Problem 152 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

(c) Physical adsorption, same as Van der Waal's, forms multi-layers.

Step 1: Understand the Process of Adsorption

Adsorption is the process where molecules from a gas or liquid adhere to the surface of a solid material. This forms an adsorbed layer on the solid's surface.

Step 2: Analyze Van der Waal's Adsorption

Van der Waal's adsorption, also known as physisorption, involves weak physical forces such as Van der Waal's forces. This form of adsorption allows for multiple layers of adsorption as the forces are not strong enough to limit the adsorption to a single layer.

Step 3: Examine Chemical Adsorption

Chemical adsorption, also known as chemisorption, involves the formation of strong chemical bonds between the adsorbate and the adsorbent. Due to the strength and specificity of these bonds, chemisorption typically results in a monolayer.

Step 4: Review Physical Adsorption

Physical adsorption (physisorption) is the same as Van der Waal's adsorption. It involves weak interactions and allows for multi-layer formation since the energy barrier is low for further adsorption beyond the first layer.

Step 5: Determine the Correct Answer

Comparing the options, both Van der Waal's and physical adsorption are forms of adsorption that can result in multi-layer formation. Chemical adsorption does not form multi-layers.

Key Concepts

Van der Waal's adsorption

In the realm of adsorption, Van der Waal's forces play a pivotal role. These forces are weak physical forces that result in an interaction between the adsorbent's surface and the adsorbate molecules. Unlike chemical bonds which are strong and specific, Van der Waal's forces are much gentler.
This gentle interaction allows molecules to easily attach and detach from the surface, leading to the potential formation of multiple layers.
Because of these weak forces, this type of adsorption is also reversible and usually occurs at low temperatures.

• Van der Waal's forces are weak.
• Allows for multiple layers of adsorption.
• Occurs at low temperatures and is reversible.

This contrasts with other adsorption types where stronger forces or bonds limit the process to a monolayer.

Chemical adsorption

Chemical adsorption, also known as chemisorption, is marked by the formation of strong chemical bonds between the adsorbate and adsorbent.
This process is highly specific, meaning that not every molecule can be adsorbed; it depends on the chemical properties and compatibility of the substances involved.

Because chemisorption involves the creation of these strong bonds, energy is required, and the process is often irreversible. This means once the adsorbate molecules are bound to the surface, they cannot freely detach.

• Formation of strong chemical bonds.
• Highly specific, non-reversible.
• Typically results in a single layer (monolayer).

This makes chemisorption distinct from Van der Waal's adsorption, which can form multi-layers due to weaker interactions.

Physical adsorption

Physical adsorption, often referred to as physisorption, is synonymous with Van der Waal's adsorption. It predominantly involves weak forces, such as Van der Waal's, similar to those in the noble gases attraction.
These attractions are not strong enough to effectively confine the process to a monolayer, allowing additional layers of adsorbate to form on top of one another.

The weak nature of these forces means physical adsorption is feasible across varying conditions, notably lower temperatures, and is usually reversible.

• Weak interactions allow for multiple layers.
• Usually occurs at low temperatures.
• Reversible process.

Thus, physical adsorption's flexibility in the multilayer creation is distinct, making it much different from the more rigorous process of chemical adsorption.