Problem 10 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The following statements relate to the adsorption of gases on a solid surface. Identify the incorrect statement among them :(a) Enthalpy of adsorption is negative (b) Energy appears as heat (c) On adsorption, the residual forces on the surface are increased (d) Entropy of adsorption is negative

Step-by-Step Solution

Answer

Option (c) is incorrect; adsorption reduces residual surface forces.

Step 1: Understanding Adsorption

Adsorption refers to the process where gas molecules are attracted and adhere to the surface of a solid. During adsorption, energy in the form of heat is released, resulting in an exothermic reaction with a negative enthalpy.

Step 2: Analyzing Heat Energy

Since adsorption is exothermic, option (b) is correct as it states that energy appears as heat during the process.

Step 3: Residual Forces on Surface

Option (c) states on adsorption, the residual forces on the surface increase; however, adsorption actually reduces these forces as the surface atoms achieve a lower energy state.

Step 4: Entropy Consideration

During adsorption, gas molecules become more ordered as they adhere to the surface, leading to a decrease in randomness. Therefore, the entropy change (option d) is negative because the system becomes more ordered.

Step 5: Identifying the Incorrect Statement

From the analysis, option (c) contradicts the nature of adsorption where surface forces decrease as adsorbates cover and stabilize the surface.

Key Concepts

Enthalpy of Adsorption

In chemistry, the enthalpy of adsorption refers to the change in heat energy when molecules or atoms attach themselves to the surface of a material. This process is always accompanied by the release of energy, making it exothermic. In simpler terms, when gases or molecules stick to surfaces, they release heat. That's why the sign of enthalpy of adsorption is negative. This signifies that energy is exiting the system, reducing its total energy level.

• Adsorption energy release indicates a stable bond formation between the substance and the surface.
• A more negative enthalpy of adsorption signifies stronger adsorption.
• The negative sign also implies that the process doesn't require energy input to occur naturally.

In mathematical notation, if the enthalpy of adsorption \( \Delta H_{ads} \) is calculated, it is usually expressed with a negative value. This process is a crucial part of many industrial applications like catalysis and purification.

Entropy of Adsorption

Entropy is a measure that describes the degree of disorder or randomness in a system. During adsorption, entropy typically decreases because the gaseous molecules that are free in space become attached to a surface. This transition makes them more ordered. Hence, the entropy of adsorption is negative because there's a decrease in randomness.

• Adsorbed molecules have restricted movement compared to their free state.
• The reduction in randomness gives a negative change in entropy.
• This ordering is crucial for processes where surface reactions or stabilization is desired.

For mathematical expression, the entropy change \( \Delta S_{ads} \) during adsorption is shown as a negative value. The negative sign indicates an increase in the system's orderliness. This understanding helps in explaining phenomena such as gas sticking to activated carbon for purification purposes.

Exothermic Reaction

An exothermic reaction involves the release of heat to the surroundings. During adsorption, as gas molecules stick to a solid surface, they release energy in the form of heat. This release ensures that adsorption is an exothermic process.

• The substance-surface interaction results in heat emission.
• Exothermic reactions generally occur spontaneously as they increase the stability of the system.
• In these reactions, the energy of the adsorbate-surface system is lower after the process.

This concept is integral to many natural and industrial processes, where the heat released during adsorption aids in maintaining the physical and chemical stabilities. It highlights why, during adsorption, there is no need for additional energy; nature itself facilitates the process through free energy release. Thus, understanding exothermic reactions helps in effectively grasping how adsorption naturally favors energy-efficient states.