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 as physisorption has a low and negative enthalpy of adsorption.

Step 1: Examine Statement (a)

Statement (a) claims that more easily liquefiable gases are adsorbed readily. In physisorption, the gases that can be liquefied easily are indeed adsorbed more readily due to stronger van der Waals forces. Hence, statement (a) is correct.

Step 2: Examine Statement (b)

Statement (b) suggests that under high pressure, physisorption results in the formation of a multi-molecular layer on the adsorbent surface. Physisorption can indeed form multiple layers because the forces involved are weak and do not saturate after forming the first layer. Thus, statement (b) is correct.

Step 3: Examine Statement (c)

Statement (c) mentions that the enthalpy of adsorption, \( \Delta H_{\text{adsorption}} \), is low and positive. In reality, physisorption usually has a low and negative enthalpy of adsorption because the process is exothermic due to the release of energy when gas molecules are adsorbed. Therefore, statement (c) is incorrect.

Step 4: Examine Statement (d)

Statement (d) states that physisorption occurs due to van der Waals forces. This is accurate since physisorption involves weak physical forces like van der Waals forces among the adsorbate molecules and the surface. Hence, statement (d) is correct.

Key Concepts

Van der Waals Forces

In the world of chemistry, van der Waals forces are gentle attractions that occur between molecules. These forces are subtler than the strong covalent or ionic bonds that hold atoms together in compounds. They arise because of temporary changes in charge distribution within molecules. You can think of this as a kind of slight magnetic pull.
These forces are essential in physisorption, which involves the physical binding of gases to a solid surface. Because van der Waals forces are relatively weak, they allow gas molecules to adhere to a surface without forming strong chemical bonds. This makes the process of physisorption reversible.

• Physisorption is predominantly driven by these weak forces.
• The strength of van der Waals forces depends on how easily the gas can be liquefied.

In summary, without van der Waals forces, physisorption simply wouldn't happen as it does. These gentle attractions ensure that different gas molecules approach and stick to surfaces, enabling processes such as multi-molecular layer formation.

Enthalpy of Adsorption

Enthalpy of adsorption is a concept that describes the energy change when a gas adheres to a solid surface. For physisorption, the key thing to remember is that this process is exothermic, which means it releases energy. Hence, the enthalpy of adsorption, typically denoted as \( \Delta H_{\text{adsorption}} \), is low and negative.
Why low and negative? In physisorption, the forces involved are weak and do not require much energy to break.

• The exothermic nature signifies that when gas molecules "nestle" onto a surface, they actually release energy.
• This energy release is what makes the process happen easily and reversibly.

So, when dealing with physisorption and evaluating energy changes, always keep in mind the low and negative nature of its enthalpy of adsorption. It's a telltale sign that the process is energy-friendly and mostly about weak physical attachments.

Multi-molecular Layer Formation

Let's explore how multi-molecular layers form during physisorption. Unlike chemisorption, where molecules bond strongly on a single layer, physisorption allows for the buildup of multiple layers. This is possible because the forces involved don't saturate or "fill up" after the first layer is formed. It’s as if these weak forces signal, "Come one, come all!" to more gas molecules.
Under the right conditions, usually high pressure, several layers can stack identically like pages in a book.

• The multi-layer formation is driven by van der Waals forces, too weak to limit growth after one layer.
• The number of layers is generally limited by the pressure and temperature conditions.

Such formations are vital for processes in separation technologies and surface science. Understanding how physisorption leads to these multi-layer arrangements is key to exploring more about surface chemistry applications.