Problem 4 - A Complete Resource Book in Chemistry for JEE Main

Question

The statement not applicable to chemisoption is that it is (a) highly specific (b) independent of temperature (c) irreversible (d) slow

Step-by-Step Solution

Answer

The statement not applicable to chemisorption is (b) independent of temperature.

Step 1: Understand the Concept

Chemisorption, or chemical adsorption, involves the formation of a chemical bond between the adsorbate and the surface. It is characterized by high specificity due to the chemical bond formation.

Step 2: Analyze Specific Options

Evaluate the options given: (a) Highly specific - This is a true statement, as chemisorption is highly specific. (b) Independent of temperature - Chemisorption usually depends on temperature as it involves activation energy. (c) Irreversible - Chemisorption is typically irreversible since chemical bonds are formed. (d) Slow - Chemisorption is generally slower than physisorption due to bond formation.

Step 3: Identify the Incorrect Statement

The statement 'independent of temperature' (option b) is incorrect for chemisorption because it generally depends significantly on temperature for bond formation due to the required activation energy.

Key Concepts

Understanding Adsorption

Adsorption is a process where molecules, known as adsorbates, accumulate on the surface of a material, called the adsorbent. This process leads to the formation of a thin film on the surface. It's important to note that adsorption only involves the binding of molecules to the surface and does not penetrate deeper into the bulk material.
There are two primary types of adsorption:

• Physisorption: This involves weak interactions, typically Van der Waals forces, leading to a reversible process.
• Chemisorption: This is characterized by the formation of a chemical bond between adsorbate and the surface, resulting in a stronger and often irreversible bond.

In chemisorption, due to the specific interactions involved, there is usually a high level of specificity for the adsorbate being bound to the adsorbent, unlike physisorption.

Explaining Chemical Bonds

Chemical bonds are the attractive forces holding atoms or molecules together. In the context of chemisorption, these bonds form at the surface level between the adsorbate molecules and the surface atoms of the adsorbent. This bond can significantly alter the characteristics of the adsorbate.
The bonds formed in chemisorption include:

• Covalent Bonds: Strong bonds formed by sharing electrons between the adsorbate and surface atoms.
• Ionic Bonds: Bonds formed due to the electrostatic attraction between oppositely charged ions.

The energy involved in forming these chemical bonds provides the distinct properties unique to chemisorption, including its usual irreversibility and specificity.

Nature of Irreversibility

In chemisorption, the formation of strong chemical bonds between adsorbate and adsorbent often leads to irreversibility. Once these chemical bonds are established, it becomes exceedingly difficult to separate the adsorbate from the surface without breaking the bonds, which requires significant energy or could even alter the material properties.
This irreversibility is a key characteristic differentiating chemisorption from physisorption, where the weaker bonds allow for a reversible process. The irreversibility means that once the adsorbate has adhered chemically to the surface, subsequent removal typically requires additional processes, possibly altering the surface or destroying the adsorbate.

Temperature Dependence of Chemisorption

Chemisorption is highly dependent on temperature. Unlike physisorption, chemisorption requires activating energy to overcome the energy barrier for reaction and bond formation.
This means that chemisorption rates increase with rising temperature up to a certain limit where the optimum reaction speed is achieved. Beyond this point, further heating might disrupt the bonds, making the process inefficient.

• At lower temperatures: The reaction may be too slow or require additional energy to proceed.
• At higher temperatures: It may reach an optimal point or begin to decrease in efficiency if bonds begin to break.

Understanding the temperature dependence is crucial for utilizing chemisorption effectively in industrial applications such as catalysis and gas capture.