Problem 87 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is not a characteristic of chemisorption? (a) adsorption is specific (b) adsorption is irreversible (c) adsorption is a multimolecular layer (d) \(\Delta \mathrm{H}\) is of the order of \(400 \mathrm{~kJ}\)

Step-by-Step Solution

Answer

Option (c) is not a characteristic of chemisorption.

Step 1: Identify Characteristics of Chemisorption

Chemisorption is a type of adsorption where a chemical reaction occurs between the surface and the adsorbate. It is characterized by being specific, often forming a monolayer, irreversible, and involving a significant change in enthalpy (\(\Delta \mathrm{H}\) in the range of 100-400 \(\mathrm{kJ/mol}\)).

Step 2: Analyze Option A

Option (a), 'adsorption is specific', matches a characteristic of chemisorption since chemisorption involves specific bonding between the adsorbate and the surface.

Step 3: Analyze Option B

Option (b), 'adsorption is irreversible', also aligns with chemisorption properties because the bonds formed during chemisorption are strong enough to make the process irreversible.

Step 4: Analyze Option C

Option (c), 'adsorption is a multimolecular layer', does not align with chemisorption as chemisorption typically forms a monolayer due to its specificity and strength of the interaction.

Step 5: Analyze Option D

Option (d), '\(\Delta \mathrm{H}\) is of the order of \(400 \mathrm{~kJ}\)', describes the significant enthalpy change typical of chemisorption, supporting chemisorption characteristics.

Step 6: Conclusion: Correct option

The option that does not match a characteristic of chemisorption is (c) 'adsorption is a multimolecular layer.' Hence, option (c) is not a characteristic of chemisorption.

Key Concepts

Understanding Adsorption

Adsorption is a process where molecules from a gas or liquid attach themselves to a solid surface. The substance that accumulates on the surface is called the adsorbate, and the material providing the surface is named the adsorbent.
When it comes to adsorption, it is important to distinguish between physisorption and chemisorption.
Chemisorption involves a chemical reaction between the surface and the adsorbate, whereas physisorption occurs through weaker van der Waals forces.

• **Chemisorption:** Typically forms a strong, specific bond, leading to characteristics like a single-layer adsorption known as a monolayer.
• **Physisorption:** Generally involves weaker forces, allowing for multiple layers of molecules to adsorb on the surface.

These distinctions are pivotal because they dictate the nature and strength of the interactions involved in the adsorption process.

Enthalpy Change in Chemisorption

Enthalpy change (HH) is a measure of the heat change during a reaction, which provides insight into the energy dynamics at play.
In chemisorption, a significant amount of energy is involved due to the strong, specific bonds formed between the adsorbate and adsorbent.
The enthalpy change for chemisorption ranges from 100 to 400 kJ/mol.
This is considerably higher than physisorption, which typically has an enthalpy change less than 100 kJ/mol.

• This high enthalpy change indicates that chemisorption often involves an irreversible process.
• The energy requirement signifies the breaking and forming of chemical bonds, unlike physisorption which just involves energy enough to overcome weak interactions.

The understanding of enthalpy change is crucial in predicting the behavior and feasibility of adsorption processes in different applications, such as catalysis.

Formation of a Monolayer in Chemisorption

A monolayer is a single, uniform layer of molecules that forms on a surface during chemisorption.
This is characteristic of the high specificity involved in chemical bonding during the process.
In chemisorption, an adsorbate forms a strong bond with the adsorbent until the entire surface is covered by a single molecular layer.

• The formation of a monolayer is fundamentally tied to the specificity and strength of chemisorptive interactions.
• Unlike physisorption, which allows for multilayer coverage due to weaker forces, chemisorption ceases to occur once a monolayer is reached.

Understanding monolayer formation is important in industries and applications where surface interactions play a key role, such as in sensor design and heterogeneous catalysis.