Question

As shown in Figure 14.23 , the first step in the heterogeneous hydrogenation of ethylene is adsorption of the ethylene molecule on a metal surface. One proposed explanation for the "sticking" of ethylene to a metal surface is the interaction of the electrons in the \(\mathrm{C}-\mathrm{C} \pi\) bond with vacant orbitals on the metal surface. (a) If this notion is correct, would ethane be expected to adsorb to a metal surface, and, if so, how strongly would ethane bind compared to ethylene? (b) Based on its Lewis structure, would you expect ammonia to adsorb to a metal surface using a similar explanation as for ethylene?

Short answer

(a) Ethane is not expected to adsorb to a metal surface, as it does not have \(\pi\) electrons available for interaction with the vacant orbitals on the surface. Hence, it would not bind to the metal surface as ethylene does. (b) Yes, ammonia can adsorb to a metal surface due to the lone pair of electrons on the nitrogen atom, which can interact with the vacant orbitals on the surface. This interaction is similar to the ethylene case, although the specific details and strength of the interaction may differ.

Step-by-step solution

Understand the adsorption mechanism of ethylene on the metal surface

According to the given information, the adsorption of ethylene on a metal surface is due to the interaction of its \(\mathrm{C}-\mathrm{C}\) \(\pi\) bond electrons with the vacant orbitals on the metal surface. Ethylene has a C=C double bond, which has a \(\sigma\) bond that holds the atoms together and a \(\pi\) bond made up of the overlapping p orbitals.

Determine the binding of ethane on the metal surface

Ethane has a C-C single bond, so it does not have \(\pi\) electrons. Instead, the single bond between the carbons is a \(\sigma\) bond. To determine if ethane would bind to a metal surface, and how strongly, we need to compare the availability of electrons for interaction with vacant orbitals on the surface. Since ethane does not have \(\pi\) electrons, it would not be expected to adsorb to a metal surface as ethylene does.

Analyzing the Lewis structure of ammonia

Ammonia has a Lewis structure consisting of one nitrogen atom bonded to three hydrogen atoms, with a lone pair of electrons on the nitrogen atom: NH3. The lone pair of electrons on the nitrogen atom can engage in interactions with vacant orbitals on the metal surface, similar to the \(\pi\) electrons in ethylene.

(a) Answer for if ethane would adsorb to a metal surface and how strongly it would bind compared to ethylene

Ethane is not expected to adsorb to a metal surface, as it does not have \(\pi\) electrons available for interaction with the vacant orbitals on the surface. Hence, it would not bind to the metal surface as ethylene does.

(b) Answer for if ammonia would adsorb to a metal surface using a similar explanation as for ethylene

Yes, ammonia can adsorb to a metal surface due to the lone pair of electrons on the nitrogen atom, which can interact with the vacant orbitals on the surface. This interaction is similar to the ethylene case, although the specific details and strength of the interaction may differ.

Key concepts

Adsorption

Adsorption is a process where a gas, liquid, or dissolved solid adheres to the surface of another substance, usually a solid. This happens at the molecular level where molecules from the fluid phase stick to the surface of the solid. In the context of heterogeneous hydrogenation, adsorption is a critical first step.
Ethylene molecules adsorb onto metal surfaces by interacting with vacant orbitals on the surface using their \( \mathrm{C}-\mathrm{C} \pi \) bond electrons. This allows them to "stick" to the surface and undergo further chemical reactions. Since ethane lacks \pi \ electrons (it only has \( \sigma\) bonds), it does not adsorb well to metal surfaces, showcasing the importance of electron availability in adsorption.

• Ethylene: Adsorbs well due to \( \pi \) electrons.
• Ethane: Does not adsorb due to lack of \( \pi \) electrons.

Metal surface interaction

The interaction between adsorbed molecules and metal surfaces is crucial in many industrial processes like hydrogenation and catalysis. These interactions often involve the transfer or sharing of electrons between the adsorbate (the molecule sticking to the surface) and the metal.
In the case of ethylene, its \( \mathrm{C}-\mathrm{C} \pi \) bond electrons interact with vacant orbitals on the metal surface, forming temporary bonds. This allows the molecule to remain on the surface until the reaction is complete.
Ammonia can also interact with metal surfaces. Its lone pair of electrons on nitrogen can engage with the vacant orbitals similar to a \( \pi \) bond, facilitating adsorption. This ability to interact with metals highlights the role of electron-rich sites in adsorption and subsequent reactions.

• Vacant Orbitals: Critical for temporary bond formation.
• Electrons: From adsorbate interact with orbitals to enable adsorption.

Lewis structure

Lewis structures are simplified representations of molecules that show how atoms are bonded together and the arrangement of electrons around them. These diagrams are essential in predicting molecular behavior like adsorption.
Ethylene's Lewis structure reveals a double bond (\( \mathrm{C=C} \)) with both \( \sigma \) and \( \pi \) bonds, enabling interaction with metal surfaces. This bonding pattern makes ethylene a good candidate for metal adsorption.
For ammonia, its Lewis structure displays three \( \mathrm{N-H} \) bonds and a lone pair on the nitrogen. This lone pair is key for metal surface interactions, acting like a \( \pi \) bond in providing electrons for surface bonding.

• Ethylene: Double bond with \( \pi \) electrons aids adsorption.
• Ammonia: Lone pair on nitrogen facilitates metal interaction.