Problem 130 - A Complete Resource Book in Chemistry for JEE Main

Question

In physisorption adsorbent does not show specificity for any particular gas because (a) Enthalpy of adsorption is low (b) It is a reversible process (c) Gases involved behave like ideal gases (d) Involved van der Waals forces are universal.

Step-by-Step Solution

Answer

The answer is (d) Involved van der Waals forces are universal.

Step 1: Understanding Physisorption

Physisorption, also known as physical adsorption, is a process where gas molecules adhere to a solid surface through weak van der Waals forces. These forces are non-specific and occur between virtually any molecules.

Step 2: Analyzing the Options

Let's analyze each option to determine which one is related to the non-specific nature of physisorption: - (a) Enthalpy of adsorption is low: This indicates weak interactions but does not explain non-specificity. - (b) It is a reversible process: Reversibility tells nothing about specificity. - (c) Gases involved behave like ideal gases: Ideal behavior is unrelated to specificity. - (d) Involved van der Waals forces are universal: These forces are present between any types of molecules, suggesting non-specificity.

Step 3: Choosing the Correct Option

Based on the analysis, option (d), "Involved van der Waals forces are universal," best explains why physisorption does not show specificity for any particular gas. The universal nature of van der Waals forces means that they can occur between any gas molecules and the adsorbent.

Key Concepts

Van der Waals forces

Van der Waals forces are a type of physical attraction that occur between all molecules, regardless of their specific properties. These forces arise from temporary changes in the distribution of electrons, which create a slight polarity within the molecules. This temporary polarity allows the molecules to attract each other weakly. Since these forces are universal, they are not unique to any particular type of molecule or element.
This means they can form between any gas and a solid surface, making them a central feature of physisorption. In this context, van der Waals forces are weak and non-specific, which explains why physisorption does not favor any specific gas over another.

• All molecules exhibit these forces.
• They are due to temporary electron distributions.
• They are weak compared to other forces like ionic or covalent bonds.

Enthalpy of adsorption

Enthalpy of adsorption refers to the change in enthalpy when a gas molecule is adsorbed onto a surface. For physisorption, this change is relatively small because the interactions are weak. The enthalpy changes reflect the energy required to overcome the interactions between gas molecules and the surface. Lower enthalpy changes are typical because the bonds being formed are not strong.
In the context of physisorption, the low enthalpy of adsorption is a key characteristic. It distinguishes physisorption from chemisorption, where the adsorption involves stronger, chemical bonding.

• Measures the heat exchanged during adsorption.
• Low for physisorption, indicating weak interactions.
• Higher for chemisorption, indicating stronger bonding.

Reversible process

In physisorption, the process is typically reversible. This means the gas molecules can be removed or released from the surface the same way they adhered. Such reversibility highlights the weak nature of the interactions involved, allowing the molecules to easily detach when conditions change, such as temperature or pressure.
Since the forces at play are weak van der Waals forces, the molecules do not chemically react with the surface, making it straightforward for them to desorb. This characteristic is important in various applications, where temporary adsorption is needed without permanent alteration of the material.

• Molecules can adsorb and desorb easily.
• Influenced by changes in temperature and pressure.
• Does not involve permanent bonding.

Ideal gases behavior

In chemistry, an ideal gas is a theoretical model that describes the behavior of gases under certain conditions. Ideal gases are assumed to have no interactions between molecules and occupy no volume. While real gases do not perfectly follow these assumptions, the model is a useful approximation for many practical purposes.
When considering physisorption, the involved gases tend to exhibit ideal gas behavior. This means their molecular interactions are minimal, further enhancing the non-specific nature of physisorption. Physisorption involves the weak, temporary forces between the gas molecules and a surface without significant deviation from ideal behavior.

• Gases behave as if there's little to no interaction.
• Assumes molecules of the gas have no volume.
• Useful for understanding physisorption's non-specific nature.