Question
Distinguish between physisorption and chemisorption, and explain why physisorption decreases with temperature while chemisorption first increases and then decreases.
Solution — Step by Step
Physisorption (physical adsorption) — the adsorbate sticks to the adsorbent through weak van der Waals forces. Enthalpy of adsorption is small (–).
Chemisorption (chemical adsorption) — the adsorbate forms an actual chemical bond with the surface. Enthalpy is large (–).
| Property | Physisorption | Chemisorption |
|---|---|---|
| Forces | van der Waals | Chemical bond |
| Enthalpy | 20-40 kJ/mol | 80-240 kJ/mol |
| Specificity | Non-specific | Highly specific |
| Layer | Multilayer | Monolayer |
| Reversibility | Reversible | Irreversible |
| Activation energy | Negligible | Significant |
Physisorption is exothermic and the bond is weak. At higher temperatures, kinetic energy of gas molecules exceeds the adsorption energy, so molecules desorb back into the gas phase. Physisorption decreases monotonically with temperature.
Chemisorption requires breaking and forming actual bonds — it has a significant activation energy. At low temperatures, molecules don’t have enough energy to overcome this barrier, so adsorption is slow.
As temperature rises, more molecules clear the activation barrier, and chemisorption increases. But once the surface is saturated and high enough kinetic energy starts driving desorption (chemisorption is also exothermic), the rate decreases again.
The net result is a maximum at some intermediate temperature.
Why This Works
The opposite trends arise because physisorption has no activation barrier (so only desorption matters at higher T), while chemisorption has a barrier (so kinetic and thermodynamic effects compete). This is a classic NEET/JEE conceptual question — every year some variant appears.
Alternative Method
Plot vs (van ‘t Hoff plot). Physisorption gives a straight line with positive slope (since is negative). Chemisorption gives a more complex curve due to the activation barrier — but at high T, the same negative- behavior takes over.
Common Mistake
Students claim chemisorption “always increases with temperature” or “is independent of temperature.” Both are wrong. The correct picture is a peak — first rise, then fall. Mark this in your notes; it is asked almost every year in NEET.