Problem 17 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

When \(\mathrm{O}_{2}\) is adsorbed on a metallic surface, electron transfer occurs from the metal to \(\mathrm{O}_{2}\). The true statement(s) regarding this adsorption is(are)(a) \(\mathrm{O}_{2}\) is physisorbed (b) Heat is released (c) Occupancy of \(\pi^{*} 2 \rho\) of \(\mathrm{O}_{2}\) is increased (d) Bond length of \(\mathrm{O}_{2}\) is increased

Step-by-Step Solution

Answer

The true statements are (b), (c), and (d).

Step 1: Understanding the Nature of Adsorption

When a molecule like \(\mathrm{O}_{2}\) is adsorbed onto a metallic surface, it can either be physisorption (weak van der Waals forces) or chemisorption (stronger chemical bonds). The transfer of electrons from the metal to \(\mathrm{O}_{2}\) suggests the formation of a chemical bond, thus indicating chemisorption, not physisorption.

Step 2: Analyzing Electron Transfer

The transfer of electrons from the metal to \(\mathrm{O}_{2}\) results in forming metal-oxygen bonds. This process is exothermic as chemical bonds release energy when formed, indicating heat is released during adsorption.

Step 3: Effect on Molecular Orbitals

As electrons are added to \(\mathrm{O}_{2}\), they populate the antibonding \(\pi^{*}\) orbitals. This increase in occupancy of the \(\pi^{*} 2p\) orbitals weakens the \(\mathrm{O}_{2}\) bond, corroborating the understanding of electron transfer dynamics.

Step 4: Implication on Bond Length

Increasing electrons in the antibonding \(\pi^{*}\) orbitals decreases bond order, which translates to an increased bond length due to weakened bonding.

Key Concepts

Electron Transfer

When oxygen (\(\mathrm{O}_{2}\)) is adsorbed onto a metallic surface, a fascinating phenomenon occurs: electron transfer. This process involves electrons moving from the metal to the \(\mathrm{O}_{2}\), leading to the formation of chemical bonds. The electrons are transferred to the lowest unoccupied molecular orbital of the \(\mathrm{O}_{2}\) molecule, facilitating chemisorption.
This electron movement is central to creating a more stable state. It forms a chemical bond between the metal and the oxygen molecule. Because electrons carry energy, their movement can be crucial in determining how and why molecules bond in certain ways under different conditions.

Antibonding Orbitals

The presence of antibonding orbitals in a molecule, such as \(\mathrm{O}_{2}\), is essential in understanding its chemical behavior. These orbitals, specifically noted as \(\pi^{*}\) orbitals when speaking about \(\mathrm{O}_{2}\), are higher in energy. Electrons that enter these orbitals decrease the overall stability of the bond.
When an electron transfer occurs during \(\mathrm{O}_{2}\) adsorption on a metallic surface, these antibonding orbitals become more populated. This increase in occupancy weakens the existing \(\mathrm{O}_{2}\) bonds, as more electrons in antibonding orbitals mean lesser bond strength between atoms within the molecule. Understanding these dynamics helps explain changes in molecular behavior and properties during adsorption.

Bond Length

The bond length is a direct consequence of interactions at the atomic level within a molecule. For \(\mathrm{O}_{2}\), when it undergoes electron transfer on a metallic surface, its bond length increases. This phenomenon can be attributed to the electrons populating the antibonding orbitals, as previously mentioned.
The increase in the number of electrons in antibonding orbitals reduces the bond order of \(\mathrm{O}_{2}\). When bond order decreases, the bond strength lessens, causing atoms to move slightly apart, resulting in increased bond length. This change is critical for understanding the molecular adjustments that occur during chemical processes such as adsorption.

Heat Release

Whenever chemical bonds are formed, energy dynamics play an important role. The process of adsorption of \(\mathrm{O}_{2}\) on a metallic surface is exothermic, meaning it releases energy in the form of heat.
As electrons transfer from the metal to \(\mathrm{O}_{2}\), bonds form that stabilize the system. This transition releases energy, contributing to the warmth that often accompanies reactions in chemistry. An exothermic reaction like this hints at the favorable nature of the chemical bonding that establishes during adsorption, making the newly formed structure more stable.

Metallic Surface Adsorption

Adsorption on a metallic surface represents a valuable process in surface science, impacting fields like catalysis and material science. The adsorption of \(\mathrm{O}_{2}\) involves the molecular interactions between the oxygen and the metallic surface. Unlike physisorption, which depends merely on weak van der Waals forces, chemisorption involves stronger bonds due to electron transfer.
Electron transfer to \(\mathrm{O}_{2}\) on a metallic surface not only leads to bond formation but alters the properties of both the adsorbate and the substrate. The strong interaction changes the surface's reactivity, transforming the \(\mathrm{O}_{2}\) from a simple diatomic molecule to a reactive participant in chemical processes.