Problem 131 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is an example of absorption? (a) Oxygen on metal surface (b) Hydrogen on finely divided nickel (c) Water on calcium chloride (d) Water on silica gel

Step-by-Step Solution

Answer

(c) Water on calcium chloride.

Step 1: Understand Absorption vs Adsorption

Absorption is a process where a substance is taken up throughout the volume of another material. In contrast, adsorption involves a substance adhering only to the surface of another material.

Step 2: Analyze Each Option

Look at each option to determine if it represents absorption or adsorption: - (a) Oxygen on metal surface: Oxygen molecules stick to the surface (adsorption). - (b) Hydrogen on finely divided nickel: Hydrogen molecules adhere to the surface of nickel (adsorption). - (c) Water on calcium chloride: Water molecules are taken up into the bulk of calcium chloride (absorption). - (d) Water on silica gel: Water molecules adhere to the surface of silica gel (adsorption).

Step 3: Identify the Example of Absorption

Based on the analysis, option (c) 'Water on calcium chloride' involves the uptake of water into the entire bulk of the calcium chloride material. This indicates absorption rather than just adherence on the surface.

Key Concepts

Understanding Absorption vs Adsorption

When exploring the concepts of absorption and adsorption, it's important to note that they are both processes involving the interaction between two substances. However, their primary difference lies in how one substance interacts with another.

• Absorption: This is a process in which a substance is fully taken in by another substance, spreading throughout its volume. It's similar to how a sponge soaks up water, where the absorbed substance permeates the absorber.


• Adhesion: On the other hand, adsorption is when a substance merely attaches itself to the surface of a material, like how dust settles on a television screen.

Absorption is crucial across various scientific and industrial fields, like removing toxins through activated charcoal or the way plants absorb nutrients from soil. Understanding these processes helps in identifying where and how to deploy materials effectively.

The Role of Chemical Processes in Absorption and Adsorption

In both absorption and adsorption, chemical processes play a pivotal role. These processes involve interactions at a molecular level that can facilitate or hinder the absorption or adsorption rates. Let's delve deeper into how these interactions differ.

• In absorption, molecules of the absorbed substance intermingle with molecules of the absorptive material. For instance, when water is absorbed by calcium chloride, it bonds with the material chemically, often generating heat in an exothermic reaction.


• In adsorption, the forces involved are usually intermolecular forces, such as van der Waals forces or hydrogen bonds. These are weaker than chemical bonds, which is why adsorption is typically reversible. A good example is how water vapors adhere to silica gel surfaces.

These chemical processes highlight why absorption often involves a more permanent uptake than adsorption. Understanding them enhances our ability to manipulate and utilize materials for desired outcomes.

Exploring Surface Chemistry in Adsorption

Surface chemistry significantly impacts adsorption, as it involves the interaction of adsorbates with the surfaces of the adsorbent. It relies on the surface area's availability and the nature of the surface binding sites.

• Surface Activity: The larger the surface area, the more sites are available for adsorption. Finely divided materials like nickel powder present more surface area than a solid chunk, enhancing the adsorption rate.


• Types of Adsorption: Adsorption can be physical (physisorption) involving weak forces or chemical (chemisorption) involving stronger chemical bonds. In the case of hydrogen on nickel, this often involves chemisorption, where a new chemical bond forms.

Surface properties, including roughness, porosity, and chemical composition, can all influence adsorption mechanisms and capacities. Studying these parameters is vital in fields like catalysis, environmental filtration, and even nanotechnology.