Surface Chemistry

Step 1: Definition.
Coagulation is the process of aggregation of colloidal particles, resulting in the settling down (precipitation) of colloids. It occurs when the stability of the colloidal system is disturbed.
Step 2: Causes of coagulation.
\begin{enumerate} \item By addition of electrolytes: Electrolyte ions neutralize the charges on colloidal particles, causing them to aggregate. \item By mutual coagulation: Mixing two oppositely charged sols can neutralize charges, causing precipitation. \item By boiling: Heating increases particle collisions and reduces stability. \item By electrophoresis: Charged particles move towards the oppositely charged electrode and get coagulated. \end{enumerate} Conclusion:
Coagulation is the destruction of colloidal stability, leading to precipitation of particles.

Step 1: Structure of soap molecules.
Soap molecules consist of two parts: - A long non-polar hydrophobic tail (hydrocarbon chain). - A polar hydrophilic head (–COO group).
Step 2: Micelle formation.
When soap is added to water, molecules arrange themselves into spherical aggregates called micelles. The hydrophobic tails remain inside, while the hydrophilic heads face water. This allows soap to trap grease and oil.
Step 3: Emulsification.
Grease and oil droplets are surrounded by soap micelles, converting them into small emulsified particles that can be washed away with water.
Conclusion:
The cleansing action of soap is due to micelle formation and emulsification, which removes dirt and oil from clothes and skin.

Step 1: Definition.
-
Lyophilic colloids: Colloids in which the dispersed phase has high affinity for the dispersion medium. They are stable and reversible.
Example: Starch sol, Gum, Gelatin.
-
Lyophobic colloids: Colloids in which the dispersed phase has little or no affinity for the dispersion medium. They are unstable and irreversible.
Example: Gold sol, Sulphur sol, As S sol.
Step 2: Reason for easy coagulation of lyophobic colloids.
Lyophobic sols are not strongly solvated and lack strong interactions with the solvent. They are stabilized mainly by charges on their particles. When an electrolyte is added, these charges are neutralized, leading to easy coagulation (precipitation).
Conclusion:
Lyophilic sols are stable due to strong solvation, while lyophobic sols are easily coagulated because they rely only on electrostatic stabilization.

(i) Electrophoresis:
Electrophoresis is a laboratory technique used to separate charged molecules (such as proteins, DNA, or RNA) by applying an electric field. The molecules are loaded into a gel matrix, and an electric current is passed through the gel. Charged particles migrate towards the electrode of opposite charge. The rate of migration depends on the size, shape, and charge of the molecule. Smaller molecules or those with a higher charge density move faster than larger or less charged ones. Electrophoresis is commonly used for DNA fingerprinting, protein analysis, and RNA separation.

(ii) Dialysis:
Dialysis is a process that separates small molecules or ions from larger molecules by passing them through a semi-permeable membrane. The membrane allows small molecules to diffuse across it while larger molecules, such as proteins, are retained. Dialysis is used in medical treatments, such as kidney dialysis, to remove waste products from blood. In laboratory applications, it is used to remove salts or other small impurities from protein solutions.

(i) True solution vs Suspension.
- True solution: Homogeneous, particle size<1 nm, particles invisible, stable (e.g., sugar solution).
- Suspension: Heterogeneous, particle size>1000 nm, visible particles, unstable (e.g., chalk in water).
(ii) Lyophilic vs Lyophobic colloids.
- Lyophilic: Solvent-loving, stable, easily formed, reversible (e.g., starch sol, gelatin).
- Lyophobic: Solvent-hating, unstable, requires special methods, irreversible (e.g., gold sol).
(iii) Multimolecular vs Macromolecular colloids.
- Multimolecular: Colloidal particles are aggregates of small molecules (e.g., gold sol).
- Macromolecular: Large molecules act as colloids (e.g., starch, proteins).

(i) Explanation.
Physical adsorption is exothermic in nature. According to Le Chatelier’s principle, increasing temperature decreases the extent of exothermic processes, hence adsorption decreases.
(ii) Explanation.
Powdered materials have more surface area exposed compared to crystalline solids. Adsorption being a surface phenomenon, greater surface area enhances adsorption.
Conclusion.
(i) Adsorption decreases with temperature.
(ii) Powdered form acts as a better adsorbent due to large surface area.

(i) Tyndall Effect:
The Tyndall effect refers to the scattering of light by colloidal particles in a medium. When a beam of light passes through a colloidal solution, the particles scatter the light, making the path of the beam visible. This effect is used to distinguish between a true solution and a colloidal solution. The scattering is more intense in colloidal solutions due to the larger particle size compared to true solutions.
(ii) Brownian Movement:
Brownian movement is the random motion of particles suspended in a fluid (liquid or gas) due to collisions with molecules of the surrounding medium. This erratic movement was first observed by Robert Brown in 1827 and provides evidence of the kinetic nature of matter. The movement is more prominent in smaller particles and decreases as the size of the particles increases.
(iii) Coagulation:
Coagulation refers to the process in which colloidal particles aggregate to form larger particles. This occurs when the charges on the colloidal particles are neutralized or when the stability of the colloidal dispersion is disturbed, typically by adding an electrolyte. Coagulation can result in the separation of the colloidal particles from the dispersion medium, as seen in the formation of a precipitate.

Step 1: Understanding lyophilic colloids.
Lyophilic colloids are "liquid-loving" colloids that form readily when mixed with the dispersion medium. They are stable and reversible in nature. Examples: starch sol, gum sol.
Step 2: Understanding lyophobic colloids.
Lyophobic colloids are "liquid-hating" colloids that do not form readily and require special methods for preparation. They are less stable and irreversible. Examples: ferric hydroxide sol, gold sol.
Step 3: Conclusion.
Thus, starch sol and gum sol are lyophilic colloids, while ferric hydroxide sol and gold sol are lyophobic colloids.

Step 1: Nature of colloidal particles.
Colloidal particles carry either a positive or negative charge. Their stability depends on the repulsion between similarly charged particles.
Step 2: Role of oppositely charged ions.
When oppositely charged ions are added, they neutralize the charge on the colloidal particles and cause coagulation.
Step 3: Order of coagulating power.
For negatively charged sols: Al >Ba >Na . For positively charged sols: [Fe(CN) ] >PO >SO >Cl .
Step 4: Conclusion.
Thus, the higher the valency of the oppositely charged ion, the greater its coagulating power.

Step 1: Nature of ferric oxide sol.
Ferric oxide sol (Fe O ) is a positively charged colloid when dispersed in water. This is because Fe ions from hydrolysis impart a positive charge to the dispersed particles.
Step 2: Verification of options.
- (A) Negative: Incorrect, ferric oxide sol is not negatively charged.
- (B) Positive: Correct, because it acquires Fe ions.
- (C) No charge: Incorrect, as colloidal particles always carry some charge.
- (D) None of these: Incorrect, as positive charge is correct.
Step 3: Conclusion.
Hence, the charge on colloidal particles of Fe O is positive.

Step 1: Understanding heterogeneous catalysis.
Heterogeneous catalysis occurs when the catalyst is in a different phase from the reactants. Usually, solid catalysts act on gaseous or liquid reactants. Step 2: Adsorption theory.
The adsorption theory explains catalysis in terms of adsorption of reactant molecules on the surface of the solid catalyst: \begin{enumerate} \item Reactant molecules are adsorbed on the surface of the catalyst. \item Adsorption weakens the chemical bonds in reactant molecules, lowering activation energy. \item The adsorbed reactants combine to form products. \item The product molecules desorb (leave) from the catalyst surface, making it free for the next reaction. \end{enumerate} Step 3: Example.
In the Haber process, and gases are adsorbed on finely divided Fe catalyst. Their bonds weaken, allowing formation of . Conclusion:
The adsorption theory provides a clear explanation for the mechanism of heterogeneous catalysis, showing how catalysts work by adsorption, bond weakening, and product desorption.