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) \(\Delta H<0\) (d) \(\Delta \mathrm{H}>0\)

Step-by-Step Solution

Answer

Option (d) \(\Delta H>0\) is not applicable to adsorption.

Step 1: Understanding Adsorption

Adsorption is a process where molecules (adsorbate) adhere to a surface (adsorbent). It usually involves weak forces like Van der Waals forces or sometimes chemical bonds. The process generally leads to a decrease in free energy (spontaneous process) and is exothermic (releases heat).

Step 1: Analyze Each Thermodynamic Quantity

For any spontaneous process, we know that the change in Gibbs free energy, \(\Delta G\) is less than zero (\(\Delta G<0\)). Hence, option (a) is applicable. For the enthalpy change, \(\Delta H\) is usually negative for adsorption because it often releases energy as heat, implying that \(\Delta H<0\). This makes option (c) applicable. A positive \(\Delta H\) would usually correspond to an endothermic process.During adsorption, the system becomes more ordered as molecules are locked in position on a surface. Hence, the change in entropy, \(\Delta S\), is typically negative, making option (b) applicable. Decrease in entropy suggests disorder decreases.

Step 2: Identify the Incorrect Choice

From Step 1, we determined that adsorption is a spontaneous and exothermic process with decreased entropy. The key issue is noting whether \(\Delta H\) should be positive or negative. \(\Delta H<0\) defines adsorption correctly because it is hotter during the release of energy, contradicting with \(\Delta H>0\). Thus, option (d) \(\Delta H>0\) is not applicable to adsorption, as adsorption does not require input of energy.

Key Concepts

Thermodynamics

Thermodynamics is the study of energy transformations in systems. It deals with concepts like energy, work, heat, and how they interact with matter.
In thermodynamics, understanding how energy changes occur in processes is crucial. During adsorption, energy changes help us predict if the process is spontaneous or requires additional energy input:

• If the process can occur on its own without any external force, it’s termed spontaneous.
• Spontaneous processes tend to move toward equilibrium, where energy is dispersed as evenly as possible.
• This energy distribution impacts properties like temperature and pressure.

Understanding how adsorption fits into thermodynamics helps us grasp why it happens the way it does.

Gibbs Free Energy

Gibbs Free Energy, often denoted as \(\Delta G\), is a measure that predicts the spontaneity of a process. If \(\Delta G < 0\), the process is spontaneous; if \(\Delta G > 0\), it’s non-spontaneous.In adsorption:

• Molecules stick to a surface, which is often spontaneous.
• A negative \(\Delta G\) indicates that adsorption releases energy, making the reaction favorable.

The formula for Gibbs Free Energy combines enthalpy and entropy changes: \(\Delta G = \Delta H - T\Delta S\).

• Here, \(\Delta H\) is the change in enthalpy (heat exchange), and \(T\Delta S\) relates to temperature and change in entropy.

This formula shows the relationship between these fundamental thermodynamic properties.

Entropy

Entropy, symbolized as \(\Delta S\), measures the level of disorder or randomness in a system. In the context of adsorption, \(\Delta S < 0\) usually indicates that the process leads to a more ordered state.During adsorption:

• Molecules become more structured as they adhere to a surface.
• This increases order, thus reducing entropy.

Essentially, when molecules bond to a surface, they lose some of their freedom to move, leading to a decrease in disorder.This concept is very important because it helps us understand why adsorption happens and ties into the larger concept of free energy as it affects spontaneity and feasibility.

Enthalpy

Enthalpy, represented as \(\Delta H\), is the total heat content of a system. It reflects how much heat energy is absorbed or released during a process.For adsorption, \(\Delta H < 0\) is typical:

• This negative enthalpy change means heat is released; it’s an exothermic reaction.
• The release of heat often signifies that the process is energetically favorable.

When adsorption is exothermic, molecules release energy as they attach to surfaces, heating the surroundings and making the process spontaneous.Understanding enthalpy helps in predicting how a process like adsorption will proceed and its impact on the environment and energy requirements.