Surface Chemistry

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.