Question
Distinguish between physical adsorption (physisorption) and chemical adsorption (chemisorption). Give their key differences in a comparison table.
Solution — Step by Step
Adsorption is the accumulation of molecules (adsorbate) on the surface of another substance (adsorbent). It is a surface phenomenon — distinct from absorption, which involves the bulk of the material.
When gas molecules land on a solid surface, they can be held by two different types of forces:
- Physisorption: Held by weak van der Waals forces (London dispersion forces)
- Chemisorption: Held by strong chemical bonds (covalent or ionic)
The type of adsorption depends on the nature of the interactions between the adsorbate and adsorbent.
- Force: Van der Waals forces (weak, non-directional)
- Enthalpy change: Low heat of adsorption (~20–40 kJ/mol)
- Reversibility: Easily reversible — gas can be released by reducing pressure or increasing temperature
- Specificity: Non-specific — a gas may adsorb on any solid
- Layer formation: Multilayer adsorption possible
- Temperature dependence: Decreases with increasing temperature (exothermic, so Le Chatelier shifts equilibrium at higher temperature)
- Activation energy: Not needed (no bond breaking/forming)
- Example: Adsorption of N₂ gas on charcoal at low temperatures
- Force: Chemical bonds (covalent/ionic)
- Enthalpy change: High heat of adsorption (~80–400 kJ/mol)
- Reversibility: Irreversible or difficult to reverse
- Specificity: Highly specific — a particular adsorbate binds to a specific adsorbent
- Layer formation: Only monolayer adsorption (one layer of chemical bonds)
- Temperature dependence: Initially increases with temperature (needs activation energy), then decreases at very high temperatures
- Activation energy: Required (bonds must be formed)
- Example: Adsorption of H₂ on nickel surface (in catalytic hydrogenation)
| Property | Physisorption | Chemisorption |
|---|---|---|
| Forces involved | Van der Waals | Chemical bond (covalent/ionic) |
| Heat of adsorption | Low (20–40 kJ/mol) | High (80–400 kJ/mol) |
| Reversibility | Reversible | Irreversible |
| Specificity | Non-specific | Highly specific |
| Layers formed | Multilayer | Monolayer only |
| Temperature effect | Decreases with T ↑ | Increases then decreases with T ↑ |
| Activation energy | Not required | Required |
| Example | N₂ on charcoal | H₂ on Ni |
At low temperatures, physisorption predominates (weaker forces can hold the molecule). As temperature increases, the molecule gains enough energy to form chemical bonds — chemisorption begins. At very high temperatures, chemisorption also decreases because the adsorbed molecule gains enough energy to desorb (leave the surface).
So with increasing temperature, the typical pattern is: physisorption → chemisorption (transition) → desorption.
Why This Works
The two types of adsorption differ because of the nature of the intermolecular forces involved. Van der Waals forces are non-directional and non-specific — they work between any two molecules at close range, giving physisorption. Chemical bonds require specific orbital overlaps and involve electron sharing or transfer — giving chemisorption with higher energy and specificity.
The monolayer limit in chemisorption makes sense: once the surface atoms have formed chemical bonds with one layer of adsorbate molecules, there are no more free surface atoms to bond with the next layer. Van der Waals forces, being long-range, can still interact with subsequent layers, enabling multilayer physisorption.
Alternative Method
You can also compare them energetically: physisorption is like temporary parking (car can leave easily); chemisorption is like building a permanent garage attachment (hard to remove). The enthalpy values (20–40 kJ vs 80–400 kJ/mol) directly reflect this difference in bond strength.
Common Mistake
Students often write that chemisorption “always increases with temperature.” This is only true up to a point. At high temperatures, the reverse reaction (desorption) becomes dominant because the adsorbed species gains enough energy to break the surface bonds and escape. The correct statement is: chemisorption initially increases with temperature (because activation energy is needed), then decreases at high temperatures.