Problem 107 - A Complete Resource Book in Chemistry for JEE Main

Question

Adsorption of gases on solid surface is generally exothermic because (a) enthalpy is positive (b) entropy decreases (c) entropy increases (d) free energy increases

Step-by-Step Solution

Answer

(b) Entropy decreases during adsorption, making it exothermic.

Step 1: Understand adsorption

Adsorption is the process where molecules of a gas or liquid accumulate on the surface of a solid, creating a film. In this context, we are discussing gases being adsorbed onto solid surfaces.

Step 2: Identify the nature of the process

Adsorption typically releases energy, meaning it is an exothermic process. Exothermic processes are characterized by a negative change in enthalpy (ΔH<0).

Step 3: Analyze the entropy change

During adsorption, the gas molecules become more ordered as they adhere to the surface. This ordering leads to a decrease in entropy (ΔS<0), because the system moves to a state of lesser disorder.

Step 4: Evaluate the Gibbs Free Energy

For a process to be spontaneous, the change in Gibbs Free Energy (ΔG) must be negative. The relationship is given by the equation ΔG = ΔH - TΔS. In adsorption, even though the entropy decreases, the negative ΔH ensures that ΔG is negative, making adsorption spontaneous.

Step 5: Eliminate incorrect options

Based on the analysis: - Option (a) is incorrect since the enthalpy change is negative, not positive. - Option (c) is incorrect as the entropy decreases during adsorption. - Option (d) is incorrect because an increase in free energy would imply a non-spontaneous process.

Step 6: Confirm the correct answer

Option (b) is correct because adsorption results in a decrease in entropy.

Key Concepts

Gases on Solid Surfaces

Adsorption is a fascinating process where gas molecules adhere to surfaces. This creates a thin layer on a solid, often altering the material's properties.
When gases settle on a solid's surface, they form what's called an 'adsorbate,' while the solid is known as the 'adsorbent.'
This process occurs because the surface of solids has incomplete chemical bonds, attracting gas molecules. This attraction causes the gas molecules to concentrate on the surface, forming a new phase.

• Adsorbents could include materials like activated carbon, silica gel, or clay.
• Common applications: pollution control, air purification, and catalysis.

Notably, adsorption differs from absorption, where substances penetrate the bulk of another material.

Exothermic Process

When thinking about adsorption, it's crucial to understand that it usually releases heat, making it an exothermic process. When gas molecules adhere to a solid surface, the potential energy in the molecular bonds changes, releasing energy as heat.
This heat release leads to a negative change in enthalpy \( (\Delta H < 0) \).
Simply put, the system loses energy due to heat being given off, contributing to why adsorption is typically spontaneous.Consider how when you touch a surface that's undergoing adsorption, it might feel warm due to this release of energy.

Entropy Change

In the context of adsorption, entropy changes play a crucial role.
Entropy is a measure of disorder or randomness in a system. When gas molecules are free and moving around, entropy is relatively high because of the disorder.
Upon adsorption, these molecules become ordered on the solid's surface. This ordering reduces disorder, leading to a decrease in entropy.

• The change in entropy \( (\Delta S) \) is negative, which means that the system is becoming more ordered.
• This concept helps understand why, despite the decrease in entropy, adsorption can still occur naturally.

Thus, the movement from higher disorder to a more ordered state signifies a reduction in system entropy.

Gibbs Free Energy

Gibbs Free Energy is a concept that combines enthalpy and entropy changes to determine if a process is spontaneous.For adsorption to occur spontaneously, the change in Gibbs Free Energy \( (\Delta G) \) must be negative.
The relationship between Gibbs Free Energy, enthalpy, and entropy is given by the equation:\[ \Delta G = \Delta H - T\Delta S \]- \( \Delta G < 0 \): The process is spontaneous.- If \( \Delta H \) is negative (exothermic) and \( \Delta S \) is negative (decreased disorder), \( \Delta G \) can still be negative due to the dominant exothermic nature.This balance of decreasing entropy and exothermic nature ensures that adsorption can spontaneously occur, making it essential to control and utilize in various industrial applications.