Catalysis - Practice Questions & MCQ

Catalyst : Substances, which accelerate the rate of a chemical reaction and themselves remain chemically and quantitatively unchanged after the reaction, are known as catalysts, and the phenomenon is known as catalysis.

Catalysis has two types

• Homogeneous Catalysis : 

  When the reactants products and the catalyst are in the same phase (i.e. liquid or gas), the process is said to be homogeneous catalysis.

  example - 

• Heterogeneous Catalysis : 

  The catalytic process in which the reactants and the catalyst are in different phases is known as heterogeneous catalysis.

  example -  

Adsorption Theory of Heterogeneous Catalysis

The mechanism involves five steps:

(i) Diffusion of reactants to the surface of the catalyst.

(ii) Adsorption of reactant molecules on the surface of the catalyst.

(iii) Occurrence of chemical reaction on the catalyst’s surface through formation of an intermediate.

(iv) Desorption of reaction products from the catalyst surface, and thereby, making the surface available again for more reaction to occur.

(v) Diffusion of reaction products away from the catalyst’s surface. 

Promoters:
Promoters are those substances that enhance the activity of a catalyst.

Poison:
Poisons are those substances that decrease the activity of a catalyst.

For example:

In this reaction, Mo act as a promoter for Fe which is a catalyst.

The important features of solid catalyst are as follows:

• Activity: The activity of a catalyst depends upon the strength of chemisorption to a large extent. The reactants must get adsorbed reasonably strongly on to the catalyst to become active. However, they must not get adsorbed so strongly that they are immobilised and other reactants are left with no space on the catalyst’s surface for adsorption. It has been found that for hydrogenation reaction, the catalytic activity increases from Group 5 to Group 11 metals with maximum activity being shown by groups 7-9 elements of the periodic table.

• Selectivity: The selectivity of a catalyst is its ability to direct a reaction to yield a particular product selectively, when under the same reaction conditions many products are possible. Selectivity of different catalysts for same reactants is different. For example, starting with H₂ and CO, and using different catalysts, we get different products.

Thus, it can be inferred that the action of a catalyst is highly selective in nature. As a result a substance which acts as a catalyst in one reaction may fail to catalyse another reaction.

The catalytic reaction that depends upon the pore structure of the catalyst and the size of the reactant and product molecules is called shape-selective catalysis. Zeolites are good shape-selective catalysts because of their honeycomb-like structures. They are microporous aluminosilicates with three-dimensional network of silicates in which some silicon atoms are replaced by aluminium atoms giving Al–O–Si framework. The reactions taking place in zeolites depend upon the size and shape of reactant and product molecules as well as upon the pores and cavities of the zeolites. They are found in nature as well as synthesised for catalytic selectivity.
Zeolites are being very widely used as catalysts in petrochemical industries for cracking of hydrocarbons and isomerisation. An important zeolite catalyst used in the petroleum industry is ZSM-5. It converts alcohols directly into gasoline (petrol) by dehydrating them to give a mixture of hydrocarbons.

Enzymes are complex nitrogenous organic compounds which are produced by living plants and animals. They are actually protein molecules of high molecular mass and form colloidal solutions in water. They are very effective catalysts; catalyse numerous reactions, especially those connected with natural processes. Numerous reactions that occur in the bodies of animals and plants to maintain the life process are catalysed by enzymes. The enzymes are, thus, termed as biochemical catalysts and the phenomenon is known as biochemical catalysis.

The following are some of the examples of enzyme-catalysed reactions:

• Inversion of cane sugar: The invertase enzyme converts cane sugar into glucose and fructose.
  
    Cane sugar                                                    Glucose                   Fructose
• Conversion of starch into maltose: The diastase enzyme converts starch into maltose.
  
           Starch                                                           Maltose