Animated Solution for Chemistry - Surface Chemistry: Amongst the following statements regarding adsorption, those that are valid are: (A) $\Delta H$ becomes less negative as adsorption proceeds. (B) On a given adsorbent, ammonia is adsorbed more than nitrogen gas. (C) On adsorption, the residual force acting along the surface of the adsorbent increases. (D) With increase in temperature, the equilibrium concentration of adsorbate increases.

Imagine you are standing in a crowded room. The people in the center are surrounded by others on all sides, perfectly balanced by the social forces around them. But the people standing at the very edge of the room have no one on one side—they have "unbalanced" or "residual" forces.

In the microscopic world of a solid surface, the exact same thing happens. The atoms deep inside the bulk of the solid are bonded symmetrically in all directions. However, the atoms on the surface are exposed. They possess dangling bonds or residual forces. These forces act like tiny magnets, desperately trying to pull passing gas molecules (the adsorbate) onto the surface to satisfy their valency. This accumulation of molecules on a surface is what we call adsorption.

Statement (A)

When a gas molecule finally lands on the surface and binds to it, a new interaction is formed. In chemistry, bond formation is universally a joyous, energy-releasing event. Therefore, adsorption is inherently an exothermic process, meaning the enthalpy change is negative ($\Delta H<0$).

But here is the fascinating part: not all surface sites are created equal. Initially, the surface is completely bare, and the "hungriest" active sites grab the gas molecules first, releasing a massive amount of heat. As the adsorption proceeds, these prime spots get filled up. The incoming molecules must now settle for weaker sites, releasing progressively less heat. Consequently, the magnitude of the heat released drops, and $\Delta H$ becomes less negative as the process continues. Statement (A) is absolutely correct.

Ammonia vs Nitrogen: Statement (B)

If we release a mixture of ammonia ($N{H}_3$) and nitrogen ($N_2$) over our solid surface, which one will stick better?

Think of ammonia as a sticky, highly interactive molecule. It is polar and capable of forming strong intermolecular forces like hydrogen bonds. Because of this, it has a high critical temperature and is easily liquefiable. Nitrogen, on the other hand, is a perfectly symmetrical, non-polar molecule that relies on very weak van der Waals forces.

The golden rule of physical adsorption is that easily liquefiable gases are adsorbed to a greater extent. The strong intermolecular forces of ammonia make it much easier for the surface residual forces to capture and hold onto it compared to the slippery nitrogen molecules. Thus, $N{H}_3$ is adsorbed more than $N_2$, making Statement (B) correct.

Statement (C)

Statement (C) claims that residual forces increase upon adsorption. This is a classic logical trap!

Why did the gas molecules come to the surface in the first place? They were pulled in by the unbalanced residual forces. Once the gas molecules bind to the surface atoms, those dangling bonds are finally satisfied. The "hunger" of the surface is quenched. Therefore, the residual forces must decrease, not increase. Statement (C) is fundamentally flawed.

The Enemy of Adsorption: Statement (D)

What happens if we turn up the heat? We already established that adsorption is an exothermic process. We can write the equilibrium as:

According to Le Chatelier's Principle, if we add heat to an exothermic system (by increasing the temperature), the system will try to consume that excess heat by shifting the equilibrium in the backward direction.

Physically, the adsorbed gas molecules gain kinetic energy from the heat, vibrate violently, and eventually break free from the surface forces, flying back into the gas phase. This process is called desorption. Therefore, with an increase in temperature, the equilibrium concentration of the adsorbate decreases. Statement (D) is incorrect.

By systematically breaking down the physics of the surface, we have determined that only statements (A) and (B) are valid. This perfectly aligns with option (c). Mastering surface chemistry is all about visualizing the microscopic tug-of-war between surface forces and thermal energy!