Question

Which of the following is not a favourable condition for physical adsorption? (a) Negative \(\Delta \mathrm{H}\) (b) High pressure (c) High temperature (d) Higher critical temperature of adsorbate

Short answer

High temperature (c) is not favorable for physical adsorption.

Step-by-step solution

Understanding Physical Adsorption

Physical adsorption, also known as physisorption, involves the adherence of gas molecules to a solid surface due to weak van der Waals forces. This process is exothermic (\(\Delta \mathrm{H} < 0\)) and generally favors low temperatures and high pressures as these conditions enhance the adherence of gas molecules to the surface.

Evaluate Each Option

Let's evaluate the given options:- (a) **Negative \(\Delta \mathrm{H}\)**: This is favorable for physical adsorption because it indicates the process releases energy, confirming attraction.- (b) **High pressure**: This increases gas molecule collisions with the surface, promoting adsorption.- (c) **High temperature**: High temperatures supply energy to adsorbed molecules, promoting their escape from the surface and reducing adsorption.- (d) **Higher critical temperature of adsorbate**: This means that the molecule is more readily adsorbed at lower temperatures.

Identify the Unfavorable Condition

From the evaluations, a high temperature does not favor physical adsorption because it decreases the tendency of gases to adsorb by providing enough energy for molecules to desorb rather than adhere to the surface.

Conclusion

Among the options, high temperature is not a favorable condition for physical adsorption as it diminishes the efficacy of the adsorption process by encouraging desorption over adsorption.

Key concepts

Exothermic Process

In the realm of physical adsorption, processes are often categorized based on their energy dynamics, specifically how energy is exchanged. An exothermic process refers to a reaction or change that releases energy, usually in the form of heat, to its surroundings. For physical adsorption, this means that when gas molecules adhere to a solid surface, energy is released. This release of energy is what makes the change exothermic.

When we say that a process has a negative \( \Delta \mathrm{H} \), it implies that the enthalpy change (or heat content) of the system decreases. In simple terms, the system loses energy, which is favorable for physical adsorption. The molecules release energy as they bind to the solid surface, resulting in a more stable configuration. This stability and energy release make surfaces generally attractive to gas molecules under cooler conditions.

Van der Waals Forces

Van der Waals forces are the subtle but essential interactions that enable the process of physical adsorption. These forces are weak and not of the same strength as covalent or ionic bonds but are nonetheless crucial for attracting gas molecules to a solid surface in the context of physisorption.

More precisely, van der Waals forces are short-range forces that arise from temporary polarity induced in molecules when electrons move. These tiny attractive forces operate when molecules are very close together.

• These interactions are not very specific;
• They do not involve electron sharing, as in covalent bonding;
• And they decrease rapidly with distance, making them significant only over short separations.

This quality makes van der Waals forces perfectly suited for physical adsorption where a non-permanent, superficial attachment occurs, just enough to hold gas molecules close to the surface without chemically altering them.

Adsorption Conditions

The effectiveness of physical adsorption is heavily influenced by the prevailing conditions under which it occurs. Several conditions considerably enhance or hinder the adsorption process:

• **Low Temperature:** Adsorption is favored at lower temperatures. High temperatures provide energy that can cause molecules to break away from the surface. This is why high temperatures are not conducive to effective adsorption.
• **High Pressure:** Increasing the pressure of the gas encourages more molecules to come into contact with the surface, enhancing adsorption. Thus, high pressure is a favorable condition.
• **Type of Adsorbate:** Molecules with higher critical temperatures can be adsorbed more effectively under the same temperature conditions compared to those with lower critical temperatures because they are more manageable and less energetically demanding to maintain closer to their liquid state.

These conditions, in conjunction with the inherent nature of the gas and surface material, determine the success and efficiency of the adsorption process.

Thermodynamics

Thermodynamics, the study of energy exchanges in physical and chemical processes, is vital for understanding adsorption from a molecular perspective. By looking at variables like temperature and pressure, thermodynamics gives insight into why certain conditions either favor or disfavor adsorption.

**Key Thermodynamic Concepts in Adsorption:**

• **Enthalpy Change (\( \Delta \mathrm{H} \)):** The sign and value of enthalpy change are significant in adsorption processes.A negative enthalpy change indicates that the process is exothermic, meaning the system releases energy, facilitating adsorption.
• **Entropy (\( \Delta \mathrm{S} \)):** Adsorption decreases the freedom of gas molecules, leading to a decrease in entropy. However, the decrease is often offset by the exothermic nature of the process.
• **Gibbs Free Energy (\( \Delta \mathrm{G} \)):** For adsorption to occur spontaneously, the Gibbs free energy of the system must decrease.Thus, even with a decrease in entropy, the process can proceed if the enthalpy change compensates to reduce free energy.

Thus, thermodynamics takes a clear picture of how the delicate balance of energy and molecular freedom dictates the favorability of adsorption under various conditions.