Surface Chemistry

Gold number is the amount (in milligrams) of aprotective colloid, which is just sufficient to prevent the coagulation of 10 mL of standard hydro gold sol, when 1 mL of $10 \% \mathrm{NaCl}$ solution is added to it.

$\because 10$ cc gold sol $=1$ cc of $10 \% ~ \mathrm{NaCl} \therefore 100 \mathrm{~mL}$ of gold sol = l cc of gelatin Therefore, gold number of gelatin is

$ \frac{1 \times 10}{100}=0.01 $

Hence, option (d) is the correct answer.

Adsorption processes are exothermic ( $\Delta H<0$ ) and degree of randomness of adsorbate particles decreases, i.e. $\Delta S<0$.

According to Gibbs' equation, $\Delta G=\Delta H-T \Delta S$

at lower temperature adsorption becomes spontaneous, i.e. $\Delta G<0$

In chemisorption, adsorption increases with temperature. Because the plot between $x / m$ and $T$ (temperature) is as follows. where, $x / m=$ mass fraction of chemisorption.

Thus, option (c) is the correct answer.

The lyophilic colloids differ in their protective power. The protective power is measured in terms of gold number. It may be noted that smaller the value of the gold number, greater will be the protecting power of protective colloids. Therefore, the reciprocal of gold number is a measure of the protective power of the colloid, gold number of gelatin, haemoglobin and sodium acetate are $5 \times 10^{-3}, 5 \times 10^{-2}$ and $7 \times 10^{-1}$, respectively, hence, the protective actions will be in the order : Haemoglobin > gelatin >sodium acetate

Adsorption is a surface phenomenon where molecules of a substance, such as a gas or a liquid, accumulate on the surface of another material, usually a solid. Let’s analyze each option to see which one cannot be explained by adsorption:

Option A: When acetic acid solution is shaken with charcoal, the concentration of acid decreases.

This can be explained by adsorption because acetic acid molecules adhere to the surface of the charcoal, reducing their concentration in the solution.

Option B: The white precipitate of Mg(OH)₂ attains a blue color when precipitated in the presence of magneson reagent.

This can be explained by adsorption because the magneson reagent might adsorb onto the surface of Mg(OH)₂, imparting a blue color to it.

Option C: An aqueous solution of NaOH attains pink color with a drop of phenolphthalein.

This cannot be explained by adsorption. The pink color is due to a chemical reaction where phenolphthalein acts as a pH indicator in a basic solution, turning pink at pH levels above 8.3. This is a chemical property of phenolphthalein rather than adsorption.

Option D: When animal charcoal is shaken with colored methylene blue solution, the solution turns colorless.

This can be explained by adsorption because the methylene blue molecules adsorb onto the surface of the charcoal, removing the color from the solution.

Of all the options, Option C cannot be explained by adsorption because it involves a chemical reaction indicating the presence of a base.

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.

Gold number of a protective colloid is a minimum weight of it in milligrams which must be added to 10 ml of a standard red gold sol. so that no coagulation of the gold sol. (i.e. change of colour from red to blue) takes place when 1 ml of 10% sodium chloride solution is rapidly added to it. Obviously, smaller the gold number of a protective colloid, the greater is the protective actioin.

It is given that the gelatin has lowest gold number thus, it would serve as a better protective colloid than colloids in the rest of the options.

From the given characteristics, the adsorption is an example of physical adsorption.

(i) Physical adsorption is multimolecular layer.

(ii) It decreases with increase in temperature.

(iii) Enthalpy of adsorption is low 20–40 kJ mol⁻¹ for physical adsorption.

(iv) Energy of activation is low because in this type of adsorption no chemical reaction takes place.

Only the transparent part of an egg has albumin.

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.

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.

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.

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.