Problem 12 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

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

Entropy decreases during adsorption, making option (b) correct.

Step 1: Understanding Adsorption

Adsorption is the process where gas molecules adhere to the surface of a solid. This process often involves the release of energy, which makes it exothermic.

Step 2: Analyzing Thermodynamic Parameters

In thermodynamics, an exothermic process is one where the enthalpy change (\( \Delta H \)) is negative, indicating that energy is released during the process. For a spontaneous process like adsorption, free energy change (\( \Delta G \)) should be negative according to the relation \( \Delta G = \Delta H - T\Delta S \), where \( \Delta S \) is the entropy change.

Step 3: Evaluating Entropy Change

During adsorption, the gas molecules lose some degree of freedom as they attach to the solid surface, which typically results in a decrease in entropy (\( \Delta S < 0 \)). This is because gases, in their natural state, have higher entropy due to more disorder and movement compared to when they are adsorbed on a surface.

Step 4: Conclusion on Energy Release

Since adsorption is exothermic, the enthalpy change \( \Delta H \) is negative. For the adsorption to remain spontaneous (negative \( \Delta G \)), the decrease in entropy (\( \Delta S \)) must be taken into account, as it aligns with our understanding of the adsorption process.

Key Concepts

Thermodynamics

Thermodynamics is a science that studies the energy and work of a system. It helps us understand how energy is transferred and transformed. In the context of adsorption, thermodynamics can explain why the process occurs and how energetically favorable it is.

The fundamental laws of thermodynamics set boundaries on what is possible and what is not.

• The First Law states that energy cannot be created or destroyed, only transformed from one form to another.
• The Second Law introduces the concept of entropy, suggesting that every energy transfer increases the disorder or randomness in a system.
• The Third Law states that as the temperature approaches absolute zero, the entropy of a perfect crystal is zero.

In adsorption, gas molecules adhere to a solid surface. This event is spontaneous, driven by the laws of thermodynamics.

A spontaneous process generally aims at minimizing the system's energy while increasing its entropy. For adsorption, even though entropy decreases as gas molecules settle, it remains spontaneous due to its characteristics as an exothermic process.

Exothermic Process

An exothermic process is one that releases energy in the form of heat. This type of process makes the surroundings warmer as the system loses internal energy.

During adsorption, gas molecules stick to the solid surface, releasing energy. This often manifests as heat, making the process exothermic.

An important sign of an exothermic reaction is the negative enthalpy change (\( \Delta H < 0 \)).

• Enthalpy (\( H \)) is a measurement of energy in a thermodynamic system. It accounts for both internal energy and the energy needed to displace the environment.
• Due to the release of energy, the enthalpy of the system decreases, indicating an exothermic nature.

An exothermic process tends to proceed without needing additional energy. This is why adsorption can occur naturally, as it doesn't require an energy input but rather releases it.

Entropy Change

Entropy refers to the level of disorder or randomness in a system. In thermodynamics, it's a measure indicating how dispersed the energy of a system is among different possible configurations.

In adsorption processes, entropy usually decreases (\( \Delta S < 0 \)). This occurs because gas molecules, which are initially in a state of high randomness, become organized on a surface.

• When gases are free, they move randomly, filling any available space, reflecting high entropy.
• Once adsorbed, these molecules are more orderly as they're bound to a specific area of the solid surface.

Despite this loss of entropy, adsorption remains spontaneous. This happens because the overall Gibbs free energy (\( \Delta G \)) becomes negative, as shown in the equation:\[ \Delta G = \Delta H - T\Delta S \]The exothermic nature (\( \Delta H < 0 \)) of adsorption offsets the entropy loss, supporting the spontaneity and natural occurrence of the process.