Question

Which of the following statements are correct about chemisorption?

• It is highly specific in nature.

• It is reversible in nature.

• It depends on nature of gas.

• It is unilayer adsorption.
The correct answer is:

• A. I \& II only
• B. I \& IV only
• C. I, II \& IV only
• D. I, III \& IV only

Hint:

To quickly remember the properties of chemisorption:

• Strong chemical bond formation.

• Highly specific in nature.

• Usually irreversible.

• Forms only a monolayer.

• Depends on the chemical nature of the gas and adsorbent.
A useful memory aid is: “Specific, Strong, Single-layer” for chemisorption.

Answer 1

The Correct Option is D

Concept: Adsorption is the phenomenon in which molecules of a gas or a liquid accumulate on the surface of a solid. Depending upon the nature of the forces involved between the adsorbent and the adsorbate, adsorption is classified into two major types:

• Physical Adsorption (Physisorption)

• Chemical Adsorption (Chemisorption)
Chemisorption involves the formation of strong chemical bonds between the adsorbate molecules and the surface of the adsorbent. Since actual chemical bonds are formed, chemisorption exhibits properties that are distinctly different from those of physical adsorption. Important characteristics of chemisorption include:

• It is highly specific in nature.

• It depends upon the chemical nature of both the adsorbent and adsorbate.

• It generally forms a single molecular layer on the surface.

• It is usually irreversible because strong chemical bonds are formed.

• It often requires activation energy and is favored at comparatively higher temperatures.
We shall now examine each statement individually.

Step 1: Analysis of Statement I
Statement I says: ``It is highly specific in nature.'' This statement is correct. In chemisorption, the adsorbate molecules form chemical bonds with the surface atoms of the adsorbent. Since chemical bond formation depends on the electronic configuration and chemical compatibility of the substances involved, only certain gases can be chemisorbed by a particular adsorbent. For example:

• Hydrogen is chemisorbed effectively by nickel.

• Nitrogen may not show the same behavior on the same surface.
Thus, chemisorption is highly selective and specific. Therefore, \[ \boxed{\text{Statement I is correct}} \]

Step 2: Analysis of Statement II
Statement II says: ``It is reversible in nature.'' This statement is incorrect. Chemisorption involves the formation of strong chemical bonds between the adsorbate and the adsorbent surface. Once these bonds are formed, considerable energy is required to break them. As a result:

• Desorption does not occur easily.

• The process generally cannot be reversed under ordinary conditions.
Therefore, chemisorption is usually regarded as an irreversible process. Hence, \[ \boxed{\text{Statement II is incorrect}} \]

Step 3: Analysis of Statement III
Statement III says: ``It depends on nature of gas.'' This statement is correct. Since chemisorption is based on chemical interactions, the extent of adsorption depends strongly upon the chemical properties of the gas being adsorbed. Different gases have different tendencies to form chemical bonds with a given surface. For example:

• Hydrogen may chemisorb readily on certain metal surfaces.

• Noble gases generally show very little chemisorption because of their inert nature.
Therefore, the nature of the gas plays a crucial role in chemisorption. Hence, \[ \boxed{\text{Statement III is correct}} \]

Step 4: Analysis of Statement IV
Statement IV says: ``It is unilayer adsorption.'' This statement is also correct. In chemisorption, once a molecule occupies an active site and forms a chemical bond, further adsorption at that same site is not possible. As a consequence, adsorption is generally restricted to only one layer of molecules. Thus chemisorption produces: \[ \boxed{\text{Monolayer (Unilayer) adsorption}} \] and not multilayer adsorption. Therefore, \[ \boxed{\text{Statement IV is correct}} \]

Step 5: Final Conclusion
Among the four statements: I = Correct II = Incorrect III = Correct IV = Correct Therefore, the correct combination is: \[ \boxed{\text{I, III and IV only}} \] Hence the correct answer is: \[ \boxed{\text{Option (D)}} \]