Adsorption is a surface phenomenon where molecules of a substance (adsorbate) accumulate on the surface of a solid or liquid (adsorbent). Depending on the nature of the forces operating between the adsorbate and the adsorbent, adsorption is classified into two types: Physical Adsorption (Physisorption) and Chemical Adsorption (Chemisorption).
| Features | Physical Adsorption (Physisorption) | Chemical Adsorption (Chemisorption) |
|---|---|---|
| Forces of Attraction | Caused by weak intermolecular van der Waals forces. | Caused by strong chemical bond formation (covalent or ionic). |
| Specificity | Not specific; any gas can be adsorbed on a solid surface to some extent. | Highly specific; occurs only if there is chemical affinity between adsorbate and adsorbent. |
| Reversibility | It is completely reversible (desorption can happen easily by decreasing pressure or increasing temperature). | It is usually irreversible. |
| Enthalpy of Adsorption | Low enthalpy of adsorption (typically 20 to 40 kJ/mol). | High enthalpy of adsorption (typically 80 to 240 kJ/mol). |
| Temperature Effect | Favored by low temperature; it decreases with an increase in temperature. | Favored by high temperature; it first increases with temperature and then decreases. |
| Activation Energy | No appreciable activation energy is required. | Requires a certain amount of activation energy. |
| Layers Formed | Results in the formation of multimolecular layers under high pressure. | Results in the formation of a unimolecular (single) layer. |
| State of Adsorbate | Adsorbate molecules retain their identity and structure. | Adsorbate molecules may undergo dissociation or structural changes. |
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