Catalysis

What is Catalysis?

Catalysis is the phenomenon in which the rate of any reaction is altered (accelerated or retarded) by the presence of a substance, which itself remains unchanged chemically in the reaction. The substance which  alters the rate of the reaction is called catalyst.

Catalytic reactions are of two types:

Homogeneous Catalysis

When the reactants and catalysts are in the same physical state  i.e. catalyst is in the same phase as the reactant is called homogenous catalysis. 

For Example

• Lead Chamber Process:
   

• Inversion of Can Sugar:
   

Heterogeneous Catalysis

A catalytic process in which the catalyst and the reactants are in different phases is called heterogeneous catalysis. This process is also known as surface catalysis or contact catalysis.

For Example

• Decomposition of H₂O₂: 
  

• Haber’s Process: 
  

What is a Catalyst?

Catalyst is a substance which alters the rate of chemical reaction (may increase or dercrese the rate) without being consumed itself during the course of reaction.

Catalysts are divided into four groups

• Positive catalyst: The substance which increases the rate of reaction is known as positive catalyst. It acts by decreasing the activation energy for reaction.
  An example of positive catalyst is the V₂O₅ used in contact process: 
  
• Negative catalyst ( Inhibitor or retarder): The substance which decreases the rate of reaction is known as positive catalyst. It acts by  increasing the activation energy for reaction.

  Example,  Acetanilide prevents oxidation of Na₂SO₃ by air so it acts as negative catalyst here.
  Similarly H₃PO₄ prevents decomposition of H₂O₂ by acting as a negative catalyst for the reaction.

• Auto catalyst: In this type of catalysis, one of the products of the reaction  acts as catalyst.
  Example, In the oxidation of oxalic acid by KMnO₄, Mn²⁺ ion formed act as catalyst and increases the rate of reaction. This is why when KMnO4 solution is added into warm solution of oxalic acid containing small amount of dil. H₂SO_4, initially there is a time lag before decolourisation occurs but the process becomes almost instantaneous as more KMnO₄ is added.

• Induced catalyst: When a chemical reaction increases the rate of another chemical reaction, it is called induced catalysis.
  Example: 
  Sodium arsenite solution is not oxidised by air. but when air is passed through a mixture of the solution of sodium arsenite and sodium sulphite, simultaneous oxidation of both takes place. Thus the oxidation of sodium arsenite is induced by oxidation of sodium sulphite. 
   

Characteristics of Catalytic Reactions

• The catalyst remains unchanged in amount and chemical composition at the end of the reaction; it may, however, undergo considerable change in physical form. 

• Catalysts are highly efficient i.e. a small quantity of the catalyst is capable of producing the desired effect. 

• The action of a catalyst is specific to a large extent. 

• The catalyst does not initiate a reaction; only accelerates the reaction that is already occurring. 

• A catalyst does not alter the final state of equilibrium in a reversible reaction. 

• A certain minimum energy must be possessed by the reactants so that they may react and produce the products. This is called the activation energy (Ea) for the reaction. A catalyst lowers the activation energy which increase the rate of the reaction. Thus, a catalyst increases the rate of a reaction by providing a pathway whose activation energy is lower than the activation energy of the uncatalysed reaction. 

Mechanism of Catalytic Action

There is no any fixed mechanism for the action of all the catalysts. This is because the catalytic reactions are of various types. However, the two theories of catalytic action which are followed are.

• Intermediate  compound formation theory of catalysis

• Adsorption theory of catalysis
   

Intermediate Compound Formation Theory

According to intermediate compound formation theory, the catalyst reacts with one of the reactants to form an unstable intermediate compound. The formation of this intermediate compound requires less energy than needed for the actual reaction. The intermediate compound formed is unstable and thus combines with the other reactant to form the final product and the catalyst is regenerated.

In simple words we can say that, a catalyst increases the arte of any reaction by providing an alternative pathway for the reaction to proceed with lower activation energy.

A large number of catalytic reactions can be explained on the basis of this theory

For example catalytic oxidation of SO₂ to SO₃ in the presence of NO as catalyst

2NO +O₂ →  2MO₂

NO₂+SO₂ →  SO₃+NO

This theory provides explanation for the fact that catalyst remains unchanged in mass and chemical composition at the end of reaction and its effectiveness even in small quantities. 
 

Adsorption Theory of Catalysis

Adsorption theory of catalysis explains the mechanism of heterogeneous catalysis mainly. The old point of view was that when the catalyst is in solid state and the reactants are in gaseous state or in solutions, the molecules of the reactants are adsorbed on the surface of the catalyst. Adsorption being an exothermic process, the heat of adsorption is taken up by the surface of catalyst, which is utilized on enhancing the chemical activity of reacting molecules.

The modern adsorption theory is the combination of intermediate compound formation theory and the old adsorption theory.

• The catalytic activity is located on the surface of the catalyst. The mechanism involves five steps

• Diffusion of reactant on the surface of catalyst

• Adsorption of reactant molecules on the surface of catalyst

• Occurrence of chemical reaction on the catalyst surface through formation of intermediates

• Desorption of reaction products away from the catalyst surface

• Diffusion of reactant products away from the catalyst surface 
   

Catalysts in Industries

Catalytic Promoters

Those substances which do not themselves act as catalyst but their presence increases the activity of a catalyst are called catalytic promoters.

Example. In the Haber’s Process, Fe is the catalyst while Mo acts as a promoter.

Catalytic Poison

The substance whose presence decreases or destroys the activity of a catalyst is called catalytic poison.

For example: the Haber’s Process, CO or H₂S acts as poison for  Fe  catalyst.

Activity of Catalyst

Activity of a catalyst is the ability of catalyst to accelerate a chemical reaction. The degree of acceleration can be as high as 10 ¹⁰ times in certain reactions.

Reaction between H₂ and O₂ to form H₂O in presence of platinum as catalyst takes place with explosive violence. In absence of catalyst, H2 and O2 can be stored indefinitely without any reaction.

Selectivity of Catalyst

Selectivity of a catalyst is its ability to direct a reaction to yield particular product (excluding other) 

For example:

Shape-Selective Catalysis by Zeolites

The catalytic reaction that depends upon the structure of pores 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. Zeolites are aluminosilicates i.e., three dimensional network silicates in which some silicon atoms are replaced by aluminium atoms. They are found in nature as well as synthesized for catalytic selectivity. Zeolites, before using as catalysts, are heated in vacuum so that the water of hydration is lost. As a result, zeolite becomes porous i.e., the cavities in the cage-like structure which were occupied by the water molecules become vacant. The size of the pores is generally 260 pm to 740 pm,  because of which only those molecules can be adsorbed in these pores whose size is small enough to enter these cavities and also leave easily. 

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. That is why these types of reactions are called ‘shape-selective catalysis’ reactions. 

Question 1: A catalyst

a. increases the rate of reaction

b. decreases the rate of reaction

c. may increase or decrease the rate of reaction

d. does not affect the rate of reaction

Question 2: A positive catalyst

a. decreases the activation energy

b. increases the activation energy

c. does not affect the rate of reaction

d. decreases the rate of reaction

Question 3: Which of the following catalysts is used in contact process?

a. V₂O₅

b. Fe

c. Co

d. Ni

Question 4: Which of the following statements regarding catalyst is incorrect?

a. The catalyst remains unchanged in amount and chemical composition at the end of the reaction; it may, however, undergo considerable change in physical form. 

b. A small quantity of the catalyst is capable of producing the desired effect. 

c. The action of a catalyst is specific to a large extent. Thus, the decomposition of KCIO₃ is catalyzed by MnO₂ but not by platinum. 

d. A catalyst does alter the final state of equilibrium in a reversible reaction. 

Related Resources

• Click here to know the syllabus of chemistry for  IIT JEE

• You can also have a look at past year papers of IIT JEE

• You can also refer to adsorption of gases on solids  and adsorption Isotherms

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