Physisorption and Chemisorption — Core Principles
Core Principles
Adsorption is a surface phenomenon where molecules (adsorbate) accumulate on a surface (adsorbent). It's distinct from absorption, which involves bulk penetration. There are two main types: physisorption and chemisorption.
Physisorption involves weak Van der Waals forces, is non-specific, reversible, forms multilayers, has a low enthalpy of adsorption (), and decreases with increasing temperature. Chemisorption involves strong chemical bonds, is highly specific, generally irreversible, forms a monolayer, has a high enthalpy of adsorption (), and typically increases with temperature initially before decreasing.
Both are exothermic processes. Understanding their differences is crucial for applications in catalysis, purification, and separation techniques.
Often confused with
Side-by-side differences the NEET paper likes to test.
| Aspect | Physisorption and Chemisorption | Chemisorption |
|---|---|---|
| Nature of forces | Weak Van der Waals forces | Strong chemical bonds (covalent, ionic) |
| Enthalpy of Adsorption ($\Delta H_{ads}$) | Low ($20-40 \text{ kJ/mol}$) | High ($80-240 \text{ kJ/mol}$) |
| Specificity | Non-specific | Highly specific |
| Reversibility | Reversible | Irreversible |
| Effect of Temperature | Decreases with increasing temperature | Increases then decreases with increasing temperature (often requires activation energy) |
| Layer Formation | Multilayer | Monolayer |
| Activation Energy | Negligible | Often significant |
| Conditions | Favored at low temperature, high pressure | Favored at relatively high temperature, often at moderate pressure |
| Example | Adsorption of $N_2$ on mica at $77\text{ K}$ | Adsorption of $H_2$ on nickel at $200\text{ K}$ |
Physisorption and chemisorption represent two distinct modes of adsorption, differentiated primarily by the nature and strength of the forces involved. Physisorption is a weak, non-specific, reversible process driven by Van der Waals forces, forming multilayers and favored at low temperatures.
Chemisorption, conversely, is a strong, highly specific, and generally irreversible process involving chemical bond formation, resulting in a monolayer and often requiring an activation energy, thus favored at higher temperatures (initially).
These fundamental differences dictate their respective applications and behaviors in various chemical and industrial processes.
Why it is tested: For NEET, understanding these differences is critical. Questions frequently test the distinguishing features, the effect of temperature and pressure, and the enthalpy changes associated with each type. Students must be able to identify which type of adsorption is occurring based on given conditions or properties, and relate them to practical examples like catalysis or gas masks.