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

Step 1: Understanding Exothermic Processes

Exothermic processes release heat, implying that the overall energy of the system decreases. This typically results from forming bonds or attractions, which are energetically favorable.

Step 2: Analyzing Enthalpy

Enthalpy change (94H) for an exothermic process is negative. A positive enthalpy change implies the process absorbs energy, which is not characteristic of adsorption.

Step 3: Exploring Entropy

Entropy (94S) during adsorption tends to decrease, as gas molecules become more ordered when adsorbed on surfaces compared to being in a gaseous state.

Step 4: Evaluating Free Energy

Free energy (94G) must decrease for a spontaneous process. In the context of adsorption, 94G = 94H - T94S. For 94G to decrease (a characteristic of spontaneous and often exothermic processes), either 94H is sufficiently negative or T94S influences it less.

Step 5: Conclusion Based on Process Characteristics

Since adsorption is exothermic due to the release of energy and the formation of bonds, the primary reason is the decrease in entropy which leads to 94G being negative, making it spontaneous.

Key Concepts

Exothermic Process

An exothermic process is a type of reaction or change where heat is released to the surroundings. This means that the energy content of the system decreases, as energy is often required to break bonds but released when new bonds form. In the context of adsorption, gas molecules form new interactions with a solid surface, which helps release energy, thus characterizing it as exothermic.
The heat release is crucial because it makes the process energetically favorable. Since energy is given off during exothermic processes, you can typically expect the system to feel warmer.

• Exothermic reactions include not just adsorption, but also processes like combustion and many chemical reactions involving bond formation.
• These processes often make the system more stable because energy is released, moving it to a lower, more stable energy state.

Entropy Change

Entropy change refers to the degree of disorder or randomness in a system. During adsorption, the entropy change (94S) is typically negative. This might seem a bit strange, since we're accustomed to thinking of entropy as increasing. However, when gas molecules adhere to a solid surface, they go from a random, high-entropy state to a more ordered arrangement.
Imagine gas molecules freely bouncing around in a container. When they make contact with a solid surface, the molecules become more structured and less random.

• Decrease in randomness equates to a decrease in entropy, making 94S negative.
• The reduced entropy is a critical indicator of the adsorption process being orderly and structured.

Free Energy Change

Free energy change (94G) is a measure of a system's ability to perform work during a process at constant temperature and pressure. For a process to occur spontaneously, the change in free energy must be negative. In adsorption, the equation 94G = 94H - T94S explains the relationship:

• 94H is the change in enthalpy, which is negative for exothermic processes and indicates energy release.
• T is the temperature, and94S is the change in entropy, which is often negative for adsorption.

In adsorption, 94G becomes negative primarily because 94H, the enthalpy release, is large enough to overcome the term -T94S, even though 94S is negative. This results in the process being spontaneous and energetically favorable.

Spontaneous Processes

Spontaneous processes occur without external influence, naturally taking place due to the conditions being energetically favorable. They are characterized by a negative free energy change (94G). For instance, once adsorption begins, it proceeds without needing additional energy input.
For a process to be spontaneous:

• 94G must be negative, a situation often achieved when 94H is negative and T94S is positive or less influential.
• Spontaneity is often linked with stability; as adsorption progresses, the system reaches a lower energy state.

Understanding that adsorption is spontaneous helps us appreciate why it happens naturally, making adsorbents such as activated carbon used practically in purifying processes.