Surface chemistry deals with the phenomenon that occurs at the surfaces or interfaces. The interface or surface is represented by separating the bulk phases by a hyphen or a slash. For example, the inetrface between a solid and a gas may be represented by solid-gas or solid/gas. Due to complete miscibility, there is no interface between the gases. The bulk phases that we come across in surface chemistry may be pure compounds or solutions. The interface is normally a few molecules thick but its area depends on the size of the particles of bulk phases. Many important phenomena, noticeable amongst these being corrosion, electrode processes, heterogeneous catalysis, dissolution and crystallization occur at interfaces. The subject of surface chemistry finds many applications in industry, analytical work and daily life situations.

Which statement is not correct for chemisorption?

It is reversible

The correct answer is option 2. It is reversible.

Let us delve into the concept of chemisorption and why the statement "It is reversible" is not correct.

Chemisorption (chemical adsorption) is a type of adsorption where the adsorbate molecules form strong chemical bonds with the surface of the adsorbent. This process is distinct from physisorption, where the adsorption is due to weak van der Waals forces.

Characteristics of Chemisorption:

Highly Specific: Chemisorption is highly specific because it involves the formation of chemical bonds, which depend on the nature of both the adsorbate and the adsorbent. Only certain molecules can chemisorb on a given surface, making the process selective and specific.

Irreversible (Not Reversible):

Chemisorption is generally irreversible. This is because the chemical bonds formed during chemisorption are strong, and breaking these bonds requires significant energy. Once a molecule is chemisorbed onto a surface, it typically stays there until a substantial amount of energy is applied to reverse the process (e.g., by heating to a very high temperature).

Formation of a Unimolecular Layer:

Chemisorption usually results in the formation of a unimolecular layer (a monolayer) on the surface of the adsorbent.  This is because the adsorption sites on the surface become saturated after the first layer of adsorbate molecules has formed strong chemical bonds.

Favoured at High Temperature:

Chemisorption is often favored at higher temperatures compared to physisorption because the process typically involves an activation energy barrier. Higher temperatures provide the necessary energy to overcome this barrier and form strong chemical bonds.

Chemical Bond Formation:

In chemisorption, the adsorbate molecules form covalent or ionic bonds with the surface atoms of the adsorbent.

These bonds are much stronger than the weak van der Waals forces involved in physisorption.

Energy Requirements:

To desorb (remove) the adsorbed molecules, the energy needed to break these chemical bonds is usually much higher than the energy required to simply reverse physisorption.  As a result, chemisorption is not easily reversible under normal conditions.

Examples:

Hydrogen on Platinum: Hydrogen gas chemisorbs on the surface of platinum by forming a strong bond with the metal atoms. This process is specific and irreversible under normal conditions.

Oxygen on Metal Oxides: Oxygen chemisorbs on metal oxides to form metal-oxygen bonds, which are stable and not easily reversed.

The statement "It is reversible" is not correct because chemisorption involves the formation of strong chemical bonds that are typically irreversible under normal conditions.
The correct answer is option 2: It is reversible.