Problem 193 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following statement is incorrect regarding physissorptions? [2009] (a) More easily liquefiable gases are adsorbed readily. (b) Under high pressure it results into multi molecular layer on adsorbent surface. (c) Enthalpy of adsorption \(\left(\Delta \mathrm{H}_{\text {adsorption }}\right)\) is low and positive (d) It occurs because of van der Waal's forces.

Step-by-Step Solution

Answer

Statement (c) is incorrect; enthalpy of physisorption is low and negative.

Step 1: Understand Physisorption

Physisorption refers to the process where adsorbates adhere to the surface of adsorbents due to weak van der Waals forces. It is a physical and reversible process distinct from chemisorption, which involves chemical bond formation.

Step 2: Analyze Statement (a)

Statement (a) claims that more easily liquefiable gases are adsorbed readily. This is true for physisorption because gases that are easily liquefiable have stronger van der Waals forces, promoting adsorption.

Step 3: Analyze Statement (b)

Statement (b) notes that physisorption under high pressure results in a multi-molecular layer on the adsorbent surface. This is also true because physisorption allows multiple layers due to weak forces involved.

Step 4: Analyze Statement (c)

Statement (c) claims that the enthalpy of adsorption (\(\Delta H_{\text{adsorption}}\)) is low and positive. This is incorrect because while the enthalpy of physisorption is indeed low, it is negative due to the release of energy.

Step 5: Analyze Statement (d)

Statement (d) suggests physisorption occurs due to van der Waals forces. This is true, as physisorption relies on these weak intermolecular forces for adsorption.

Key Concepts

Van der Waals forces

Van der Waals forces are fundamental to the concept of physisorption. These forces are weak intermolecular attractions that occur due to temporary fluctuations in electron density around molecules or atoms. They allow molecules to temporarily stick to each other without forming permanent chemical bonds. This is why van der Waals forces are key to processes like physisorption where gases adhere to surfaces.

Understanding these forces is crucial because they explain why physisorption is a physical process, not involving any chemical changes. The strength of the van der Waals forces explains why physisorption can occur rapidly and why it is reversible.

• Weak forces making interactions possible without chemical bonds.
• Leads to quick and reversible processes.
• Strong enough to allow gases like oxygen or carbon dioxide to adhere to surfaces.

While they may not be as forceful as chemical bonds, van der Waals forces are mighty enough to generate temporary attraction, making physisorption possible.

Enthalpy of adsorption

The enthalpy of adsorption \( \Delta H_{\text{adsorption}} \) is a crucial parameter in understanding adsorptive processes. For physisorption, it is known to be low and negative. This reflects the minimal energy involved in the process, which is typically a release of energy rather than an absorption.

When gases adhere to a surface, they release some energy due to the weak interaction forces. This energy is what is measured as the enthalpy of adsorption. Unlike chemisorption that has higher enthalpy values due to bond formation, physisorption simply involves weak interactions.

• Low enthalpy values indicate weak forces and energy release.
• Negative value signifies that energy is released during adsorption.
• Distinguishes physisorption from chemisorption that involves higher energy changes.

The insight offered by enthalpy measurements is vital to predict and understand how gases will behave in different adsorptive environments.

Multi-molecular layers

In the context of physisorption, multi-molecular layers refer to the ability of the adsorbate to form multiple layers on the adsorbent surface under the right conditions, like high pressure. Given the weak van der Waals forces involved, there is no restriction to the number of layers formed, unlike in chemisorption where only a single monolayer forms.

High pressure pushes molecules closer to each other, promoting the formation of additional layers. This characteristic is important when considering industrial applications where vast surface areas and interaction capacity are necessary.

• Occurs due to the nature of weak van der Waals forces allowing extra layers.
• High pressure conditions can encourage layer formation.
• Provides more surface interaction sites, useful in applications like carbon capture.

The ability to form multiple layers enhances the adsorption capacity and is a defining feature of physisorption.

Adsorption process

The adsorption process is a surface-based phenomenon where atoms, ions, or molecules from a gas or liquid accumulate on the surface. In physisorption, this process is driven mainly by intermolecular forces like van der Waals forces, distinguishing it from chemical adsorption or chemisorption.

Physisorption is favored by

• low temperatures, which enhance molecular interactions without breaking bonds,
• the physical nature of interactions making the process reversible.
• Typical conditions like high pressures can promote adsorption volumes.

Learning about the adsorption process helps us to optimize conditions for maximizing adsorption in environmental and industrial processes, such as removing pollutants or enhancing catalytic actions. It also highlights the dynamic nature of surface interaction, governed not by chemical affinities, but by physical interactions.