Which of the following statements is not applicable to chemisorption?

A. It is independent of temperature.

B. It is highly specific

C. It is slow

D. It is irreversible

Choose the correct answer from the options given below:

A only

The correct answer is option 4. A only.

Let us carefully analyze each of the characteristics of chemisorption (also known as chemical adsorption) in detail and explain why statement A is incorrect

Statement A: "It is independent of temperature."

The statement is incorrect. Chemisorption depends strongly on temperature. This is because chemisorption involves the formation of chemical bonds between the adsorbate (the substance being adsorbed) and the surface. The process typically requires activation energy to overcome, which means it can be slower at low temperatures and accelerates at higher temperatures as more molecules gain the energy required to form these bonds. However, at very high temperatures, desorption may also occur because the bonds may break due to the energy supplied. Therefore, chemisorption initially increases with temperature but may decrease beyond an optimal temperature.

Reasoning: Since chemisorption involves actual chemical reactions, it is not like physical adsorption (physisorption), which is less dependent on temperature. The rate of chemisorption is highly temperature-dependent, unlike what is suggested in this statement. This makes Statement A fals, as temperature plays a crucial role in chemisorption.

Statement B: "It is highly specific."

The statement is true. Chemisorption is highly specific because the process depends on the formation of chemical bonds, which only happens between specific adsorbate molecules and specific surfaces. For example, certain metals will chemisorb oxygen atoms, while others will not. The specificity arises because the nature of chemical bonding is dependent on the electronic structure of both the adsorbate and the surface atoms.

Reasoning: The formation of specific chemical bonds means that not every molecule can adsorb onto every surface. This distinguishes chemisorption from physisorption, which is more general and can happen with a wider range of adsorbates and surfaces

3. Statement C: "It is slow."

The statement is true. Chemisorption is generally slower than physisorption because it involves the formation of chemical bonds. In physisorption, weaker van der Waals forces are at play, which allows the process to happen almost instantaneously. Chemisorption, on the other hand, may require activation energy and is limited by the speed at which the chemical bonds can form.

Reasoning: The slowness of chemisorption comes from the need to overcome energy barriers associated with bond formation. Thus, compared to physical adsorption (which can happen quickly), chemisorption is typically slower.

4. Statement D: "It is irreversible."

The statement is true. Chemisorption is generally irreversible under normal conditions. This is because chemical bonds formed between the adsorbate and the surface are quite strong. Once the adsorbate forms these chemical bonds, it usually requires significant energy to desorb or break these bonds, making it difficult to reverse the process. In contrast, physisorption is easily reversible because it involves weaker forces.

Reasoning: Since chemical bonds are harder to break than physical attractions (like van der Waals forces), chemisorption is typically irreversible unless external conditions, such as high temperature or chemical reactions, provide enough energy to break the bonds.

Conclusion:

Statement A is incorrect because chemisorption is dependent on temperature. The process accelerates as temperature increases (due to the need for activation energy) and may decrease at very high temperatures due to desorption. Statements B, C, and D are true and align with the nature of chemisorption.

Thus, the correct answer is option 4: A only.