Adsorption | Surface Chemistry
Please watch: "General Principles and Processes of Isolation of Elements 12th One Shot | Bihar Board | Metallurgy" • General Principles and Processes of Isolat... -~- Adsorption is a surface phenomenon in which molecules or atoms (adsorbate) from a gas or liquid accumulate on the surface of a solid or liquid (called the adsorbent). It’s different from absorption, where a substance penetrates uniformly into the bulk of another material. In adsorption, the substance only adheres to the surface. 🧪 Types of Adsorption Physical Adsorption (Physisorption) Caused by weak van der Waals forces. Low heat of adsorption (~20–40 kJ/mol). Reversible and occurs at low temperature. Common example: adsorption of gases like CO₂ or N₂ on activated charcoal. Chemical Adsorption (Chemisorption) Involves formation of chemical bonds between adsorbate and adsorbent. High heat of adsorption (~80–400 kJ/mol). Irreversible and usually occurs at high temperature. Example: adsorption of hydrogen on nickel during hydrogenation. ⚙️ Factors Affecting Adsorption Nature of Adsorbent and Adsorbate: Rough or porous surfaces increase adsorption. Surface Area: Greater surface area → more adsorption. Temperature: For physisorption: decreases with temperature. For chemisorption: increases with temperature (up to a point). Pressure: For gases, adsorption increases with pressure. Concentration (for liquids): Higher concentration → greater adsorption. 📉 Adsorption Isotherms These describe how the amount adsorbed varies with pressure (at constant temperature). Freundlich Isotherm: 𝑥 / 𝑚 = 𝑘 𝑝 1 / 𝑛 x/m=kp 1/n where 𝑥 / 𝑚 x/m = mass of gas adsorbed per unit mass of adsorbent 𝑝 p = equilibrium pressure 𝑘 , 𝑛 k,n = constants Langmuir Isotherm: 𝜃 = 𝑏 𝑃 1 + 𝑏 𝑃 θ= 1+bP bP where 𝜃 θ = fraction of surface covered 𝑏 b = adsorption coefficient 𝑃 P = pressure 💡 Applications of Adsorption Activated charcoal in gas masks (adsorbs poisonous gases). Catalysis: Adsorption of reactants on catalyst surfaces. Chromatography: Separation of mixtures. Softening hard water: Adsorption of Ca²⁺, Mg²⁺ on zeolites. Drug delivery: Controlled adsorption/desorption in pharmaceutical systems.
