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Adsorption is the accumulation of molecules (adsorbate) on a surface (adsorbent). It is a surface phenomenon, distinct from absorption (bulk phenomenon). Adsorption is always exothermic: $delta_H$ < 0 (surface energy decreases) and $delta_S$ < 0 (gas loses freedom). For spontaneity: $delta_G$ = $delta_H$ - T*$delta_S$ < 0. At low T, |$delta_H$| > |T*$delta_S$|, so adsorption is spontaneous. At high T, T*$delta_S$ term dominates, making $delta_G$ positive — desorption occurs. Two types: physisorption (van der Waals, 20-40 kJ/mol, multilayer, reversible, non-specific) and chemisorption (chemical bonds, 80-240 kJ/mol, monolayer, irreversible, specific). Physisorption decreases monotonically with temperature. Chemisorption may initially increase (activation energy needed) then decrease. Factors affecting adsorption: (1) Nature of adsorbent — porous, high surface area preferred (activated charcoal). (2) Nature of adsorbate — easily liquefiable gases adsorb better (higher critical temperature = stronger intermolecular forces). (3) Temperature — generally decreases adsorption. (4) Pressure — increases adsorption. Common adsorbents: activated charcoal, silica gel, alumina, zeolites. Applications: gas masks (charcoal), chromatography, water purification, heterogeneous catalysis.