Question

Physical adsorption of a gaseous species may change to chemical adsorption with (a) Increase in temperature (b) Decrease in temperature (c) Decrease in surface area of adsorbent (d) Increase in surface area of adsorbent

Short answer

(a) Increase in temperature

Step-by-step solution

Understanding Adsorption Types

Physical adsorption (also known as physisorption) involves weaker van der Waals forces, whereas chemical adsorption (or chemisorption) involves stronger covalent bonds. Physisorption usually occurs at lower temperatures while chemisorption is favored at higher temperatures.

Analyzing Temperature's Effect

Increasing the temperature provides energy that can overcome the energy barrier associated with forming chemical bonds, making chemical adsorption more favorable. Conversely, decreasing the temperature tends to favor physical adsorption due to the lack of sufficient energy to form strong bonds.

Considering Surface Area Effects

While surface area can affect the amount of adsorption, it is more directly related to the capacity of an adsorbent rather than the type of adsorption. Therefore, changes in surface area do not typically affect the transition from physical to chemical adsorption.

Identifying the Correct Choice

Based on the analysis, an increase in temperature would lead to a transition from physical to chemical adsorption as it provides the energy needed for forming stronger chemical bonds.

Key concepts

Physical adsorption

Physical adsorption is a process where gases or liquids adhere to a surface due to weak van der Waals forces. These forces are much weaker compared to chemical bonds, which means that physical adsorption usually occurs without any significant alteration to the adsorbent material or the adsorbate. Physical adsorption is also commonly referred to as physisorption.

This type of adsorption is generally favored at lower temperatures. As the temperature decreases, the kinetic energy of the adsorbed molecules decreases, making it easier for them to settle on the surface. At the same time, there's no significant energy required to maintain the adsorption, as it relies on naturally occurring weak interactions.

In practical applications, physical adsorption is advantageous when temporary adhesion is required without altering the chemical structure of the adsorbate or adsorbent.

Chemical adsorption

Chemical adsorption, also known as chemisorption, involves the formation of strong chemical bonds between the adsorbent and the adsorbate. This occurs when the adsorbed species forms a new compound or share electrons with the surface. Because these bonds are as strong as or stronger than intramolecular bonds, chemisorption is highly specific and often irreversible.

Unlike physical adsorption, chemisorption generally occurs at higher temperatures, where thermal energy can overcome energy barriers required for bond formation. This energy allows atoms or molecules to rearrange or react on the surface, leading to strong and permanent attachments. Hence, its occurrence is more selective and dependent on the chemical nature of both the surface and the adsorbate.

In industrial applications, chemisorption is crucial for catalytic processes where strong bonds facilitate reactions on catalyst surfaces.

Temperature effect on adsorption

Temperature plays a crucial role in determining whether physical or chemical adsorption predominates. As temperature increases, the energy available to molecules increases as well. This added energy can sometimes convert physisorption into chemisorption due to the ability to overcome activation energy barriers needed for forming new chemical bonds.

• At low temperatures, physical adsorption is favored as molecules have insufficient energy to form strong covalent bonds.
• As temperatures rise, the increased energy results in a shift towards chemical adsorption as molecules overcome energy barriers to form strong bonds with the surface.

It is important to note that excessive temperature may lead to desorption, where adsorbed species lose their attraction to the surface and separate.

Surface area in adsorption

The surface area of an adsorbent significantly affects the capacity of adsorption. A larger surface area means there are more sites available for molecules to adhere to, thereby increasing the total amount of adsorbed substance. However, it does not necessarily change the nature of adsorption from physical to chemical.

• Increased surface area results in greater physisorption or chemisorption opportunities due to more available sites.
• While surface area enhances adsorption capacity, it does not inherently change physical adsorption into chemical adsorption.

In practical terms, materials with high surface areas, such as activated carbon or nanoporous materials, are excellent at adsorbing large amounts of adsorbates, making them useful in filtering and purification processes.