Question

Which forms multi-layers during adsorption? (a) Van der Waal's adsorption (b) Chemical adsorption (c) Physical adsorption (d) All of these

Short answer

Option (c) Physical adsorption.

Step-by-step solution

Understand the Types of Adsorption

Adsorption is a process where molecules from a gas or liquid adhere to a solid or liquid surface. It occurs in two main forms: physical adsorption (also known as physisorption) and chemical adsorption (also known as chemisorption).

Multiple Layers Formation in Adsorption

In physisorption, molecules are held on the surface by weak Van der Waals forces, allowing for the possibility of forming multiple layers, as the forces are not specific to just one layer. In chemisorption, molecules form a strong chemical bond with the surface, typically limiting adsorption to a monolayer.

Identify Adsorption in Each Option

Option (a) "Van der Waals adsorption" refers to physisorption, which can form multiple layers. Option (b) "Chemical adsorption" refers to chemisorption, which does not form multiple layers. Option (c) "Physical adsorption" also refers to physisorption. Option (d) "All of these" is incorrect because chemical adsorption does not form multiple layers.

Conclude the Correct Answer

The correct options that relate to adsorption forming multiple layers are (a) Van der Waals adsorption and (c) Physical adsorption, as both relate to physisorption.

Key concepts

Physisorption

Physisorption, also known as physical adsorption, is a type of adsorption that involves weak interactions between the adsorbate and the surface. These interactions are typically governed by Van der Waals forces, which are weak and non-specific. Because these forces do not require any particular bonding between the atoms or molecules involved, they allow multiple layers of molecules to accumulate on the surface.

This layering occurs because each additional layer of adsorbate molecules is only slightly influenced by the adsorbate beneath it. This process can continue as long as there is gas or liquid to supply more molecules, and as long as there is sufficient attraction by the surface. It is important to understand this difference because it shows why physisorption can sometimes lead to significant accumulations of material on surfaces, unlike chemisorption, which tends to restrict itself to a single layer.

• Weak Van der Waals interactions
• Non-specific, allowing for multiple layers
• Reversible adsorption due to weak bonds

Chemisorption

Chemisorption is a different mechanism from physisorption. It refers to adsorption where a chemical bond is formed between the adsorbate and the surface. These bonds are typically much stronger than the Van der Waals forces seen in physisorption. Because they are strong and involve specific chemical interactions, chemisorption is normally limited to a single layer of molecules.

The reason for this single layer limit is that once a molecule is chemically bonded to a surface, it often occupies specific sites and actively limits further layers of molecules from bonding to those same sites. Chemisorption is also typically irreversible or requires significant energy to reverse due to the strength of these bonds.

• Strong chemical bonds
• Typically results in a single layer of atoms or molecules
• Often irreversible due to the energy required to break bonds

Van der Waals Forces

Van der Waals forces are the underlying mechanism that allows physisorption to occur. These forces are weak intermolecular attractions that can act between molecules or across larger surfaces. They are a blend of different types of interactions like dispersion forces, dipole-dipole interactions, and induced dipole interactions.

Despite their individual weakness, Van der Waals forces are key to the phenomenon of physisorption because they can atrract many molecules to a surface without the need for direct chemical bonding. This allows for accumulation of layers since each molecule only needs to experience a weak attraction to attach itself.

• Weak and non-specific
• Includes dispersion forces and dipole interactions
• Facilitates the formation of multiple layers in physisorption