Animated Solution for Chemistry - Surface Chemistry: The correct statement(s) about surface properties is (are)

The study of surface chemistry bridges the gap between bulk properties and atomic-level interactions. This problem from JEE Advanced beautifully intertwines multiple concepts: the classification of colloids, the thermodynamics of adsorption, the kinetic properties of colloidal particles, and the relationship between a gas's critical temperature and its tendency to be adsorbed. Let's embark on a detailed journey through each of these statements to uncover the physical truths they hold.

Colloids and Emulsions

The first statement claims that a cloud is an emulsion. To evaluate this, we must return to the fundamental classification of colloids. A colloidal system consists of two phases: the dispersed phase (the particles) and the dispersion medium (the continuous phase in which particles are distributed).

An emulsion is specifically defined as a colloidal dispersion where both the dispersed phase and the dispersion medium are liquids. A classic everyday example is milk, where liquid fat globules are dispersed in liquid water.

Now, let's look up at the sky. What is a cloud? A cloud is formed when tiny liquid water droplets are suspended in the atmosphere, which is a mixture of gases. Therefore, the dispersed phase is liquid, and the dispersion medium is gas. This specific type of colloid is known as an aerosol. Because a cloud is an aerosol and not an emulsion, statement (A) is incorrect.

Thermodynamics of Adsorption

Statement (B) delves into the thermodynamics of adsorption. Adsorption is the phenomenon where molecules of a gas or liquid accumulate on the surface of a solid.

Imagine gas molecules darting around freely in a container. They possess high translational kinetic energy and a high degree of randomness. When these molecules strike a solid surface and get adsorbed, they are essentially "pinned" down. They lose their freedom to move in three dimensions, restricting their motion to the 2D surface. This significant loss of randomness means that the change in entropy of the system is negative:

Furthermore, why do the molecules stick in the first place? They stick because they form bonds with the surface atoms. In physisorption, these are weak Van der Waals forces; in chemisorption, they are actual chemical bonds. Regardless of the type, bond formation is inherently an energy-releasing process. The system moves to a lower, more stable energy state, releasing heat into the surroundings. Therefore, the process is exothermic, meaning the change in enthalpy is negative:

Since both enthalpy and entropy decrease during adsorption, statement (B) is absolutely correct.

Brownian Motion

Statement (C) brings us to the kinetic properties of colloids, specifically Brownian motion. If you observe a colloidal solution under an ultramicroscope, you will see the particles executing a continuous, random, zig-zag motion. This is Brownian motion.

What causes it? It arises from the unequal and unbalanced collisions between the fast-moving molecules of the dispersion medium and the colloidal particles.

Does this motion depend on the size of the particles? Physics tells us that a smaller mass will experience a greater acceleration for a given force ($F=ma$). Smaller colloidal particles are more easily jostled by the medium's molecules, leading to faster and more pronounced Brownian motion. Conversely, larger particles are more sluggish.

Does it depend on viscosity? Yes. A highly viscous medium acts like a thick syrup, offering more resistance to the movement of particles, thereby dampening the Brownian motion.

Because Brownian motion heavily depends on the size of the particles (smaller size = more motion), statement (C) is incorrect.

Critical Temperature and Adsorption

The final statement, (D), compares the adsorption of two gases: ethane and nitrogen. It provides their critical temperatures ($T_c$): $563\text{ K}$ for ethane and $126\text{ K}$ for nitrogen.

What is the physical significance of critical temperature? The critical temperature is the temperature above which a gas cannot be liquefied, no matter how much pressure is applied. A higher critical temperature indicates that the gas molecules have stronger intermolecular forces of attraction.

Gases with stronger intermolecular forces (like ethane) are more easily liquefiable. When such gases come into contact with a solid surface, these same strong intermolecular forces allow them to interact more strongly with the surface via Van der Waals forces. Consequently, they are adsorbed to a much greater extent.

Since $T_c(\text{ethane})>T_c(\text{nitrogen})$, ethane will be adsorbed more strongly and in greater amounts on the same mass of activated charcoal at a given temperature. Thus, statement (D) is correct.

By systematically breaking down the physical principles behind each option, we have determined that adsorption is an exothermic process accompanied by a decrease in entropy, and that gases with higher critical temperatures are more readily adsorbed. This makes options (B) and (D) the correct choices.