Mix Examples-Surface Chemistry Questions in English

(A) The correct option is $A$. When water is added to cement,an exothermic reaction occurs. During this process,the cement reacts with water to form a gelatinous mass which slowly sets into a hard mass having a three-dimensional network structure involving $-Si-O-Si-$ and $-Si-O-Al-$ chains.

(B) $AgBr$ is most sensitive to light and undergoes photochemical reduction.
$2AgBr \xrightarrow{\text{Light}} 2Ag + Br_2$

(D) surface phenomenon is a process that occurs at the interface of two phases (e.g.,solid-liquid,solid-gas,liquid-gas).
$1$. Heterogeneous catalysis occurs at the surface of the catalyst.
$2$. Corrosion is a surface process occurring on the metal surface.
$3$. The equilibrium between water and its vapor in a closed vessel involves the interface between liquid and gas phases.
$4$. Crystallization is a bulk process where solute particles arrange themselves into a crystal lattice throughout the solution,not just at the surface.
Therefore,crystallization is not a surface phenomenon.

(C) The correct answer is $(C)$.
Indicator type silica gel used as a dehumidifier contains $Co^{2+}$ ions.
When dry,it is blue in colour,and upon absorbing moisture,it becomes pink due to the formation of hydrated cobalt chloride.

(C) In photography,the undecomposed $AgBr$ is removed from the photographic film or plate using sodium thiosulphate (hypo) solution. This process is known as fixing. The reaction is as follows:
$AgBr + 2Na_2S_2O_3 \rightarrow Na_3[Ag(S_2O_3)_2] + NaBr$

(A) Blood is a negatively charged colloid due to the presence of negatively charged ions on the surface of blood cells. Therefore,the statement that blood is a positively charged sol is incorrect.
Soap solution contains ionic micelles as colloidal particles,which form above the $CMC$ (Critical Micelle Concentration).
Blood is purified by the process of dialysis,which removes waste and unwanted particles from the blood.
Chemical adsorption (chemisorption) typically increases initially with temperature due to the activation energy required for bond formation,but it decreases at higher temperatures as the exothermic process becomes less favorable.

(D) Total mass of fatty acid $= 0.27 \ g$ in $100 \ mL$ of solution.
Mass of fatty acid in $10 \ mL$ of solution $= 0.27 \times \frac{10}{100} = 0.027 \ g$.
Volume of fatty acid $= \frac{\text{Mass}}{\text{Density}} = \frac{0.027 \ g}{0.9 \ g \ cm^{-3}} = 0.03 \ cm^3$.
Area of the monolayer on the watch glass $= \pi r^2 = 3 \times (10 \ cm)^2 = 300 \ cm^2$.
Height of the monolayer $= \frac{\text{Volume}}{\text{Area}} = \frac{0.03 \ cm^3}{300 \ cm^2} = 10^{-4} \ cm$.
Converting to meters: $10^{-4} \ cm = 10^{-4} \times 10^{-2} \ m = 10^{-6} \ m$.

(B) The lubricating action of an oil depends on its ability to remain in the liquid state under operating conditions. Substances with high vapour pressure (e.g.,gasoline) evaporate quickly,whereas substances with low vapour pressure remain as a stable film between surfaces. Therefore,an oil with low vapour pressure provides better lubrication.

(D) The quenching of fluorescence is a process where the intensity of fluorescence is decreased due to various factors.
It primarily depends on the concentration of the fluorescent molecules (as higher concentration leads to self-quenching) and the frequency of collisions between the fluorescent substance and the quencher molecules.
Therefore,both the concentration of fluorescent molecules and the collisions of the fluorescent substance contribute to the quenching process.

(A) The setting of cement involves the hydration of its constituents,such as tricalcium silicate $(Ca_3SiO_5)$ and dicalcium silicate $(Ca_2SiO_4)$.
This hydration process is highly exothermic in nature,releasing a significant amount of heat.

(D) Surface phenomena are processes that occur only at the interface or surface of a substance.
$1$. Adsorption is a surface phenomenon where particles accumulate on the surface.
$2$. Surface tension is a property of the surface of a liquid.
$3$. Viscosity is a bulk property of a fluid (liquid or gas) representing its resistance to flow,not a surface phenomenon.
$4$. Absorption is a bulk phenomenon where a substance is distributed throughout the entire volume of another substance.
Therefore,both Absorption and Viscosity are not surface phenomena.

(A) Soap molecules are surfactants,which contain both a hydrophilic head and a hydrophobic tail. $ \newline $ When soap is added to water,these molecules accumulate at the surface,disrupting the cohesive forces between water molecules. $ \newline $ This disruption significantly reduces the surface tension of water,which is why soap is effective for cleaning.

(A) $1$. Adsorption is a spontaneous process and is exothermic $(\Delta H < 0)$. Since gas molecules are adsorbed on the surface,their randomness decreases,leading to a decrease in entropy $(\Delta S < 0)$. Thus,statement $[A]$ is correct.
$2$. The extent of adsorption of a gas on a solid adsorbent is directly proportional to its critical temperature $(T_c)$. Since $T_c(\text{ethane}) = 563 \ K > T_c(\text{nitrogen}) = 126 \ K$,ethane is adsorbed more readily. Thus,statement $[B]$ is correct.
$3$. $A$ cloud is an aerosol where liquid droplets are dispersed in a gas. An emulsion is a colloid of liquid in liquid. Thus,statement $[C]$ is incorrect.
$4$. Brownian motion depends on both the size of the particles and the viscosity of the medium. Smaller particles exhibit more vigorous motion. Thus,statement $[D]$ is incorrect.

(A) The correct matches are as follows:
$(a)$ Starch is hydrolyzed by the enzyme Diastase $(iii)$.
$(b)$ Cane sugar (sucrose) is hydrolyzed by the enzyme Invertase $(iv)$.
$(c)$ Zeolites are well-known shape-selective catalysts $(i)$.
$(d)$ Maltose is converted to glucose by the enzyme Maltase $(ii)$.
Therefore,the correct sequence is $(a)-(iii), (b)-(iv), (c)-(i), (d)-(ii)$.

(C) Statement $a$ is incorrect: The protective power of a lyophilic sol is inversely proportional to its gold number. $A$ lower gold number indicates higher protective power.
Statement $b$ is incorrect: According to the Hardy-Schulze rule,the coagulating power of an ion increases with the increase in the magnitude of the charge on the ion. For negative sols,the order should be $Al^{3+} > Ba^{2+} > Na^{+}$.
Statement $c$ is incorrect: $A$ cloud is a liquid in gas type of colloid (aerosol).
Statement $d$ is correct: Physical adsorption is non-specific in nature and can form multiple layers on the surface of the adsorbent at high pressure.

(B) Initial surface area of the cube with edge length $L_1 = 1 \ cm$ is $A_1 = 6 \times (L_1)^2 = 6 \ cm^2$.
The edge length of the smaller cube is $L_2 = 1 \ nm = 10^{-7} \ cm$.
The number of smaller cubes $n$ is given by the ratio of volumes: $n = \frac{(L_1)^3}{(L_2)^3} = \frac{1^3}{(10^{-7})^3} = 10^{21}$.
The total surface area of all $n$ smaller cubes is $A_2 = n \times 6 \times (L_2)^2 = 10^{21} \times 6 \times (10^{-7})^2 = 10^{21} \times 6 \times 10^{-14} = 6 \times 10^7 \ cm^2$.
The ratio of the total surface area of the smaller cubes to the initial cube is $\frac{A_2}{A_1} = \frac{6 \times 10^7}{6} = 10^7$.

(D) Adsorption is a spontaneous process accompanied by a decrease in enthalpy $(\Delta H < 0)$ and a decrease in entropy $(\Delta S < 0)$. Since $\Delta G = \Delta H - T\Delta S$,for the process to be spontaneous,$\Delta G$ must be negative,which is satisfied when $|\Delta H| > |T\Delta S|$. Thus,statement $(A)$ is correct.
$(B)$ Chemisorption involves the formation of chemical bonds between the adsorbate and the adsorbent. Gases that react strongly with the adsorbent surface exhibit chemisorption. Thus,statement $(B)$ is correct.
$(C)$ According to the Freundlich adsorption isotherm,$\frac{x}{m} = k \cdot p^{1/n}$. If the slope $(1/n)$ is non-zero,the amount of gas adsorbed $(\frac{x}{m})$ depends on the pressure $(p)$. Therefore,adsorption is not independent of pressure. Thus,statement $(C)$ is incorrect.
$(D)$ The gold number is defined as the minimum mass in milligrams of a protective colloid required to prevent the coagulation of $10 \ mL$ of a gold sol by the addition of $1 \ mL$ of $10 \% \ NaCl$ solution. The gold number for potato starch is approximately $20-25$,not $0.15$. Thus,statement $(D)$ is incorrect.
Therefore,statements $(C)$ and $(D)$ are not correct.

(C) $(i)$ In the oxidation of oxalic acid with $KMnO_4$ in an acidic medium,$Mn^{2+}$ ions are produced which act as an autocatalyst. Thus,statement $(i)$ is correct.
$(ii)$ $CdS$ is a negatively charged sol. It is precipitated by the addition of cations (like $Ba^{2+}$),not anions like $Cl^{-}$. Thus,statement $(ii)$ is incorrect.
$(iii)$ Protective power is inversely proportional to the gold number. Given gold numbers: $A = 0.03$,$B = 25$,$C = 0.25$. Order of protective power: $A (0.03) > C (0.25) > B (25)$. Thus,statement $(iii)$ is correct.
$(iv)$ Physisorption is a reversible process,whereas chemisorption is irreversible. Thus,statement $(iv)$ is incorrect.