Adsorption is the accumulation of molecules (adsorbate) at the surface of a substance (adsorbent). It is fundamentally different from absorption — adsorption is a 2D surface process while absorption is a 3D bulk process. The combined process is sorption.

Physisorption vs. Chemisorption

The two types differ at every level. Physisorption is governed by van der Waals forces (enthalpy 20–40 kJ/mol), is non-specific, reversible, forms multiple layers, requires no activation energy, and decreases monotonically with temperature. The temperature behavior is crucial for NEET: since physisorption is exothermic, raising temperature shifts the equilibrium toward desorption (Le Chatelier's principle), decreasing the extent of physisorption.

Chemisorption involves chemical bond formation (enthalpy 80–240 kJ/mol), is highly specific, irreversible, forms only a monolayer, requires appreciable activation energy, and shows a maximum in the extent vs. temperature graph. At low temperatures, molecules lack enough energy to overcome the activation energy barrier, so chemisorption is low. As temperature increases, more molecules have sufficient energy → chemisorption increases. At very high temperatures, the chemisorbed bonds break and desorption dominates → chemisorption decreases.

Freundlich Isotherm

The Freundlich adsorption isotherm is an empirical equation: x/m = kP^(1/n), with 0 < 1/n < 1. This means adsorption increases sub-linearly with pressure. The logarithmic form is: log(x/m) = log k + (1/n) log P. A plot of log(x/m) versus log P gives a straight line: slope = 1/n and y-intercept = log k. The isotherm is valid only at intermediate pressures — it does not predict saturation (at very high pressures it incorrectly predicts x/m → ∞). The Langmuir isotherm corrects this by including a saturation term.

Factors Affecting Adsorption

Surface area is the most important factor — greater surface area means more active sites. Activated charcoal (specific surface area 1000–3000 $m^{2}$/g) is far more effective than ordinary charcoal. Among gases, those with higher critical temperature (more easily liquefied, stronger intermolecular forces) are adsorbed more strongly. Temperature: physisorption decreases; chemisorption shows a maximum. Pressure: adsorption generally increases with pressure up to saturation.

Thermodynamic Proof of Exothermicity

Using $\Delta G$ = $\Delta H$ − T$\Delta S$: adsorption is spontaneous ($\Delta G$ < 0); during adsorption, molecules move from 3D bulk to 2D surface, losing translational freedom → $\Delta S$ < 0. For $\Delta G$ < 0 with $\Delta S$ < 0: $\Delta H$ must be negative (exothermic) and |$\Delta H$| > T|$\Delta S$|. This proves all adsorption is exothermic. Conversely, at very high temperatures, T|$\Delta S$| eventually exceeds |$\Delta H$|, making $\Delta G$ > 0 — adsorption becomes non-spontaneous.

Applications

Gas masks (activated charcoal adsorbs toxic gases), water purification (adsorbs organic molecules and chlorine), sugar decolorization (animal charcoal removes coloring), silica gel desiccation (adsorbs moisture), and heterogeneous catalysis (chemisorption at catalyst surface activates reactants).