Question
(NEET 2023 style.) Compare physical adsorption (physisorption) and chemical adsorption (chemisorption) in terms of nature of forces, enthalpy of adsorption, reversibility, and dependence on temperature. Also explain why physisorption decreases with increasing temperature while chemisorption initially increases.
Solution — Step by Step
| Property | Physisorption | Chemisorption |
|---|---|---|
| Forces | Van der Waals (weak) | Chemical bonds (strong) |
| 20–40 kJ/mol | 80–240 kJ/mol | |
| Reversibility | Reversible | Irreversible |
| Specificity | Non-specific | Highly specific |
| Activation energy | Low (often zero) | Often high |
| Layer formation | Multilayers | Monolayer only |
Physisorption is exothermic. By Le Chatelier’s principle, increasing temperature shifts the equilibrium “gas + surface ⇌ adsorbed gas” backward — desorbs the adsorbate. Higher kinetic energy of gas molecules makes them harder to “stick” via weak van der Waals forces. So adsorption decreases with increasing temperature.
Chemisorption forms a chemical bond, which requires activation energy to break the gas molecule’s existing bonds and form new bonds with the surface. At low T, the rate is too slow even though the equilibrium favours adsorption. As T increases, more molecules cross the activation barrier, so adsorption rises.
At high T, even chemisorption is exothermic (). Once the activation energy is overcome, further temperature increase shifts equilibrium backward (Le Chatelier again). The graph: rises, peaks, then decreases.
Why This Works
The key insight: physisorption is only thermodynamics-controlled (no significant activation barrier), so the equilibrium-based Le Chatelier argument applies directly. Chemisorption is first kinetics-controlled (activation barrier limits rate at low T), then thermodynamics-controlled (equilibrium limits extent at high T). This dual nature gives the characteristic peaked curve.
The same reasoning explains why catalysts (chemisorption-based) often have an “optimal temperature” — below which they are too slow, above which they desorb too easily.
Memorize the “ladder” of adsorption energies:
- Physisorption: 20–40 kJ/mol (like hydrogen bonds)
- Chemisorption: 80–240 kJ/mol (like single covalent bonds)
This 4×–6× difference is what determines all other properties — strength of binding determines reversibility, multilayer formation, specificity.
Alternative Method
Use the kinetic-molecular picture. Gas molecules have a Maxwell-Boltzmann distribution of speeds. For physisorption, only “slow” molecules stick — at higher T, fewer slow molecules exist, so adsorption decreases. For chemisorption, only “fast enough to react” molecules stick — at higher T, more fast molecules exist, so adsorption increases (until equilibrium kicks in).
Students think chemisorption always increases with temperature. Wrong. It rises, peaks, and falls. The peak temperature depends on the activation energy and of the specific system.
Final answer: physisorption — weak forces, low ΔH, reversible, decreases with T. Chemisorption — strong bonds, high ΔH, irreversible, peaks at intermediate T.