Question

The adsorption of solids, from a solution is called (a) chemical adsorption (b) physical adsorption (c) positive adsorption (d) negative adsorption

Short answer

The adsorption of solids from a solution is called positive adsorption.

Step-by-step solution

Understanding Adsorption

Adsorption is the process where molecules from a liquid or gas adhere to a solid surface. It's important to differentiate between types such as chemisorption (chemical) and physisorption (physical), depending on the forces involved.

Identifying Positive and Negative Adsorption

In the context of solutions, positive adsorption occurs when the concentration of a solute is higher on the surface of a solid than in the bulk solution, whereas negative adsorption is when the concentration on the surface is lower than in the bulk.

Analyzing the Question

The question asks for the category of adsorption when it occurs from a solution. This specifically relates to the concentration of the solute on the surface of the solid compared to the solution.

Making the Choice

Since we're dealing with adsorption from a solution where the concentration of solute may increase on the solid's surface, it points towards positive adsorption. Therefore, option (c) positive adsorption is the process described.

Key concepts

Chemical Adsorption

Chemical adsorption, also known as chemisorption, is a complex process where molecules adhere to a surface through strong chemical bonds. Unlike physical adsorption, which involves weak van der Waals forces, chemical adsorption involves the sharing or transfer of electrons.
This results in the formation of a new chemical species on the surface. Because chemical adsorption is usually irreversible, once a molecule attaches via chemical bonds, it requires significant energy to break these bonds and remove the molecule from the surface.
Chemical adsorption is characterized by:

• High specificity, as it only occurs at specific active sites on the surface.
• Formation of strong chemical bonds.
• Typically exothermic reactions, meaning they release heat.
• Possibly involving chemical changes or reaction on the adsorbent surface.
• Dependency on temperature, often increasing with a rise in temperature, up to a point where desorption may occur.

Chemical adsorption plays a crucial role in various applications like catalysts, sensors, and filtration systems.

Physical Adsorption

Physical adsorption, or physisorption, occurs when molecules or atoms adhere to a surface through weaker and more temporary van der Waals forces, rather than chemical bonds.
This type of adsorption is typically reversible and doesn't involve any modification of the properties of the adsorbate molecules.
Physical adsorption is known for:

• Weak interaction forces (van der Waals forces) between the adsorbate and the surface.
• Reversibility, meaning desorption can occur easily, often just by changing pressure or temperature conditions.
• Less specificity compared to chemical adsorption, allowing many different molecules to be adsorbed.
• Being exothermic, but with much lower energy changes compared to chemisorption.
• A temperature dependency that typically shows decreases in adsorption capacity with increasing temperature.

Physical adsorption is significant in processes such as gas storage, air purification, and a variety of separation technologies.

Positive Adsorption

Positive adsorption occurs when the concentration of a solute is higher on the surface of a solid compared to its concentration in the bulk solution.
This means that more solute molecules prefer to be adsorbed on the surface rather than remain in the solution.
Key characteristics of positive adsorption include:

• An increased concentration of solute molecules at the solid surface.
• Often observed in systems where adsorbate molecules have more affinity towards the adsorbent.
• It can be both physical or chemical adsorption, depending on the interaction forces involved.
• Efficient in removing substances from solutions, which could be beneficial in filtration or water purification.

Positive adsorption is also commonly exploited in various industrial processes, such as the purification of liquids and gases, as well as in studies related to surface chemistry and material science.

Negative Adsorption

In negative adsorption, the concentration of a solute is less on the surface of a solid than it is in the surrounding solution.
This typically happens when the solute and adsorbent have minimal affinity for each other.
Characteristics of negative adsorption include:

• A decreased concentration of solute on the surface compared to the bulk solution.
• Often occurs in systems where the solute prefers to stay in solution rather than adsorbing onto the solid surface.
• Can occur when the surface is saturated or when there is repulsion between the adsorbent and adsorbate molecules.
• Might be enhanced or reduced based on changes in system conditions like temperature or pH.

While negative adsorption isn't utilized as much as positive adsorption, understanding it is crucial for scenarios involving competitive adsorption and desorption processes.