Catalysis-Characteristics, Types, Examples, Practice problems and FAQs

Many of the great discoveries made in the past were in response to a social or economic need of the time. The demand and supply of food was a topic of huge importance. In order to meet the demands, major developments in agricultural technology were required. to produce more food have been effective in alleviating many of the difficulties. However, it was quite clear that without synthetic fertilizer or pesticides, the developments in agricultural technology would scarcely have been possible. 

In order to overcome this issue, Haber promptly got to work. He found the method to convert atmospheric nitrogen and hydrogen to form ammonia. The method used by Haber was very slow. After several years BASF, a German company, found the industrial method for large scale production of ammonia. 

It uses various substances that enhance the reaction by an appreciable amount. One such substance was iron, Fe. Also, it allows the reaction to occur at a lower temperature, thus reducing the cost of production. Such substances are known as catalysts. Let's study in detail all about catalysts.

Table of contents

• Definition
• Characteristics
• Types of catalyst
• Promoters
• Poisons
• Autocatalysis
• Practice problems
• Frequently Asked Questions-FAQs

Definition

A chemical substance which can alter the rate of a reaction, by providing an alternative pathway is known as a catalyst. 

It can be better understood with the following example.

Normally, KClO₃ when heated to a high temperature( 653 K - 873 K)undergoes the decomposition reaction but when catalyst Mno₂ is used, reaction occurs at a low temperature and rapidly at lower temperature( 473 k - 633K) . 

Characteristics

1. During the reaction, the mass and chemical composition of a catalyst remains unchanged. Its physical state, on the other hand, can alter. For example, in the decomposition reaction of KClO₃   granular Mno₂ is left as a powder at the end of the reaction.
2. Greater the surface area of the catalyst greater is the rate of reaction. Therefore finely divided catalysts are used to enhance the rate of reaction. 
3. Catalysts are reaction specific.  A substance that catalyses one reaction, fails to catalyse other reactions.
4. The efficiency of a catalyst is maximum at a particular temperature, known as optimum temperature. At any temperature above or below the optimum temperature, the activity of the catalyst decreases. Biological enzymes also work only at a particular pH. The pH at which the activity of enzymes will be maximum is known as optimum pH. 
5. A catalyst cannot initiate the reaction.
6. A catalyst does not alter the enthalpy of a reaction.

Types of catalysts

Catalysts can be classified on the following basis:

1. On the basis of phases

• Homogenous catalyst

In homogeneous catalyst, both the catalysts and reactants are present in the same phase. This type of reaction is known as homogeneous catalysis.

Example-

1. Hydrolysis of ester, in acidic medium (in H₂SO₄(aq)                                          

2. Oxidation of sulphur dioxide, in presence of NO(g) as catalyst                       

3. Hydrolysis of sugar in acidic medium(in H₂SO₄(aq)

• Heterogeneous catalysts

In heterogeneous catalyst, catalysts and reactants are present in a different phase. This type of reaction is known as heterogeneous catalysis.

Example-

1. Decomposition of decomposition in presence of MnO₂(S), as a catalyst. 

2. Formation of SO₃, in presence of pt(s), as a catalyst.                          

3. Formation of ammonia, Fe(s), as a catalyst. 

4. Formation of NO, in presence of pt(s), as a catalyst. 

5. Reduction of vegetable oil to form vegetable ghee in presence of Ni(s), as a catalyst.                     

2. On the basis of impact on the rate of reaction

• Positive catalyst 

Positive catalyst increases the reaction rate by reducing the activation energy barrier. 

• Negative catalyst or Inhibitor

Negative catalyst decreases the reaction rate by increasing the activation energy barrier. 

Promoters

Substances that are not catalysts themselves but their presence can increase the catalytic activity of a catalyst.

Example- 

1. In the reduction of vegetable oil to form vegetable ghee in presence of Ni(s), as a catalyst and Cu as a promoter.                                 

2.  In the formation of ammonia, Fe(s), as a catalyst and MO as a promoter.

Poisons/Anti-catalyst

Substances that are not catalysts themselves but their presence decreases the activity of a catalyst. Poisoning is due to preferential adsorption of a chemical species (i.e poison) on the surface of the catalyst.

Example-

1. In the formation of ammonia, Fe(s) and CO/H₂S 

2.    Formation of SO₃, in presence of AS₂S₃ and

3. Reduction of acyl chloride into aldehyde using Pd/BaSO₄ as a catalyst and Quinoline (C₉H₇N) as a poison

Autocatalysis

When one of the products of a reaction behaves as a catalyst for that reaction and increases the rate of reaction. 

Example-

Generally, in ester hydrolysis H₂SO₄ are used as catalysts. But in this reaction, acetic acid, which is formed in the product is acting as a catalyst.

Practice problems

Q 1. A homogeneous catalyst is one in which reactants and products are in ____.

(A) same phase
(B) different phase
(C) may be in the same or different phase
(D) None of the above

Answer: (A)
Homogeneous catalyst, both the catalysts and reactants are present in the same phase. This type of reaction is known as homogeneous catalysis.

Q 2. Which of the following is the correct statement for the given reaction.

(A) HCL act a catalyst for this reaction
(B) It is an example of autocatalysis
(C) Both (A) and (B)
(D) None of the above

Answer: (C)

Given reaction acidic hydrolysis of ester, in which H₂SO₄ act a catalyst. But in the absence of HClCH₃COOH act as a catalyst. Such a reaction in which the product itself plays the role of catalyst, is an example of autocatalysis. 

Q 3. Substances that are not catalysts themselves but their presence can increase the catalytic activity of a catalyst, are known as

(A) Positive catalysts
(B) Promoters
(C) Both (A) and (B)
(D) None of the above

Answer: (B)
Substances that are not catalysts themselves but their presence can increase the catalytic activity of a catalyst, are known as promoters. On the other hand, positive catalysts are those substances that increase the reaction rate by reducing the activation energy barrier. 

Which of the following is the correct statement for the given reaction.

(A) Fe act as catalyst
(B) CO/H₂Sact as poison
(C) Both (A) and (B)
(D) None of the above

Answer: (C)

The given reaction is a Haber’s process. In Haber’s process, Fe is used as a catalyst and CO/H₂S is used to inhibit the reaction, whereby it acts as a poison. 

Frequently Asked Questions-FAQs

Q 1. How does molybdenum act as a promoter in Haber’s process?

Answer: Haber’s process is the process used in the large scale production of ammonia. 

The first hydrogen gets adsorbed on the surface of nickel, then, MO enters the lattice and increases the surface area of Nickel and bond length of H -H and bond energy decreases. Thus, the covalent bond is much weakened and cleaves readily​.

Because the promoter, H₂ reacts with N₂ faster and ammonia is formed faster. Thus, promoter enhances the activity of the catalyst.

Q 2. Does catalyst has any impact on equilibrium?

Answer: Catalyst cannot change the equilibrium state but it helps to attain equilibrium quickly. It changes the rate of forward and backward reaction equally. 

Q 3. Can catalyst initiate a reaction?

Answer: A catalyst cannot initiate a reaction. However, there are situations when the activation energy is so high that in practice, a reaction may not begin until a catalyst greatly lowers the activation energy. 

For example, a mixture of hydrogen and oxygen does not react at room temperature, but the reaction occurs at lower temperatures. In the presence of Pt black, this happens very quickly. 

Q 4. Does the amount of catalyst matters in a reaction?

Answer: In most reactions, a small amount of catalyst is sufficient to catalyse the infinite amount of reaction.  1 g of Platinum is suffciient to catalyse 109 of H₂O₂ decomposition position. But in some cases, the rate of reaction is dependent on the amount of catalyst used.

One such expectation is acidic or alkaline hydrolysis of ester, where the concentration of H⁺ or OH^- effects the rate of reaction. 

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