What is Catalysis?
The process of increasing the rate of chemical reaction by adding a substance which does not take part in the reaction is called catalysis and the substance which is added and increases the rate of reaction is called a catalyst. A very small amount of catalyst is required to alter the rate of reaction. For example, in the reaction of converting hydrogen peroxide into water and oxygen gas, potassium permanganate is used as a catalyst which increases the rate of reaction.
2H₂O₂ \[\overset{\text{Potassium permanganate}}{\rightarrow}\] 2H₂O + O₂
Types of Catalysis
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On the basis of phases of catalysts and reactants, catalysis can be divided into following two types –
Homogeneous Catalysis
Heterogeneous Catalysis
What is Homogeneous Catalysis and Catalyst?
The catalyst who is present in the same phase as of the reactants in the reaction is called homogeneous catalyst and this type of catalysis process is called homogeneous catalysis.
Examples of Homogeneous Catalysis and Catalysts –
1. Hydrolysis of Sugar – In hydrolysis of sugar reactants sugar (sucrose solution) and water are used in liquid states and the catalyst sulfuric acid is also used in the liquid state. Reaction is given below –
C₁₂H₂₂O₁₁₍ₗ₎ + H₂₍ₗ₎ \[\overset{\text{H₂SO₄₍ₗ₎}}{\rightarrow}\] C₆H₁₂O₆₍ₗ₎ + C₆H₁₂O₆₍ₗ₎
Sucrose Glucose Fructose
2. Hydrolysis of the Ester – In hydrolysis of the ester, ester is taken in liquid state with water (liquid) for the reaction in presence of catalyst hydrochloric acid which is also taken in liquid state. Reaction is given below –
CH₃COOCH₃₍ₗ₎ + H₂O₍ₗ₎ \[\overset{\text{HCl₍ₗ₎}}{\rightarrow}\] CH₃COOH₍ₗ₎ CH₃OH₍ₗ₎.
What is Heterogeneous Catalysis and Catalysts?
The catalyst whose phase differs from that of the reactants in the reaction is called heterogeneous catalyst and this type of catalysis process is called heterogeneous catalysis.
Examples of Heterogeneous Catalysis and Catalysts –
1. In Haber’s process of formation of ammonia, nitrogen and hydrogen are used in gaseous forms while catalyst iron is used in solid form.
\[N_{2(g)}\] + 3\[H_{2(g)}\] \[\overset{\text{Fe₍ₛ₎}}{\rightarrow}\] 2NH₃
2. Formation of Sulfuric Acid – In this process sulfur dioxide (gas) is oxidized to sulfur trioxide (gas) by heterogeneous catalysis in presence of solid V2O5 catalyst. Then sulfur trioxide is hydrolyzed to sulfuric acid.
\[SO_{2(g)}\] + \[O_{2(g)}\] \[\overset{\text{V₂O₅₍ₛ₎}}{\rightarrow}\] 2 \[SO_{3(g)}\]
What is Adsorption Theory of Heterogeneous Catalysis?
Modern Adsorption theory of heterogeneous catalysis is the mixture of moderate compound hypothesis and the old adsorption hypothesis or old adsorption theory. Old adsorption theory lacked specificity so there was a need for modern adsorption theory.
According to adsorption theory of heterogeneous catalyst, there are free valencies in the catalyst on which reactant molecules get attached. The mechanism of adsorption theory of heterogeneous catalysis involves following steps –
Step 1. Diffusion of reactant molecules
Step 2. Adsorption
Step 3. Intermediate complex formation
Step 4. Desorption
Step 5. Diffusion of product molecules
Step 1. Diffusion of Reactant Molecules – In this step reactant molecules get diffused towards the surface of the catalyst.
Step 2. Adsorption – In this step reactant molecules get adsorbed on the surface of the solid catalyst or form loose bonds with the free valencies of the catalyst.
Step 3. Intermediate Complex Formation – In this step adsorbed reactant molecules on the surface of the catalyst react with each other and form new stronger bonds with each other which leads to the formation of an intermediate.
Step 4. Desorption – In this step intermediate converts into product as it loses its affinity towards the catalyst. The product molecule gets desorbed from the surface of the catalyst.
Step 5. Diffusion of Product Molecules – In this step desorbed product molecules from the surface of the catalyst get diffused away from the catalyst.
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This ends our coverage on Adsorption Theory of Heterogeneous Catalysis. We hope you enjoyed learning and were able to grasp the concept. We hope after reading this article you will be able to answer questions based on this topic. If you are looking for solutions to NCERT Textbook problems based on this topic, then log on to Vedantu website or download Vedantu Learning App. By doing so, you will be able to access free PDFs of NCERT Solutions as well as Revision notes, Mock Tests and much more.
FAQs on Adsorption Theory of Heterogeneous Catalysis
1. What is the adsorption theory of heterogeneous catalysis?
The adsorption theory of heterogeneous catalysis explains how a reaction rate is accelerated by a solid catalyst when the reactants are in a different phase (usually gaseous or liquid). According to this theory, reactant molecules are adsorbed onto the surface of the catalyst, where the chemical reaction takes place. The products then desorb from the surface, freeing it up for a new cycle. This process effectively provides an alternative reaction pathway with a lower activation energy.
2. What are the five main steps involved in the adsorption theory of heterogeneous catalysis?
The mechanism of heterogeneous catalysis, as explained by the adsorption theory, involves five sequential steps:
- Diffusion of reactants: Reactant molecules diffuse from the bulk to the surface of the catalyst.
- Adsorption: Reactant molecules are held onto the catalyst's surface, specifically at the active sites, through weak van der Waals forces or chemical bonds (chemisorption).
- Chemical Reaction: The adsorbed reactants rearrange and form an activated complex, which then transforms into product molecules on the catalyst surface.
- Desorption of products: The newly formed product molecules leave the catalyst's surface.
- Diffusion of products: The desorbed product molecules diffuse away from the surface, making it available for more reactants.
3. Can you provide a real-world example of the adsorption theory in action?
A classic example is the hydrogenation of ethene using a nickel catalyst. In this process:
- Ethene (C₂H₄) and hydrogen (H₂) gas molecules are adsorbed onto the surface of the solid nickel (Ni) catalyst.
- This adsorption weakens the double bond in ethene and the bond in hydrogen.
- Hydrogen atoms then add across the weakened double bond, forming ethane (C₂H₆) on the surface.
- Finally, the ethane molecule, having a weaker affinity for the nickel surface, desorbs and diffuses away.
4. What is the role of 'active sites' in the adsorption theory of heterogeneous catalysis?
Active sites are specific points on the surface of a catalyst, such as peaks, cracks, or corners, that exhibit high catalytic activity. According to the theory, adsorption does not happen uniformly across the entire surface. Instead, reactant molecules preferentially bind to these active sites because they have free valencies or a specific geometric arrangement that promotes the reaction. The presence and nature of these sites are crucial for both the activity and selectivity of the catalyst.
5. How does heterogeneous catalysis, as explained by the adsorption theory, differ from homogeneous catalysis?
The primary difference lies in the phase of the reactants and the catalyst. In heterogeneous catalysis, the catalyst is in a different phase from the reactants (e.g., a solid catalyst for gaseous reactants). The adsorption theory is specific to this type because it requires a surface for the reaction to occur. In homogeneous catalysis, the catalyst and reactants are in the same phase (e.g., all are in the same liquid solution). The mechanism here does not involve surface adsorption but typically proceeds through the formation of an intermediate compound.
6. Why is the adsorption of reactants on the catalyst surface an essential step for the reaction to occur?
Adsorption is essential for two main reasons. Firstly, it brings the reactant molecules into close proximity, increasing their concentration on the catalyst surface compared to the bulk phase. Secondly, and more importantly, the interaction with the catalyst's active sites weakens the chemical bonds within the reactant molecules. This strain on the bonds lowers the overall activation energy required for the reaction to proceed, thereby significantly increasing the reaction rate.
7. What is the key difference between adsorption and absorption in the context of catalysis?
The key difference is where the molecules accumulate. Adsorption is a surface phenomenon where molecules of a substance stick to the surface of a solid or liquid. In contrast, absorption is a bulk phenomenon where molecules are drawn into the bulk or interior of the material. Heterogeneous catalysis relies specifically on adsorption because the reaction needs to happen on the surface, where active sites can weaken bonds and facilitate the reaction before the products leave.
8. What are the key features of a good catalyst according to the adsorption theory?
According to the adsorption theory, a good catalyst must balance two important features:
- Activity: This refers to the catalyst's ability to significantly increase the rate of a chemical reaction. Activity depends on the strength of chemisorption; the bond must be strong enough to weaken reactant bonds but not so strong that the product cannot desorb.
- Selectivity: This is the catalyst's ability to direct a reaction to yield a specific desired product, while minimising side reactions that could lead to unwanted byproducts. Selectivity is often determined by the specific shape and nature of the active sites on the catalyst.
