Surface Chemistry Class 12 Notes

Key Definitions in Surface Chemistry

Adsorption

The accumulation of molecular species at the surface rather than in the bulk of a solid or liquid.

Adsorbate

The substance which is adsorbed on the surface of another substance.

Adsorbent

The surface on which adsorption takes place.

Desorption

The process of removing an adsorbed substance from a surface.

Catalysis

The phenomenon of increasing the rate of a chemical reaction by adding a substance called a catalyst.

Colloids

Heterogeneous systems in which one substance is dispersed as very fine particles in another substance, with particle size ranging from 1 nm to 1000 nm.

Lyophilic Colloids

Colloids where the dispersed phase has a strong affinity for the dispersion medium (solvent loving), e.g., starch in water.

Lyophobic Colloids

Colloids where the dispersed phase has little or no affinity for the dispersion medium (solvent hating), e.g., metal sols.

Micelles

Aggregates of surfactant molecules that spontaneously form in solution above a certain concentration (Critical Micelle Concentration, CMC) and temperature (Krafft temperature).

Adsorption: The Surface Phenomenon and its Types

Adsorption is a surface phenomenon where molecules of a substance (adsorbate) are attracted and held to the surface of another substance (adsorbent). This process results in a higher concentration of the adsorbate on the surface compared to the bulk. It's an exothermic process, meaning heat is released during adsorption, leading to a decrease in the system's enthalpy ($\Delta H < 0$). Since molecules become restricted to the surface, the entropy of the system decreases ($\Delta S < 0$). For spontaneous adsorption, the Gibbs free energy change ($\Delta G = \Delta H - T\Delta S$) must be negative. Since both $\Delta H$ and $\Delta S$ are negative, adsorption is favorable at lower temperatures.

There are two main types of adsorption:

1. Physisorption (Physical Adsorption):

• Caused by weak van der Waals forces between the adsorbate and adsorbent.
• It is non-specific in nature; any gas can be adsorbed on any solid, though to varying extents.
• It is reversible; desorption occurs when temperature is increased or pressure is decreased.
• The enthalpy of adsorption is low (20-40 kJ/mol), comparable to the heat of liquefaction of gases.
• Forms multilayer adsorption; the adsorbed layer can be several molecules thick.
• Favored by low temperature and high pressure.

1. Chemisorption (Chemical Adsorption):

• Caused by strong chemical bonds (covalent or ionic) between the adsorbate and adsorbent.
• It is highly specific; occurs only when there is a possibility of chemical bond formation.
• It is generally irreversible; desorption requires very high temperatures, which can break the chemical bonds.
• The enthalpy of adsorption is high (80-240 kJ/mol), similar to that of chemical reactions.
• Forms a monolayer adsorption; only one layer of adsorbate molecules is formed.
• Favored by high temperature (initially increases with temperature due to activation energy, then decreases) and high pressure.

Factors affecting adsorption include the nature of the adsorbate (easily liquefiable gases adsorb more), nature of the adsorbent (porous and finely divided solids have greater surface area), temperature, and pressure. Adsorption isotherms (Freundlich, Langmuir) describe the relationship between the amount of gas adsorbed by an adsorbent and pressure at a constant temperature.

Physisorption vs. Chemisorption: A Quick Contrast

Catalysis: Speeding Up Reactions

Colloidal State: The World Between Solutions and Suspensions

The colloidal state represents a heterogeneous system where the size of dispersed particles is intermediate between true solutions and suspensions (typically 1 nm to 1000 nm). These systems exhibit unique properties due to their large surface area to volume ratio.

Classification of Colloids:

1. Based on Physical State of Dispersed Phase (DP) and Dispersion Medium (DM): Like true solutions, colloids can be solid-in-liquid, liquid-in-gas, etc. Examples include sols (solid in liquid), emulsions (liquid in liquid), gels (liquid in solid), aerosols (solid/liquid in gas).

1. Based on Nature of Interaction between DP and DM:

• Lyophilic Colloids: 'Solvent loving'. Formed by substances like gum, starch, proteins which directly mix with a suitable liquid (DM). They are quite stable and reversible. Examples: starch sol, protein sol.
• Lyophobic Colloids: 'Solvent hating'. Formed by substances like metals, metal sulphides, which do not directly mix with DM. Special methods are required for their preparation. They are less stable and irreversible. Examples: gold sol, ferric hydroxide sol.

1. Based on Type of Particles of Dispersed Phase:

• Multimolecular Colloids: Consist of aggregates of a large number of atoms or smaller molecules (e.g., gold sol, sulphur sol). Particles held by van der Waals forces.
• Macromolecular Colloids: Consist of large molecules (macromolecules) having colloidal dimensions (e.g., starch, nylon, proteins).
• Associated Colloids (Micelles): Substances which behave as strong electrolytes at low concentrations but aggregate to form colloidal particles (micelles) at higher concentrations (above Critical Micelle Concentration, CMC) and temperature (Krafft temperature). Examples: soaps and detergents (e.g., sodium stearate, C₁₇H₃₅COONa).

Preparation of Colloids:

• Condensation methods: For lyophobic colloids, involve bringing smaller units to colloidal size (e.g., by chemical reactions, excess cooling, solvent exchange).
• Dispersion methods: For lyophobic colloids, involve breaking down larger particles to colloidal size (e.g., mechanical dispersion, electrical disintegration/Bredig's Arc method, peptization).

Purification of Colloids:

• Dialysis: Separation of crystalloids from colloids by diffusion through a semi-permeable membrane.
• Electro-dialysis: Dialysis accelerated by an electric field.
• Ultrafiltration: Separating colloidal particles from the solvent and soluble solutes by specially prepared filters.

Properties of Colloidal Solutions:

• Tyndall Effect: Scattering of light by colloidal particles, making the path of light visible. This happens because particle size is comparable to the wavelength of light.
• Brownian Movement: Continuous zigzag motion of colloidal particles due to collision with DM molecules, which prevents particles from settling.
• Electrophoresis (or Cataphoresis): Movement of charged colloidal particles under the influence of an electric field towards the oppositely charged electrode.
• Coagulation/Flocculation: The process of precipitation of colloidal particles by adding an electrolyte. The charge on colloidal particles is neutralized by oppositely charged ions of the electrolyte. Hardy-Schulze rule states that the coagulating power of an electrolyte is directly proportional to the valency of the effective ion (the ion carrying charge opposite to that of the colloidal particle). For example, for a negatively charged sol, the order of coagulating power is Al³⁺ > Ba²⁺ > Na⁺.
• Emulsions: Colloidal solutions of two immiscible liquids (e.g., oil in water, water in oil). They are stabilized by emulsifying agents.

Coagulation Example (Hardy-Schulze Rule)

• {"heading":"Example: Comparing Coagulating Power","description":"A negatively charged arsenic sulphide (As₂S₃) sol needs to be coagulated. Which of the following electrolytes would be most effective: NaCl, BaCl₂, or AlCl₃?\n\nSolution:\nAccording to the Hardy-Schulze rule, for a negatively charged sol, the coagulating power depends on the charge of the cation. The ions provided are Na⁺, Ba²⁺, and Al³⁺.\nThe valencies are: Na⁺ (1), Ba²⁺ (2), Al³⁺ (3).\nTherefore, Al³⁺ has the highest positive charge and thus the highest coagulating power. AlCl₃ would be the most effective electrolyte for coagulating the negatively charged As₂S₃ sol."}

Key Points to Remember for Exams

• Adsorption is a surface phenomenon, adsorption isotherms (Freundlich, Langmuir) define its extent.
• Physisorption is weak, reversible, multilayered; Chemisorption is strong, irreversible, monolayered.
• Catalysts lower activation energy and provide an alternative reaction pathway, but do not change the equilibrium constant.
• Heterogeneous catalysis involves active sites on the catalyst surface and follows an adsorption mechanism.
• Enzymes are highly specific biochemical catalysts, often explained by the Lock and Key mechanism.
• Colloidal particles have sizes between 1 nm and 1000 nm, exhibiting properties like Tyndall effect and Brownian movement.
• Lyophilic colloids are stable and reversible; lyophobic colloids are less stable and require stabilizing agents.
• Hardy-Schulze rule: greater the valency of the flocculating ion, greater is its coagulating power.
• Micelles are associated colloids formed by surfactants above CMC and Krafft temperature.

Exam Tip: Mastering Surface Chemistry

For Surface Chemistry, definitions and distinctions are paramount. Be prepared to define all key terms (adsorption, desorption, catalysis, micelles, etc.) and clearly differentiate between physisorption and chemisorption, or lyophilic and lyophobic colloids. Practice writing these differences in tabular format for maximum clarity. Understand the underlying principles of the Hardy-Schulze rule for coagulation and the factors affecting enzyme activity. A common trap is confusing adsorption with absorption – remember, adsorption is a surface phenomenon, absorption is bulk. Focus on mechanisms for catalysis (especially heterogeneous and enzyme catalysis) and the applications of colloids.

Practice Questions with Solutions

• Q: What is the main difference between adsorption and absorption? A: Adsorption is a surface phenomenon where molecules accumulate on the surface, while absorption is a bulk phenomenon where molecules are uniformly distributed throughout the bulk of a substance.
• Q: Why is adsorption an exothermic process? A: Adsorption is exothermic because the attractive forces developed between adsorbate and adsorbent lead to a decrease in the potential energy of the system, releasing energy as heat.
• Q: State the Hardy-Schulze rule. For a positively charged sol, which ion (Cl⁻, SO₄²⁻, PO₄³⁻) would be most effective for coagulation? A: Hardy-Schulze rule states that the greater the valency of the oppositely charged ion, the greater is its coagulating power. For a positively charged sol, PO₄³⁻ would be most effective due to its higher negative charge (3⁻).
• Q: Give two applications of adsorption. A: Applications include gas masks for adsorbing poisonous gases, removal of colouring matter from solutions using activated charcoal, and heterogeneous catalysis.

Frequently Asked Questions