Surface Chemistry

Analyzing each statement in regard to colloids:

(I) Lyophobic colloids are not formed by simple mixing of dispersed phase and dispersion medium. This statement is correct. Lyophobic colloids (hydrophobic when water is the dispersion medium) have molecules or particles that do not have an affinity towards the dispersion medium. They require special methods to be prepared because the dispersed phase does not naturally mix with the dispersion medium. For example, high-energy inputs like ultrasonication, high-speed stirring, or the addition of stabilizing agents are often needed to create and stabilize these colloids.

(II) For emulsions, both the dispersed phase and the dispersion medium are liquid. This statement is also correct. Emulsions are a type of colloid where both the dispersed phase and the dispersion medium are liquids. These two liquids are immiscible or not capable of mixing together without the help of emulsifying agents (like surfactants) which stabilize the emulsion.

(III) Micelles are produced by dissolving a surfactant in any solvent at any temperature. This statement is incorrect for two reasons. First, micelle formation specifically depends on a critical concentration known as the critical micelle concentration (CMC) above which micelles begin to form in solution. Second, temperature also affects the formation of micelles, as the critical micelle concentration can change with temperature. Not any solvent or any temperature condition will result in micelle formation.

(IV) Tyndall effect can be observed from a colloidal solution with dispersed phase having the same refractive index as that of the dispersion medium. This statement is incorrect. The Tyndall effect is the scattering of light by particles in a colloid or another fine dispersion. For the Tyndall effect to be observed, there must be a significant difference in the refractive index between the dispersed particles and the dispersion medium. If the dispersed phase and the dispersion medium have the same refractive index, the light will not scatter, and the Tyndall effect will not be observed.

Given these analyses, the correct set of statements is Option A: (I) and (II).

Methylene blue adsorbs onto activated charcoal from its aqueous solution at 25°C in a process known as physical adsorption (physisorption).

(i) Physical adsorption of methylene blue on activated charcoal can occur at all temperatures, but it is more pronounced at lower temperatures. This means Option (A) is incorrect.

(ii) In the bulk, forces between molecules are balanced. However, molecules on the surface experience unbalanced or residual attractive forces, leading to higher surface energy. These unbalanced forces are responsible for van der Waals attractions between the adsorbent (activated charcoal) and the adsorbate (methylene blue). The formation of these weak attractions reduces the energy of surface molecules, resulting in a decrease in the system's enthalpy. Therefore, Option (B) is correct.

(iii) The extent of physisorption decreases as temperature increases because the weak van der Waals forces weaken at higher temperatures, reducing the adsorbate's ability to adhere to the adsorbent surface. Consequently, Option (C) is incorrect.

(iv) Physical adsorption is a reversible process. As the temperature decreases, the attractive forces between the adsorbent and adsorbate strengthen, increasing the extent of physisorption. Thus, Option (D) is incorrect.

As Sb$_2$S$_3$ is a negative sol, so, Al$_2$(SO$_4$)$_3$ will be the most effective coagulant due to higher charge density on Al$^{3+}$ in accordance with Hardy-Schulze rule.

Order of effectiveness of cations: Al$^{3+}$ > Ca$^{2+}$ > Na$^{+}$ > NH$^{+}_{4}$

The surfactant CH$_3$(CH$_2$)$_{15}$N$^+$(CH$_3$)$_3$Br$^-$ forms micelles at lowest concentration.

Critical concentration for micelle formation decreases as the molecular weight of hydrocarbon chain of surfactant grows because its true solubility diminishes and the tendency of surfactant molecule to associate increases. The molecular weight is maximum in case of CH$_3$(CH$_2$)$_{15}$N$^+$(CH$_3$)$_3$Br$^-$.

In colloidal surfactants, critical micelle concentration (CMC) is the concentration of surfactants above which micelles. Once micelles are formed, the polar surfactants are not free to conduct electricity. Also, a micelle is much larger than a monomer it diffuses more slowly through solution and so is a less efficient charge carrier.

Statements (b) and (c) are correct whereas statements (a) and (d) are incorrect.

(a) Process of precipitating colloidal sol is known as coagulation.

(b) Concentration of colloidal solutions is small due to large molar mass and thus, their colligative properties are very small compared to true solutions. Hence, these solution freezes at higher temperature.

(c) Micelles are formed at the critical micelle concentration (CMC) which depends to temperature.

(d) Micelles and macromolecular colloids are two different types of colloid.

Option (A) : Correct. Adsorption is a surface phenomenon in which the concentration of the adsorbate increases only on the surface. It is always accompanied by a decrease in the entropy ($\Delta$S). Since adsorption is an exothermic process, the enthalpy change ($\Delta$H) of the system is also negative.

Option (B) : Correct. Easily liquefiable gases, that is, gases with high critical temperatures, are more readily adsorbed because van der Waals forces are stronger near the critical temperature. Thus, the adsorption of ethane will be more than that of nitrogen.

Option (C) : Cloud is an aerosol in which liquid is dispersed phase and gas is dispersion medium. Whereas, emulsion is liquid in liquid colloidal system.

Option (D) : Incorrect. Brownian movement varies with the size of the particles and viscosity of the solution. Smaller particles with lower viscosity show faster movement.

Since, adsorption involves electron transfer from metal to O₂, it is chemical adsorption not physical adsorption, hence (a) is incorrect. Adsorption is spontaneous which involves some bonding between adsorbent and adsorbate, hence exothermic. The last occupied molecular orbital in O₂ is $\pi$*2p. Hence, electron transfer from metal to oxygen will increase occupancy of $\pi$*2p molecular orbitals. Also increase in occupancy of $\pi$*2p orbitals will decrease bond order and hence increase bond length of O₂.

In the case of physiorption, with the increase of temperature and pressure the rate of adsorption decreases because according to Le Chatelier's principle, increase of temperature and pressure will shift the equilibrium to the left

Adsorbate + Adsorbent $\rightleftharpoons$ Adsorption + Heat

This is shown in Graphs I and III; whereas in the case of chemisorption, there is a formation of strong bond between the adsorbate and the adsorbent and so the rate of adsorption increases with increase in temperature (Graphs II and IV).

The stability of lyophobic colloidal particles is generally due to two main factors:

Preferential Adsorption: The same charge (either positive or negative) is preferentially adsorbed onto the surface of lyophobic particles, preventing them from coming close to each other and precipitating. For instance, when a dilute solution of silver nitrate is mixed with potassium iodide, it forms a white precipitate of silver iodide. The iodide ions are preferentially adsorbed onto the silver iodide particles, making the sol negatively charged. These negatively charged particles repel each other, thus preventing precipitation.

Electrostatic Layers: Negatively charged sol particles attract positive charges from the surrounding medium, forming a second layer of positive charges around them. This results in two layers of opposite charges, known as the fixed layer and the diffused layer. The potential difference between these oppositely charged layers is referred to as electrokinetic potential or zeta potential.

Here is a visual illustration of the concept (known as Helmholtz double layer):

This double layer of opposite charges around the colloidal particles is crucial in maintaining their stability.

(A)

Adsorption, whether it is chemisorption or physisorption, is an exothermic process. In physisorption, gas molecules are adsorbed onto the surface of the adsorbent by van der Waals forces. When the attractive forces between the gas and the solid adsorbent are stronger than those between the gas molecules themselves, energy is released, making the process exothermic.

In chemisorption, a chemical bond is formed between the adsorbent and the adsorbate. This bond formation releases energy, making the process exothermic.

Thus, Option A is correct.

(B)

At low temperatures, gases such as hydrogen or oxygen can accumulate on the solid surface (adsorbent) through a process called physisorption, which has a low enthalpy of adsorption (20-40 kJ/mol). With increasing temperature, oxygen or hydrogen can react with metal surfaces (adsorbent) to form metal oxides or metal hydrides, a process known as chemisorption, with an enthalpy of chemisorption between 80-240 kJ/mol.

Thus, Option B is correct.

(C)

Physisorption is a low enthalpy process that is favorable at low temperatures, while chemisorption has a high enthalpy process and occurs more readily at higher temperatures. Therefore, the statement that physisorption increases with increasing temperature and chemisorption decreases with increasing temperature is incorrect.

Thus, Option C is incorrect.

(D)

Chemisorption occurs at higher temperatures because the energy of the reactant molecules (adsorbate and adsorbent) must be sufficient to overcome the potential energy barrier to form the product. However, once chemical bonds are formed between the reactant molecules, a significant amount of energy is released. Therefore, chemisorption is more exothermic than physisorption.

Thus, Option D is correct.

Note : Physisorption arises due to weak forces of attraction between the adsorbent and the absorbent molecules.