperature.

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