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

The incorrect statements are in option (c) 1,2 and 4.

Step 1: Identify Chemical Adsorption Characteristics

In chemical adsorption (chemisorption), the interaction between the adsorbent and adsorbate is strong, involving chemical bonds. Enthalpy changes are significant and typically greater than -20.0 ext{kJ mol}^{-1} . Furthermore, chemisorption forms a monomolecular layer, due to strong specific interactions between the surface and adsorbate.

Step 2: Check Each Option with Characteristics

Option (1) matches chemisorption, as it has high adsorption enthalpy. Option (2) is incorrect, since van der Waals forces are associated with physisorption, not chemisorption. Option (3) aligns with chemisorption as it generally involves a monomolecular layer. Option (4) is incorrect because a multimolecular layer is typical for physisorption, not chemisorption.

Step 3: Determine Which Options Are Incorrect

From the analysis: statement (2) is incorrect because of the mention of van der Waals forces, and statement (4) is incorrect because of the reference to multimolecular layers. Options (1) and (3) align with typical chemisorption characteristics.

Step 4: Identify the Incorrect Combination

Since statement (2) and statement (4) are incorrect, the correct choice will be the one that includes these incorrect statements — (c) 1,2 and 4.

Key Concepts

Chemisorption

Chemisorption, also known as chemical adsorption, involves the formation of strong chemical bonds between the adsorbate and the adsorbent. This process requires a significant amount of energy due to these bonds being quite strong.
The interaction results in a monolayer on the surface, which is generally specific to the type of material involved.
This specificity and the strong bonding distinguish chemisorption from other types of adsorption such as physisorption. It is typically characterized by high adsorption enthalpy, often greater than -20 ext{kJ mol}^{-1}. . Such strong interactions lead to a single, firmly attached layer of molecules on the surface.

Physisorption

Physisorption, or physical adsorption, is characterized by weak van der Waals forces, making it quite different from chemisorption. Unlike chemisorption, physisorption involves lower energy changes, resulting in the formation of multiple layers, rather than a single monolayer.
This process is not very specific, as it does not involve the creation of actual chemical bonds.
Instead, the adsorbate molecules are held by comparatively weaker forces, allowing them to easily accumulate or be removed from the surface. Due to its low energy requirements, physisorption is generally reversible and occurs readily at low temperatures.

Adsorption Enthalpy

Adsorption enthalpy reflects the energy change during the process of adsorption, representing how tightly the adsorbate molecules adhere to the surface of the adsorbent.
In chemisorption, this value is relatively high, owing to the strong chemical bonds formed, typically being greater than -20 ext{kJ mol}^{-1} .
On the other hand, during physisorption, the adsorption enthalpy is much lower due to weaker interactions. This value can provide insight into the nature of the adsorption process—whether physical or chemical.

Monomolecular Layer

A monomolecular layer refers to a single layer of molecules adsorbed onto a surface. In chemisorption, this is a characteristic feature due to the strong and specific nature of the bonding involved.
The chemical bonds formed in chemisorption generally allow only a single layer of molecules to be attached to the surface.
This contrasts with physisorption, where multiple layers can form due to weaker attractive forces. The formation of a monomolecular layer during chemisorption is often indicative of the process's specificity and strength.

Van der Waals Forces

Van der Waals forces are weak intermolecular forces that play a significant role in physisorption. They include attractions between molecules that do not form strong chemical bonds, unlike those seen in chemisorption.
These forces allow for the formation of multimolecular layers, as the interactions are not strong enough to limit adsorption to a single monolayer.
Since they are weak, van der Waals forces are often associated with processes requiring lower energy and tend to be more reversible. They allow adsorbates to easily attach and detach from the surface, which is why physisorption is less permanent than chemisorption.