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 \(\pi_{2 p}^{*}\) of \(\mathrm{O}_{2}\) is increased (D) bond length of \(\mathrm{O}_{2}\) is increased

Short answer

The TRUE statements regarding the adsorption of \(\mathrm{O}_{2}\) on a metallic surface are (B) heat is released, (C) occupancy of \(\pi_{2 p}^{*}\) of \(\mathrm{O}_{2}\) is increased, and (D) bond length of \(\mathrm{O}_{2}\) is increased.

Step-by-step solution

- Understand Adsorption Types

There are two main types of adsorption: physisorption, which involves weak van der Waals forces, and chemisorption, which involves the formation of chemical bonds. When electron transfer is involved, it typically indicates chemisorption because it implies a change in the electronic structure of the adsorbed molecule.

- Energy Changes in Adsorption

Adsorption is generally an exothermic process; this means that when a molecule is adsorbed on a surface, energy is released in the form of heat.

- Consideration of Molecular Orbitals

When \(\mathrm{O}_{2}\) receives electrons, they are filled into the antibonding \(\pi_{2 p}^{*}\) molecular orbitals. Filling antibonding orbitals weakens the bond. Thus, increasing the occupancy of these orbitals would increase bond length.

- Analyze the Statements

Based on the principles outlined in the previous steps, assess each statement: (A) is false because electron transfer indicates chemisorption not physisorption; (B) is true because adsorption typically releases heat; (C) is true because electron transfer to \(\mathrm{O}_{2}\) increases occupancy of its antibonding orbitals; (D) is likely true as the antibonding orbitals are being filled, the bond length of \(\mathrm{O}_{2}\) is expected to increase.

Key concepts

Physisorption vs Chemisorption

Understanding the distinction between physisorption and chemisorption is fundamental in surface chemistry. Physisorption, often the initial stage of adsorption, involves the adherence of molecules to a surface due to weak intermolecular forces, like van der Waals attractions. It's typically reversible and occurs at low temperatures.

In contrast, chemisorption is a much stronger form of adsorption because it involves the formation of chemical bonds between the adsorbate - the molecule being adsorbed - and the substrate - the surface it's being adsorbed onto. This type of adsorption is usually irreversible, requires higher energies to occur, and is highly specific to the interacting surfaces. When we observe electron transfer as in the example of \(\mathrm{O}_{2}\) being adsorbed onto a metallic surface, we're looking at a clear case of chemisorption, as new chemical bonds are being formed.

Exothermic Adsorption Process

Adsorption is generally an exothermic process, which means it releases heat. This release occurs because the potential energy of the adsorbate decreases as it forms a more stable configuration on the adsorbent surface. Heat release during adsorption can range from moderate to significant, depending on the nature of the bonds being formed.

In physisorption, the heat released is relatively low compared to chemisorption because the interactions involved are weaker. In chemisorption, the energy released can be quite substantial due to the strength of the chemical bonds. This release of energy is a critical factor in the industrial application of adsorption processes, such as in catalysis, where exothermic adsorption heats can affect reaction rates.

Molecular Orbitals in Adsorption

The concept of molecular orbitals is crucial to understanding how adsorption alters the electronic structure of molecules. Molecular orbitals come in two types - bonding and antibonding. Bonding orbitals serve to maintain the molecule's stability by keeping electrons between atoms, while antibonding orbitals, often denoted with an asterisk as \(\pi_{2 p}^{*}\), can destabilize the bond if electrons are added to them.

In the case of \(\mathrm{O}_{2}\) adsorption on a metallic surface, electrons from the metal fill the antibonding orbitals of \(\mathrm{O}_{2}\). When these orbitals are populated, they reduce the bond strength between the oxygen atoms, owing to the repulsive force they introduce, which in turn affects other properties of the molecule, such as bond length.

Bond Length Alteration in Adsorbed Molecules

When molecules are adsorbed onto a surface and interact with the surface atoms, there can be a change in the bond lengths within the adsorbed molecules. The alteration in bond length is particularly noticeable during chemisorption, where the strong interactions with the surface could lead to a significant change in the electronic distribution within the molecule.

For instance, when \(\mathrm{O}_{2}\) is chemisorbed and its antibonding orbitals are filled with electrons, the bond length between the oxygen atoms increases. This is because the added electrons in the \(\pi_{2 p}^{*}\) orbitals generate electron-electron repulsion which outweighs the bonding interactions, pushing the atoms apart. This phenomenon highlights how the adsorption process not only affects the chemical reactivity of the adsorbate but also its physical properties.