Problem 85 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following statement is wrong about chemisorption? (1) it is reversible in nature (2) it is specific in nature (3) it first increases with increase in temperature and then pressure (4) it involves the formation of a compound on the surface of the solid adsorbent (a) 1 only (b) 2 and 3 (c) 1 and 3 (d) 2 and 4

Step-by-Step Solution

Answer

The wrong statements are (1) and (3); choice (c) is correct.

Step 1: Understanding Chemisorption

Chemisorption, or chemical adsorption, is the process where a gas molecule or atom forms a chemical bond with the surface of a solid. This results in a surface compound characterizing chemisorption as more specific and stronger compared to physisorption. Due to the strength of these new bonds, chemisorption is usually irreversible.

Step 2: Analyzing Statement 1

Statement (1) claims that chemisorption is reversible in nature. Given that chemisorption involves stronger chemical bonds, it is typically irreversible. Hence, statement (1) is incorrect.

Step 3: Analyzing Statement 2

Statement (2) claims that chemisorption is specific in nature. Chemisorption is indeed specific because it depends on the chemical identity of both the adsorbate and the adsorbent to form chemical bonds. Therefore, statement (2) is correct.

Step 4: Analyzing Statement 3

Statement (3) states that chemisorption first increases with an increase in temperature and then pressure. Chemisorption typically requires activation energy, thus initially increasing with temperature until an optimum point. However, there is no similar pressure dependency described, making this statement problematic in its accuracy.

Step 5: Analyzing Statement 4

Statement (4) suggests that chemisorption involves the formation of a compound on the solid adsorbent's surface. This is accurate, as chemisorption creates chemical bonds forming a new compound on the surface.

Step 6: Identifying the Wrong Statement

Based on the analysis, statement (1) is inaccurate because chemisorption is generally irreversible, and part of statement (3) is misleading. Therefore, the correct answer is option (c) 1 and 3.

Key Concepts

Adsorption

Adsorption is the process where molecules, atoms, or ions from a gas or liquid adhere to a solid surface. It differs from absorption, where substances permeate or dissolve in a solid or liquid.
Adsorption can be classified into two main types: **physisorption** and **chemisorption.** Physisorption is characterized by weak van der Waals forces, while chemisorption involves strong chemical bonds.
In chemisorption, the gas molecules form chemical bonds with the solid surface, resulting in a stronger and more specific interaction. This specificity is due to the necessity of particular conditions for the adsorbate to bond with the adsorbent, like the right shape and electronic structure.

Chemical Bonds

Chemical bonds are the forces that hold atoms and molecules together. They can be covalent, ionic, metallic, hydrogen bonds, or van der Waals forces, each with different characteristics and strengths.
In chemisorption, the type of bond formed is primarily covalent, where electrons are shared between the adsorbate and the adsorbent surface.
This formation of new chemical bonds results in the altered properties of the adsorbate, with the possibility of forming a surface compound. The strength and nature of these bonds are what set chemisorption apart from physical adsorption, making it more energetically favorable but less reversible.

Irreversibility

Irreversibility in chemisorption refers to the difficulty in reversing the process to separate the adsorbate from the adsorbent.
This characteristic arises due to the formation of strong chemical bonds. Once these bonds form, breaking them often requires significant energy, unlike in physisorption where weaker forces allow for easier reversibility.
Because of this, chemisorption is utilized in processes where a strong and permanent attachment is beneficial, such as catalysis, whereas reversibility in physisorption is preferred in separations and purifications.

Temperature Dependence

Temperature plays a critical role in the process of chemisorption. Unlike physisorption, which generally decreases with temperature, chemisorption often requires an initial increase in temperature to overcome the activation energy barrier.
This activation energy is necessary for forming strong chemical bonds between adsorbate molecules and the solid surface.

• As temperature rises, the rate of chemisorption can increase, up to an optimum point.
• Beyond this point, excessive temperature may break the bonds, decreasing adsorption efficiency.

Thus, the precise control of temperature is crucial to optimize chemisorption for industrial and laboratory applications.