Question

Which among the following is true for chemisorption?

• A. Heat of adsorption is in the range of 20 - 40 $\mathrm{kJ\ mol}^{-1}$.
• B. It is multimolecular layered.
• C. van der Waals forces are involved.
• D. It is favoured at high temperature up to certain limit.

Hint:

Think of chemisorption as a real chemical reaction taking place on a surface. Just like most chemical reactions need heat to overcome an activation energy barrier to get started, chemisorption requires higher initial temperatures than physical adsorption to proceed effectively!

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks to identify the correct physical or chemical characteristic of chemical adsorption (chemisorption) from the given choices.

Step 2: Key Formula or Approach:
Chemisorption involves the formation of true chemical bonds (covalent or ionic) between the adsorbate molecules and the adsorbent surface. This fundamentally differentiates it from physical adsorption (physisorption), which relies on weak intermolecular forces.

Step 3: Detailed Explanation:
Let's analyze each statement systematically:
Option (A) states that the heat of adsorption is between 20 - 40 $\mathrm{kJ\ mol}^{-1}$. This low energy value is a hallmark of physisorption. Chemisorption involves bond formation and has a much higher heat of adsorption, typically ranging from 80 - 240 $\mathrm{kJ\ mol}^{-1}$.
Option (B) describes a multimolecular layer structure. This happens only in physisorption because van der Waals forces can extend past the first layer. Chemisorption stops once all active surface sites form a chemical bond, making it strictly monomolecular (a single layer).
Option (C) mentions van der Waals forces. These weak attraction forces are responsible for physisorption, whereas chemisorption requires chemical overlapping or electron transfer.
Option (D) states that it is favored at high temperatures up to a certain limit. Chemisorption is an activated process that requires an initial activation energy. Raising the temperature provides this energy, increasing the initial rate and extent of adsorption. However, because adsorption is globally exothermic, extremely high temperatures eventually cause desorption by breaking the chemical bonds. Therefore, this statement is perfectly true.

Step 4: Final Answer:
The correct statement for chemisorption is that it is favored at high temperatures up to a certain limit, which matches option (D).