Class 12 Chemistry Surface Chemistry Viva Questions With Answers

Physisorption (Physical Adsorption) and Chemisorption (Chemical Adsorption) are two frequently asked types of adsorption. The key differences are:

• Forces Involved: Physisorption involves weak van der Waals forces, while Chemisorption involves strong chemical bonds.
• Enthalpy of Adsorption: Physisorption has a low enthalpy (20-40 kJ/mol), whereas Chemisorption has a high enthalpy (80-240 kJ/mol).
• Specificity: Physisorption is not specific and can occur on any surface. Chemisorption is highly specific and only occurs if a chemical bond can be formed.
• Reversibility: Physisorption is reversible because the weak forces can be easily overcome by increasing temperature or decreasing pressure. Chemisorption is generally irreversible due to the formation of stable chemical bonds.

The two defining properties of colloidal solutions frequently tested in exams are:

• Tyndall Effect: This is the scattering of light by colloidal particles when a beam of light is passed through a colloidal solution. The path of the light becomes visible. This phenomenon is used to distinguish a true solution from a colloidal one, as true solutions do not scatter light.
• Brownian Movement: This refers to the continuous, random, zig-zag motion of colloidal particles. It is caused by the unbalanced bombardment of the colloidal particles by the smaller molecules of the dispersion medium. This movement provides stability to the colloid by preventing the particles from settling down.

Lyophilic and Lyophobic sols are two types of colloidal solutions based on the interaction between the dispersed phase and dispersion medium.

• Lyophilic Sols (Solvent-loving): These are colloids where the dispersed particles have a strong affinity for the dispersion medium. They are stable and reversible. Example: Starch or gelatin dissolved in water.
• Lyophobic Sols (Solvent-hating): In these colloids, the dispersed particles have little to no affinity for the dispersion medium. They are unstable and require a stabilising agent. Example: A sol of gold or sulphur in water.

The most important difference is that Lyophilic sols are easily formed by simple mixing, while Lyophobic sols require special methods for their preparation.

The Hardy-Schulze rule is a crucial concept for understanding the coagulation or precipitation of colloidal solutions. The rule states:

• The oppositely charged ions are effective for causing coagulation of a sol.
• The coagulating power of an electrolyte is directly proportional to the valency (charge) of the active ion (the ion carrying the opposite charge to the colloid).

For example, to coagulate a negative sol like arsenious sulphide (As₂S₃), the coagulating power of cations is in the order: Al³⁺ > Ba²⁺ > Na⁺. This principle is often a part of questions asking how to precipitate a given sol.

Adsorption is always exothermic because it is a spontaneous process where randomness decreases. According to thermodynamic principles:

• During adsorption, gas or solute molecules are held on a surface, which restricts their movement. This leads to a decrease in the entropy of the system (ΔS is negative).
• For any process to be spontaneous, the change in Gibbs free energy (ΔG) must be negative.
• As per the Gibbs-Helmholtz equation, ΔG = ΔH - TΔS.

Since ΔS is negative, the term (-TΔS) becomes positive. For ΔG to be negative, the change in enthalpy (ΔH) must be negative and its magnitude must be greater than TΔS. A negative ΔH indicates that the process is exothermic.

Heterogeneous catalysis, where the catalyst is in a different phase from the reactants, works through a mechanism explained by the modern adsorption theory. The steps are:

1. Diffusion of reactants: Reactant molecules move towards the surface of the catalyst.
2. Adsorption: Reactants are adsorbed onto the catalyst's active sites, weakening their bonds.
3. Formation of Intermediate: A chemical reaction occurs on the surface, forming an activated complex or intermediate.
4. Desorption of products: The product molecules detach from the catalyst surface.
5. Diffusion of products: The product molecules diffuse away from the surface, freeing up the active sites for more reactants.

A classic example is the hydrogenation of ethene using a Nickel catalyst, where ethene and hydrogen adsorb on the Nickel surface, react, and then ethane desorbs.

The cleansing action of soap is a key application of surface chemistry, specifically involving associated colloids (micelles).

• Soap Structure: A soap molecule has two parts: a long hydrophobic (water-repelling) hydrocarbon tail and a short hydrophilic (water-attracting) ionic head.
• Micelle Formation: In water, above a specific concentration called the Critical Micelle Concentration (CMC), soap molecules aggregate to form a spherical structure called a micelle. The hydrophobic tails point inwards, away from water, while the hydrophilic heads point outwards.
• Cleansing Action: The oily or greasy dirt, being hydrophobic, is trapped in the core of the micelle. When water is rinsed away, the entire micelle, along with the trapped dirt, is washed away. This action only happens above the CMC.

The classification of colloids into multimolecular, macromolecular, and associated types is based on the nature of their dispersed particles:

• Multimolecular Colloids: These are formed by the aggregation of a large number of small atoms or molecules (diameter < 1 nm) which are held together by weak van der Waals forces. Example: Sulphur sol (S₈ molecules aggregate) or Gold sol.
• Macromolecular Colloids: In this type, the dispersed particles are themselves large molecules (macromolecules) like polymers, which have sizes in the colloidal range when dissolved in a suitable solvent. Example: Starch, cellulose, proteins, or nylon in a suitable solvent.
• Associated Colloids (Micelles): These are substances that behave as normal, strong electrolytes at low concentrations but exhibit colloidal properties at higher concentrations due to the formation of aggregates (micelles). Example: Soaps and detergents in water above their CMC.

The effect of temperature on physisorption and chemisorption is a critical point of distinction and a common higher-order thinking question.

• Physisorption: It is an exothermic process involving weak van der Waals forces. According to Le Chatelier's principle, an increase in temperature shifts the equilibrium in the backward direction (desorption). Therefore, physisorption decreases with an increase in temperature.
• Chemisorption: It is also an exothermic process, but it involves the formation of chemical bonds which requires a certain activation energy. Therefore, chemisorption first increases with temperature as the heat supplies the necessary activation energy. After reaching a maximum, it begins to decrease because the overall process is exothermic.