Problem 9 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

A particular adsorption process has the following characteristics : (i) It arises due to van der Waals forces and (ii) it is reversile. Identify the correct statement that describes the above adsorption process:(a) Adsorption is monolayer. (b) Adsorption increases with increase in temperature. (c) Enthalpy of adsorption is greater than \(100 \mathrm{~kJ} \mathrm{~mol}^{-1}\) (d) Energy of activation is low.

Step-by-Step Solution

Answer

The correct statement is (d) Energy of activation is low.

Step 1: Understand the Given Characteristics

The problem states that the adsorption process is due to van der Waals forces and is reversible. This means that the process is physical adsorption (physisorption), which involves weak forces and is usually reversible in nature.

Step 2: Analyze Characteristic of Physisorption

Physisorption is typically characterized by low enthalpy of adsorption, as it involves weak van der Waals forces. The enthalpy change is usually less than \( 40 \text{ kJ mol}^{-1} \), indicating that option (c) is incorrect since it states greater than 100 \text{ kJ mol}^{-1}.

Step 3: Evaluate the Effect of Temperature

Physisorption decreases with an increase in temperature because it is an exothermic process. This means that option (b), which states adsorption increases with temperature, is incorrect.

Step 4: Determine the Activation Energy

The activation energy for physisorption is generally low, as weak forces are involved and the molecules do not need to overcome a significant energy barrier. Thus, option (d) is plausible.

Step 5: Determine if Monolayer is Likely

Physisorption can occur in multiple layers, which means monolayer adsorption is not exclusive to this process. Therefore, option (a) is not necessarily true for this type of adsorption characterized by van der Waals forces.

Key Concepts

Van der Waals forces

Van der Waals forces are gentle attractions between atoms or molecules that determine numerous physical properties in material science. These forces are important in physisorption, signifying the weak, non-covalent interactions that permit the attachment of a gas or liquid molecule to a solid surface.
These forces emerge due to temporary dipoles generated by fluctuating electrons within atoms or molecules. As the electrons shift and create these momentary forces, they enable gases or liquids to form thin layers on surfaces without creating strong chemical bonds.
In the context of adsorption, these forces are responsible for holding the adsorbate molecules close to the surface in a reversible and generally multi-layer fashion. Since van der Waals forces are quite weak compared to chemical bonds, the molecules can easily detach from the surface upon slight changes in conditions such as temperature or pressure. This characteristic makes processes involving van der Waals forces highly adaptable and significant in industries like catalysis, gas storage, and separation processes.

Reversible adsorption

Reversible adsorption refers to the capability of an adsorbate to detach from the adsorbent surface and return to the bulk phase. This process is typical of physisorption, contrasting with chemisorption, where stronger covalent bonds are usually formed, making the process less reversible.

• Weak Interactions: The reversible nature is due to weak forces like van der Waals that do not create strong, permanent bonds with the surface.
• Temperature Influence: With increasing temperature, the adsorbate molecules gain kinetic energy, which makes detaching from the surface easier and hence is more reversible.
• Applications: This makes reversible adsorption useful in gas purification systems, where the ability to easily regenerate the adsorbent by altering pressure or temperature is vital.

Reversible adsorption is important for batch and cyclic operations in various industries, allowing for the reusability of materials and sustainable processes.

Enthalpy of adsorption

Enthalpy of adsorption, often known as the heat of adsorption, refers to the heat change that occurs when a mole of a substance is adsorbed. In physisorption, this enthalpy change is relatively low, typically less than 40 kJ/mol, because only weak van der Waals forces are involved.
This low enthalpy indicates that physisorption is an almost ambient condition phenomenon, where minimal energy is required to both adsorb and desorb the molecules. This makes such adsorption processes exothermic, releasing heat but not enough to disrupt significant energy barriers.
For students looking to understand this concept, remember that the lower the enthalpy, the less 'sticky' the adsorption, aligning with the reversible and weak nature of such processes. This differentiates physisorption from chemisorption, where the enthalpy of adsorption is significantly higher due to the formation of stronger and more permanent chemical bonds.