NCERT Books for Class 12 Chemistry Chapter5 - Surface Chemistry

To score full marks, present the differences in a clear tabular format. For a 3-mark question, provide at least four distinct points of comparison. Key points to include are:

• Nature of Forces: Weak van der Waals' forces (Physisorption) vs. strong chemical bonds (Chemisorption).
• Enthalpy of Adsorption: Low, around 20-40 kJ/mol (Physisorption) vs. High, around 80-240 kJ/mol (Chemisorption).
• Reversibility: Reversible in nature (Physisorption) vs. Irreversible in nature (Chemisorption).
• Specificity: Not specific (Physisorption) vs. Highly specific (Chemisorption).
• Temperature Dependence: Favoured at low temperatures (Physisorption) vs. Favoured at high temperatures (Chemisorption).

Lyophilic colloids (solvent-loving) are more stable than lyophobic colloids (solvent-hating) for two primary reasons:

1. Solvation: The dispersed particles in lyophilic colloids are extensively solvated (hydrated if water is the medium). This layer of solvent around the particle prevents them from coming close and aggregating.
2. Charge: Like lyophobic colloids, these particles also carry a charge which causes repulsion between them.

A Higher Order Thinking Skills (HOTS) question could ask: 'An electrolyte is added in small quantity to a lyophobic sol and in a large quantity to a lyophilic sol to cause coagulation. Explain the difference in behaviour.'

The Hardy-Schulze rule is crucial for understanding the coagulation of colloidal solutions. The rule states two key things:

• The ion responsible for causing coagulation is the one carrying a charge opposite to that of the colloidal particles.
• The coagulating power of an electrolyte is directly proportional to the valency (charge) of the active ion.

For example, to coagulate a negative sol like arsenious sulphide (As₂S₃), the coagulating power of cations would be in the order: Al³⁺ > Ba²⁺ > Na⁺. This rule is frequently tested in 1-mark or 2-mark questions where you have to arrange electrolytes in order of their coagulating power.

From the topic of Catalysis, expect questions that test your understanding of its types and features. Important question formats include:

• Differentiating between homogeneous and heterogeneous catalysis with one example of each.
• Explaining the concepts of activity and selectivity of solid catalysts with industrial examples, such as the synthesis of different products from CO and H₂ using different catalysts.
• Short notes on shape-selective catalysis by zeolites, a topic often asked for 2 marks.

The Freundlich Adsorption Isotherm is considered empirical because it is based on experimental observations and lacks a strong theoretical foundation. Its major limitation, often a point of focus in exam questions, is its failure at high pressures. The equation suggests that adsorption will increase indefinitely with pressure, but experimentally, adsorption reaches a saturation point where the surface is fully covered. For an exam question, you must state that the Freundlich isotherm is valid only over a limited range of pressure and fails to predict behaviour at saturation pressure.

The cleansing action of soap is a prime application of micelle formation, a key concept in Surface Chemistry. For a 3-mark question, you should explain:

• A soap molecule has a hydrophilic (water-loving) head and a hydrophobic (oil-loving) tail.
• Above the Critical Micelle Concentration (CMC), these molecules aggregate to form micelles, with the hydrophobic tails pointing inwards and hydrophilic heads outwards.
• The oily dirt on clothes is trapped in the hydrophobic core of the micelle.
• When rinsed with water, the entire micelle, along with the dirt, is washed away due to the interaction of the hydrophilic heads with water. This entire process is called emulsification.