Problem 19 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

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

Step-by-Step Solution

Answer

Correct statements are (a), (b), and (d).

Step 1: Analyzing Adsorption Exothermicity

Adsorption is a process where a gas accumulates on the surface of a solid. By nature, adsorption releases energy, which means it is an exothermic process. Therefore, statement (a) 'Adsorption is always exothermic' is correct.

Step 2: Physisorption vs. Chemisorption Transformation

Physisorption involves weak van der Waals forces and can potentially transform into chemisorption, which involves stronger chemical bonds, given sufficient thermal energy or temperature. As temperature increases, the energy barrier for bond formation in chemisorption can be overcome. Statement (b) 'Physisorption may transform into chemisorption at high temperature' is indeed correct.

Step 3: Temperature Effects on Adsorption Types

Physisorption is typically exothermic and decreases with rising temperature due to decreased gas-solid interactions, according to Le Chatelier’s principle. Therefore, physisorption does not increase with temperature. Chemisorption, however, decreases with rising temperature, because its exothermic reaction becomes less favorable. Thus, statement (c) is incorrect.

Step 4: Energy Dynamics of Chemisorption

Chemisorption involves stronger interactions than physisorption, usually due to chemical bonding, making it more exothermic. However, it often requires higher activation energy, which means it occurs more slowly. This makes statement (d) 'Chemisorption is more exothermic than physisorption, however it is very slow due to higher energy of activation' correct.

Key Concepts

Exothermic Processes in Adsorption

Adsorption is the process where molecules of a gas adhere to the surface of a solid. This interaction is usually exothermic, meaning it releases energy in the form of heat. Here's why.
During adsorption, molecules that are free in the gas phase are attracted to the solid surface. This attraction leads to the release of energy, which is a hallmark of exothermic processes.

• In exothermic processes, the system's overall energy decreases as energy is released to the surroundings.
• Adsorption typically results in heat being given off because the interactions stabilize the gas molecules on the surface.

The exothermic nature of adsorption implies that as the temperature of the system increases, the tendency for adsorption might decrease according to Le Chatelier's principle. In essence, because energy is already being given off, forcing more energy into the system can result in a reduced effectiveness of adsorption.

Understanding Physisorption

Physisorption is a type of adsorption characterized by weak forces, specifically van der Waals forces, which hold the gas molecules to the solid surface. Unlike chemisorption, physisorption does not involve the formation of chemical bonds.

• Physisorption is generally reversible and occurs at lower temperatures.
• The weak nature of these interactions allows physisorption to be a rapid process requiring little energy to begin.

As temperature increases, the gas molecules gain enough kinetic energy to overcome the weak attractive forces, often resulting in a decrease in physisorption. This makes high temperatures less favorable for physisorption compared to chemisorption. However, given enough thermal energy, physisorption can sometimes transform into chemisorption. This transformation can occur because the extra energy helps gas molecules overcome the activation energy barrier needed to form stronger bonds.

Exploring Chemisorption

Chemisorption involves the formation of strong chemical bonds between the adsorbate (the substance being adsorbed) and the surface of the adsorbent (the material on which the substance is adsorbing).

• Chemisorption typically requires higher energy to activate the formation of these bonds.
• It is usually slower compared to physisorption due to these energy demands.

Although chemisorption is more exothermic due to the strength of the chemical bonds formed, it still decreases with increasing temperature as its exothermic nature becomes less favorable. At lower temperatures, chemisorption is more likely to take place as the gas molecules are more apt to form stable bonds with the surface. Thus, even though both physisorption and chemisorption are exothermic, chemisorption often requires a specific range of conditions—such as lower temperatures and/or activation energy being met—for the process to occur effectively.