Problem 127 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following is not applicable to the phenomenon of adsorption? (a) \(\Delta \mathrm{G}<0\) (b) \(\Delta \mathrm{S}<0\) (c) \(\Delta \mathrm{H}<0\) (d) \(\Delta H>0\)

Step-by-Step Solution

Answer

The incorrect statement, not applicable to adsorption, is (d) \(\Delta H>0\).

Step 1: Understanding Adsorption

Adsorption is a surface phenomenon where molecules from a gas or liquid adhere to a solid or liquid surface. It is typically exothermic, meaning it releases heat.

Step 2: Analyzing Thermodynamic Parameters

During adsorption, the Gibbs free energy change (9G) is negative (9 ext{G}<0) because the process is spontaneous. The enthalpy change (9H) is usually negative (9 ext{H}<0) because it is exothermic, indicating heat is released. The entropy change (9S) is also negative (9 ext{S}<0) since the system becomes more ordered as molecules are concentrated on the surface.

Step 3: Determining the Opposing Statement

Option (d) suggests that the enthalpy change (9H) during adsorption is greater than zero, implying the process is endothermic, which contradicts the characteristic nature of adsorption. Therefore, option (d) 9H>0 is not applicable to adsorption.

Key Concepts

Thermodynamic Parameters

In the study of adsorption, understanding thermodynamic parameters is crucial. These parameters include Gibbs free energy change (\( \Delta \mathrm{G} \)), enthalpy change (\( \Delta \mathrm{H} \)), and entropy change (\( \Delta \mathrm{S} \)). Each of these plays a significant role in dictating whether the process is spontaneous, exothermic, or endothermic.

• Gibbs Free Energy (\( \Delta \mathrm{G} \): This measures the available energy to perform work during a process. A negative \( \Delta \mathrm{G} \) implies that the reaction is spontaneous and likely to happen without additional energy.
• Enthalpy Change (\( \Delta \mathrm{H} \): Often determines if a process is releasing or absorbing heat. It is key in classifying processes as exothermic or endothermic.
• Entropy Change (\( \Delta \mathrm{S} \): Represents the disorder or randomness in a system. A negative entropy change indicates more order, a common trait in adsorption where molecules align on a surface.

Understanding these parameters helps in predicting and controlling the outcome of adsorption processes in research and industrial applications.

Enthalpy Change

Enthalpy change (\( \Delta \mathrm{H} \)) is a vital concept when discussing adsorption. It tells us whether heat is released or absorbed during the process. In adsorption, \( \Delta \mathrm{H} \) is typically negative, signifying that it is an exothermic process.

• Exothermic Nature: Since adsorption is exothermic (\( \Delta \mathrm{H} < 0 \)), it releases heat. This heat release is due to the strong interaction between the adsorbate molecules and the surface.
• Why is \( \Delta \mathrm{H} \) Negative? During adsorption, energy is required to hold the molecules on a surface, but the system loses overall energy by releasing heat, hence the negative \( \Delta \mathrm{H} \).
• Misconceptions: An option suggesting that \( \Delta \mathrm{H} > 0 \), as in endothermic processes, would not apply to typical adsorption scenarios.

Grasping the concept of enthalpy change helps in understanding how energies influence adsorption, making it an essential thermodynamic parameter.

Entropy Change

Entropy change (\( \Delta \mathrm{S} \)) in adsorption reflects the changes in the disorder of the system. Often, \( \Delta \mathrm{S} \) is negative because adsorption leads to a more ordered system.

• Orderliness of Adsorption: Adsorption causes molecules to stick to surfaces, reducing their freedom to move, decreasing entropy. This order contrasts from their original, more disordered state.
• Negative Entropy Change: With \( \Delta \mathrm{S} < 0 \), the system's entropy lessens as adsorption brings about organization in how molecules occupy space.
• Interpreting \( \Delta \mathrm{S} \): An understanding of entropy change is significant as it balances out the heat and ordering processes involved in adsorption.

A clear grasp of how entropy change operates in adsorption tells us about the disorder within the system and how this affects overall thermodynamic stability.