Understanding Chemisorption Properties

Chemisorption is a type of adsorption that involves a chemical reaction between the surface and the adsorbate. New chemical bonds are formed at the surface. This process is distinct from physisorption, which only involves weaker van der Waals forces.

Analyzing Each Statement on Chemisorption

Let's examine each statement provided in the question to determine its applicability to chemisorption:

(A) It is independent of temperature

Chemisorption typically involves an activation energy, similar to chemical reactions. Initially, as temperature increases, the rate and extent of chemisorption often increase because more molecules gain enough energy to overcome the activation barrier. However, at very high temperatures, the adsorbate molecules may gain enough energy to break the chemical bonds with the surface, leading to desorption, and thus the amount adsorbed decreases. Therefore, chemisorption is highly dependent on temperature, not independent of it. The relationship is often not simple, showing an initial increase followed by a decrease.

(B) It is highly specific

Chemisorption requires the formation of chemical bonds between the adsorbent surface and the adsorbate molecule. This means that it can only occur when there is a specific chemical affinity or reactivity between the two substances. For example, oxygen chemisorbs on metals, but not on noble gases. This requirement for specific chemical interaction makes chemisorption highly specific.

(C) It is slow

Because chemisorption involves the formation of chemical bonds and often requires overcoming an activation energy barrier, the process can be relatively slow, especially compared to physisorption which is often rapid. The rate depends on various factors including temperature, pressure, and the nature of the adsorbent and adsorbate.

(D) It is irreversible

The formation of chemical bonds in chemisorption is generally a strong interaction. Reversing this process (desorption) requires breaking these chemical bonds, which typically requires significant energy (often supplied by high temperature or low pressure). Under normal conditions, the desorption is very slow or negligible, making chemisorption largely irreversible compared to physisorption, which is readily reversible by changing temperature or pressure.

Identifying the Statement Not Applicable to Chemisorption

Based on the analysis:

• Statement (A) says chemisorption is independent of temperature. This is incorrect; chemisorption is highly dependent on temperature.
• Statement (B) says chemisorption is highly specific. This is correct.
• Statement (C) says chemisorption is slow. This is generally correct.
• Statement (D) says chemisorption is irreversible. This is largely correct.

The statement that is NOT applicable to chemisorption is that it is independent of temperature.

Comparing Chemisorption and Physisorption

To further understand chemisorption, it's useful to compare it with physisorption:

Revision Table: Key Chemisorption Properties

Here's a summary of the key properties of chemisorption:

The statement "It is independent of temperature" is the one that does not apply to chemisorption.

Additional Information on Chemisorption

Chemisorption is a crucial phenomenon in many surface processes, particularly in heterogeneous catalysis. The catalytic activity of a solid surface often depends on the ability of reactant molecules to chemisorb onto the surface and form activated intermediates. The strength of the chemisorption bond plays a critical role in determining the reaction pathway and rate.

The activation energy for chemisorption is the minimum energy required for the adsorbate molecule to react with the surface to form a chemical bond. This energy barrier explains why the rate of chemisorption is often temperature-dependent. The surface structure and composition of the adsorbent significantly influence the chemisorption process, affecting both the rate and the sites available for chemical bond formation.