Question

The correct statement(s) pertaining to the adsorption of a gas on a solid surface is (are) (1) Adsorption is always exothermic (2) Physisorption may transform into chemisorption at high temperature (3) Physisorption increases with increasing temperature but chemisorption decreases with increasing temperature (4) Chemisorption is more exothermic than physisorption, however it is very slow due to higher energy of activation.

Short answer

The correct statements are (1), (2), and (4).

Step-by-step solution

- Understanding Adsorption

Adsorption is the process where molecules of a gas or liquid accumulate on the surface of a solid or liquid. It is generally an exothermic process because energy is released when bonds are formed between the adsorbate molecules and the surface of the adsorbent.

- Evaluating Point 1

Adsorption is always exothermic as energy is released when the gas particles adhere to the solid surface. Hence, statement (1) is correct.

- Transforming Physisorption to Chemisorption

At high temperatures, physisorption, which involves weak van der Waals forces, can transform into chemisorption, where stronger chemical bonds are formed. This makes statement (2) correct.

- Temperature Dependency

Physisorption generally decreases with increasing temperature, while chemisorption involves overcoming an activation energy barrier and can decrease or increase depending on the specific case. This means statement (3) is incorrect.

- Comparing Exothermic Nature

Chemisorption typically releases more energy (is more exothermic) than physisorption, but it occurs more slowly due to the higher activation energy required. Therefore, statement (4) is correct.

Key concepts

Physisorption

Physisorption is a type of adsorption where gas molecules adhere to the surface of a solid through weak Van der Waals forces. These are the same kinds of forces that cause molecules to stick together in a liquid. Because these forces are weak, physisorption is usually reversible. This means the gas can easily detach from the surface if conditions change.
Unlike strong chemical bonds in chemisorption, physisorption only involves physical attachment. This process is generally fast and doesn’t require a lot of energy to happen. However, it is highly dependent on temperature. At higher temperatures, the motion of gas molecules increases, making it harder for them to stay attached to the surface. Thus, physisorption usually decreases with an increase in temperature.

• Weak Van der Waals forces are involved.
• Generally a fast and reversible process.
• Highly sensitive to temperature changes.

Chemisorption

Chemisorption is another form of adsorption, but it involves much stronger chemical bonds. In this process, gas molecules form a chemical bond with the surface of a solid, which is what makes chemisorption stronger and generally more permanent than physisorption. Because chemical bonds are stronger, a lot more energy is released when these bonds form, making chemisorption very exothermic.
However, chemisorption isn't as fast as physisorption. It needs higher activation energy to start because breaking existing bonds or forming new chemical bonds takes more energy. Once it starts, it releases a significant amount of energy. Due to the necessity of this high activation energy, chemisorption can be slower and more selective than physisorption. It can happen readily or require additional energy to proceed, such as higher temperature or a catalyst.

• Involves strong chemical bonds.
• More exothermic than physisorption.
• Slower due to higher activation energy.

Exothermic Reaction

An exothermic reaction is a chemical reaction that releases energy in the form of heat. In the context of adsorption, both physisorption and chemisorption are exothermic processes, meaning they release heat when molecules adhere to a surface.
The reason this happens is because energy is released when new bonds form between the gas molecules and the solid surface. While both types of adsorption release energy, chemisorption generally releases more energy than physisorption because chemical bonds are stronger than Van der Waals forces.

• Energy is released in the form of heat.
• Both physisorption and chemisorption are exothermic.
• Chemisorption releases more energy due to forming stronger bonds.

Understanding that adsorption is always exothermic helps explain why certain conditions favor either physisorption or chemisorption. It also clarifies why chemisorption requires higher activation energy to start but releases more energy once it occurs.