Question

The heat evolved in physisorption lies in the range (in \(\mathrm{kJ} / \mathrm{mol}\) ) of : (a) \(20-40\) (b) \(40-100\) (c) \(100-200\) (d) \(200-400\)

Short answer

The heat evolved in physisorption typically ranges within a few kJ/mol. The correct answer is therefore (a) 20-40 kJ/mol.

Step-by-step solution

Understanding Physisorption

Physisorption is a type of adsorption that occurs when gas or liquid molecules adhere to the surface of a solid due to weak van der Waals forces. The amount of heat evolved during physisorption is generally very low compared to chemisorption because the forces involved are relatively weak.

Identify the Range of Heat Evolved

Knowing that physisorption involves weak attractions, we can rule out any high heat evolution values. For physisorption, the heat evolved is typically within a few kilojoules per mole because no chemical bonds are formed or broken.

Choose the Correct Option

The ranges provided in the options suggest an increasing amount of heat evolved. Since physisorption involves the lower end of energy changes, we select the range that represents the lowest energy changes which are typical for this type of adsorption.

Key concepts

van der Waals forces

Van der Waals forces play a crucial role in various interactions at the molecular level and are particularly significant in the process of physisorption. These forces are a type of weak intermolecular attraction that can occur between atoms, molecules, or surfaces.

There are three main types of van der Waals forces: Keesom forces, arising between polar molecules that have permanent dipole moments; Debye forces, between polar and nonpolar molecules where a temporary dipole is induced; and London dispersion forces, which are temporary fluctuating dipoles that arise even between nonpolar molecules due to electron movement.

These forces are not as strong as covalent or ionic bonds, which is why the physisorption process involving van der Waals forces is characterized by low enthalpy changes and is typically reversible. In the context of the given exercise, this explains why the heat evolved during physisorption is relatively small when compared to processes that involve stronger chemical bonding such as chemisorption.

adsorption

Adsorption refers to the process whereby molecules of a gas or liquid adhere to the surface of a solid or liquid. This surface phenomenon is actually the accumulation of molecules at the boundary, not to be confused with absorption, which is the penetration of substances throughout the bulk of the solid or liquid.

There are two main types of adsorption: physisorption, which is caused by van der Waals forces and is typically weak and reversible, and chemisorption, which involves stronger chemical bonding and is often irreversible. Factors affecting adsorption include the surface area, porosity of the adsorbent, temperature, and the nature of the adsorbate.

Understanding the principles of adsorption is essential in many practical applications such as heterogeneous catalysis, water purification, and air filtration. In these scenarios, the adsorbent's physical properties are maximized to ensure efficient trapping and removal of unwanted particles.

enthalpy of adsorption

The enthalpy of adsorption, also known as the heat of adsorption, denotes the change in enthalpy when a substance gets adsorbed onto a surface. It is a crucial concept as it provides insight into the strength of the interaction between the adsorbate and adsorbent.

In physisorption, the enthalpy of adsorption is relatively low (approximately in the range of 20-40 kJ/mol, as suggested by the correct answer in the given exercise), implying that the adsorbate molecules are loosely bound to the adsorbent by weak van der Waals forces. In contrast, chemisorption involves stronger, chemical bond formation, and therefore a significantly higher enthalpy change (typically greater than 40 kJ/mol).

This heat release can be measured calorimetrically, and its value is directly tied to process efficiency in industrial applications like catalysis. In practical situations, a lower heat of adsorption is preferred for reactions where quick and reversible adsorption-desorption cycles are desirable.