Question

The heat evolved in chemisorption lies in the range (in \(\mathrm{kJ} / \mathrm{mol}\) ) of : (a) 80 to 240 (b) 20 to 40 (c) 40 to 80 (d) 20 to 100

Short answer

The heat evolved in chemisorption lies in the range of 80 to 240 kJ/mol.

Step-by-step solution

Identify the Type of Adsorption

Chemisorption involves the formation of strong chemical bonds between the adsorbate and the surface of the adsorbent. This process is typically characterized by the release of a significant amount of energy.

Review Typical Energy Range for Chemisorption

The energy released during chemisorption is usually comparable to that of a chemical reaction. Therefore, the heat evolution for chemisorption is expected to be higher than physisorption, which is a physical adsorption process involving weaker forces like van der Waals forces.

Select the Correct Range

Based on the nature of the bonds formed during chemisorption and the significant energy released, the correct range of heat evolved should typically be higher than that for physisorption. Among the options given, the range that best corresponds to the energy usually released during chemical reactions is between 80 to 240 kJ/mol.

Key concepts

Understanding the Adsorption Process

The adsorption process is a fundamental concept in surface chemistry where particles, atoms, or molecules from a gas, liquid, or dissolved solid (collectively known as the adsorbate) adhere to a surface of a solid or a liquid (the adsorbent), creating a film of the adsorbate on the surface. Unlike absorption where the substance is fully integrated into the bulk material, adsorption denotes a surface phenomenon.

There are two primary types of adsorption:

• Physisorption: Physical adsorption primarily involving weaker intermolecular forces like van der Waals forces.
• Chemisorption: Chemical adsorption that involves the formation of strong chemical bonds at the adsorbent's surface.

Although both types result in the adhesion of molecules to surfaces, chemisorption is distinct because it is a more permanent and specific type of adsorption that forms stronger, more localized bonds, akin to a chemical reaction.

Chemical Bonds in Adsorption

Chemisorption is characterized by the formation of chemical bonds between the adsorbate and adsorbent surfaces. This bond formation can be seen as a chemical reaction where the adsorbate's molecules or atoms lose their individual integrity to some extent and start sharing or transferring electrons with the adsorbent's surface atoms.

Depending on the adsorbate and adsorbent materials involved, the bonding in chemisorption may involve covalent bonds, ionic bonds, or even metallic bonds. The strength of these bonds is comparable to that found in actual chemical compounds. Because of these strong interactions, chemisorption usually happens at specific sites on the adsorbent surface and often requires activation energy to initiate, much like a chemical reaction.

Heat Evolution in Chemisorption

The exothermic nature of the chemisorption process—that is, its tendency to release heat—plays a significant role in understanding its energetics. Similar to exothermic chemical reactions, when a chemical bond is formed in chemisorption, energy is released. This heat evolution can be considerable and is a tell-tale sign of chemisorption as opposed to physisorption, which involves much less energy exchange.

The amount of heat evolved during chemisorption is typically in the range of 80 to 240 kJ/mol, which is significant enough to be compared with the energy changes in chemical reactions. This release of energy also affects the kinetics and thermodynamics of the process, making chemisorption generally more stable and less reversible than physisorption.