Problem 16 - A Complete Resource Book in Chemistry for JEE Main

Question

What is the name of a phenomenon in which both adsorption and absorption takes place? (a) chemisorption (b) physisorption (c) desorption (d) sorption

Step-by-Step Solution

Answer

The phenomenon is called sorption, option (d).

Step 1: Understanding the Concepts

First, let's understand the concepts involved in the question. Adsorption refers to the process where molecules stick to the surface of a solid or liquid. Absorption refers to the process where molecules are taken up by the volume of a solid or liquid.

Step 2: Defining the Phenomenon

The phenomenon where both adsorption and absorption occur simultaneously is called sorption. Sorption is a general term that encompasses both adsorption and absorption.

Step 3: Identifying the Correct Option

In the provided options, (a) chemisorption, (b) physisorption, and (c) desorption refer to specific types of adsorption or its reverse process. Therefore, the only option that encapsulates both adsorption and absorption is (d) sorption.

Key Concepts

Adsorption

Adsorption is a surface-based process where molecules from a gas or liquid adhere to a solid or liquid surface. Imagine a sponge attracting tiny particles to its outer layer. That's adsorption. However, unlike absorption, the molecules do not penetrate into the material. Instead, they just settle on the surface.

Factors such as surface area and temperature significantly influence adsorption. A larger surface provides more space for molecules to attach. Meanwhile, lower temperatures generally enhance adsorption because the molecules have less energy to leave the surface. Common adsorption applications include activated charcoal in water filters and silica gel packs used to keep things dry.

Adsorption is often divided into two types based on the nature of the force that attracts the molecules: physisorption and chemisorption.

Absorption

Absorption involves the uptake of molecules into the bulk or interior of a solid or liquid. To visualize it, think of a sponge soaking up water. Here, the water is absorbed into the sponge, not just on its surface.

In absorption, molecules disperse throughout another substance, penetrating internally. This process is highly dependent on the nature of the absorbing medium and the absorbed substance.

For example, when ammonia gas is absorbed by water, it becomes part of the liquid, forming a solution. Absorption plays a critical role in many biological and chemical processes, such as the absorption of nutrients through the intestinal wall in the human body.

Physisorption

Physisorption is a type of adsorption characterized by the physical attraction between molecules and a surface. This form of adsorption is generally weak, relying on Van der Waals forces, which are the forces that occur between atoms and molecules.

Since physisorption depends on these weak forces, it is usually reversible. For instance, when heating a material, physisorbed molecules may simply evaporate away.

• It often happens at low temperatures.
• The process can occur quickly, as there's no change in chemical structure.
• It's non-directional and non-specific, meaning it doesn't depend as much on the chemical nature of the adsorbent and adsorbate.

A common example of physisorption is the condensation of vapors on cold surfaces.

Chemisorption

Chemisorption involves a chemical reaction between the surface and the adsorbate, leading to a stronger bond than physisorption. This process results in the formation of a new chemical compound. Such bonds are often covalent, meaning they involve the sharing of electron pairs between atoms.

Chemisorption is typically an irreversible process because the created bonds are strong. This type of adsorption requires higher temperatures to promote the necessary chemical reactions.

• The adsorption often depends on the surface's chemical properties.
• It's usually slower than physisorption, as forming a chemical bond takes more time.
• Each adsorbent site typically holds only a single molecule, due to specificity.

An example of chemisorption is the hydrogen molecules reacting with metal surfaces, leading to hydrogenation reactions. This process is fundamental in catalysis, where substances increase the rate of chemical reactions.