Adsorption arises due to the fact that the surface particles of the adsorbent are not in the same environment as the particles inside the bulk. The extent of adsorption increases with the increase of surface are per unit mass of the adsorbent at a given temperature and pressure. Another important factor featuring adsorption is the heat of adsorption. During adsorption, there is always a decrease in residual forces of the surface, therefore, it is invariably an exothermic process or \(\Delta H\) and \(\Delta S\) are always negative. There are mainly two types of adsorption of gases on solids. In physisorption, the attractive forces are mainly van der Waals forces while in cemisorption, actual bonding occurs between the particles of adsorbate and adsorbent. Generally, easily liquifying gases are adsorbed more easily on the surface of a solid as compared to the gases which are liquified with difficulty. Freundlich gave an emperical relationship between the quantity of gas adsorbed by unit mass of solid adsorbent and pressure, at a particular temperature.

Entropy during adsorption is negative because

the disorderness in the system decreases.

The correct answer is option 2. the disorderness in the system decreases.

Entropy (\( \Delta S \)) is a thermodynamic quantity that measures the degree of disorder or randomness in a system. The greater the number of possible microstates (arrangements of particles) in a system, the higher the entropy. A positive entropy change (\( \Delta S > 0 \)) indicates an increase in disorder, while a negative entropy change (\( \Delta S < 0 \)) indicates a decrease in disorder.

Adsorption is a process where molecules (adsorbate) from a gas or liquid phase adhere to the surface of a solid (adsorbent). The process typically involves the interaction between the surface atoms of the solid and the adsorbate molecules, which results in the formation of a bond or attraction.

Molecular Perspective:

Before Adsorption: The gas or liquid molecules are in a highly disordered state. They possess high translational entropy because they can move freely throughout the volume of the gas or liquid phase. The molecules have a large number of possible microstates, leading to high entropy.

During Adsorption: When these molecules adsorb onto a solid surface, they lose much of their freedom of movement. The translational motion of the molecules is restricted because they are now attached to specific sites on the solid surface. The number of possible microstates decreases because the molecules can no longer move freely as they did in the gas or liquid phase.

After Adsorption: The adsorbed molecules are in a more ordered state compared to their previous free state. The overall disorder of the system decreases, which corresponds to a decrease in entropy.

Why is Entropy Change Negative?

Decrease in Freedom: The adsorbed molecules are no longer free to move around as they are bound to specific sites on the adsorbent surface. This reduction in freedom leads to fewer possible microstates.

Order Increase: The transition from a disordered state (in the gas or liquid phase) to an ordered state (adsorbed on the surface) results in an overall increase in order.

Entropy and Order: Since entropy is a measure of disorder, and adsorption leads to an increase in order (decrease in disorder), the entropy change for the system is negative.

Example: Adsorption of Gas on Charcoal

Consider a gas like \( \text{CO}_2 \) being adsorbed onto the surface of charcoal:

Before Adsorption: \( \text{CO}_2 \) molecules in the gas phase are highly disordered, moving freely in all directions with high kinetic energy.

During Adsorption: When \( \text{CO}_2 \) molecules adhere to the surface of charcoal, they lose much of their freedom to move. They are now in a fixed position relative to the surface of the charcoal.

After Adsorption: The gas molecules are much more ordered, as they are now bound to specific sites on the charcoal surface. This results in a decrease in the entropy of the system.

Conclusion: The entropy during adsorption is negative because the disorderness in the system decreases when molecules adsorb onto a surface. This is due to the reduction in the number of possible microstates available to the adsorbed molecules, leading to an increase in order and a corresponding decrease in entropy. Thus, option 2. the disorderness in the system decreases is the correct explanation for why entropy is negative during adsorption.