Problem 120 - A Complete Resource Book in Chemistry for JEE Main

Question

Choose the incorrect statement for chemical adsorption (1) value of adsorption enthalpy is above \(-20.0 \mathrm{kJmol}^{-1}\) (2) van der Waals forces exist between the adsorbent and the adsorbate (3) usually monomolecular layer is formed on the adsorbent (4) multimolecular layer may be formed on the adsorbent (a) 1,2 and 3 (b) 2,3 and 4 (c) 1,2 and 4 (d) \(1,2,3\) and 4

Step-by-Step Solution

Answer

Option (c) 1, 2, and 4 are incorrect.

Step 1: Understand Chemical Adsorption

Chemical adsorption, also called chemisorption, involves the formation of a chemical bond between the adsorbent and the adsorbate. This usually results in a higher adsorption enthalpy because bond formation or bond rearrangement often requires or releases more energy than physical processes like van der Waals interactions.

Step 2: Analyze Enthalpy of Adsorption

For chemical adsorption, the enthalpy of adsorption is typically much greater than \(-20.0 \mathrm{kJmol}^{-1}\). The value mentioned (\(-20.0 \mathrm{kJmol}^{-1}\)) is usually associated with physical adsorption, which is weaker.

Step 3: Consider Intermolecular Forces

In physical adsorption, van der Waals forces are significant; however, in chemical adsorption, stronger chemical bonds are involved. Therefore, van der Waals forces are not the primary interactions in chemical adsorption.

Step 4: Assess Monomolecular vs. Multimolecular Layers

Chemical adsorption typically results in a monomolecular layer because each adsorbate molecule forms a chemical bond with the adsorbent surface. Multimolecular layers are more characteristic of physical adsorption due to weaker interactions allowing further layers to form.

Step 5: Identify the Incorrect Statements

Based on the understanding of chemical adsorption: - (1) is incorrect because chemisorption typically has higher enthalpy. - (2) is incorrect because van der Waals forces are not dominant. - (4) is incorrect because multimolecular layers are associated with physisorption, not chemisorption.

Step 6: Choose the Correct Option

The incorrect statements are 1, 2, and 4. Therefore, the correct choice is option (c) 1, 2, and 4.

Key Concepts

Enthalpy of Adsorption

When discussing chemical adsorption, or chemisorption, one of the key factors to understand is the enthalpy of adsorption. This property reflects the heat change associated with the adsorption process. Unlike physical adsorption, which typically exhibits lower enthalpy values (often less than \(-20 \, \mathrm{kJ \cdot mol^{-1}}\)), chemisorption involves the formation of chemical bonds between the adsorbate and adsorbent. This bond formation requires or releases a significant amount of energy, leading to much higher enthalpy values, usually above \(-20 \, \mathrm{kJ \cdot mol^{-1}}\).
It's critical to note that higher enthalpy values in chemisorption indicate a stronger and more stable interaction between the molecules involved.

• This is because breaking a chemical bond requires substantial energy.
• The formation/release of these bonds significantly impacts the heat exchange during the process.

Understanding enthalpy helps in distinguishing between chemisorption and physisorption, especially when comparing the strength and permanence of the adsorption process.

Intermolecular Forces

In the world of adsorption, the type of intermolecular forces plays a significant role in determining the nature of the adsorption process. Physical adsorption, or physisorption, relies on weak van der Waals forces to attach molecules to the surface. These are the same forces that allow geckos to climb walls. They are essential for temporary interactions, contributing to lower enthalpy changes.
On the other hand, chemical adsorption, or chemisorption, primarily involves the formation of stronger, specific chemical bonds which can include covalent or ionic interactions. These forces are much stronger than van der Waals forces and result in more stable attachments. Due to the strength of these bonds:

• Chemsorption is generally less reversible than physisorption.
• Molecules often require substantial energy to detach.

Hence, when examining intermolecular forces, it's clear that they dictate not only how molecules adhere to surfaces but also how strong and lasting these interactions will be.

Monomolecular Layer Formation

In chemical adsorption, a fascinating outcome is the formation of a monomolecular layer on the adsorbent's surface. This occurs because each molecule of the adsorbate forms a chemical bond with the surface, maximizing the surface coverage and utilization. In contrast, physical adsorption allows for multilayers due to the weaker forces involved, enabling subsequent layers to pile on top.
Monomolecular layers are a hallmark of chemisorption, marking the precise interaction between specific molecules and surface sites. This precision leads to:

• High specificity in adsorbent-adsorbate interactions.
• Optimization of surface area without additional layers distorting the surface characteristics.

Therefore, understanding monomolecular layer formation is crucial for interpreting how surfaces interact with their environments and tailor these surfaces for specific reactions or applications.