Physisorption and Chemisorption: The Two Faces of Adsorption
Imagine a wall. If you throw a tennis ball at it, the ball might stick for a moment because of static cling or a bit of moisture — but a gentle tap knocks it off. That's one kind of sticking. Now imagine you throw a blob of superglue at the same wall. It bonds, changes the wall's surface, and you'd need to scrape it off. That's a different kind of sticking entirely.
Adsorption — the accumulation of a substance (the adsorbate) on the surface of another (the adsorbent) — happens in exactly these two ways. The first is physisorption (physical adsorption). The second is chemisorption (chemical adsorption).
Physisorption: The Gentle Touch
Physisorption is driven by van der Waals forces — the same weak, universal attractions that make gases deviate from ideal behaviour. These forces are present between any two molecules that are close enough, so physisorption is non-specific: any gas will physisorb on any solid surface, given the right conditions.
Because the forces are weak, the process is reversible. The adsorbate can be removed easily by lowering the pressure or raising the temperature — no chemical change occurs. The heat released (enthalpy of adsorption) is small, typically in the range of 20–40 kJ/mol.
Physisorption is like condensation: it increases as temperature decreases and as pressure increases. At low temperatures, gas molecules have less kinetic energy and are more easily trapped by the surface.
Key characteristics at a glance:
| Property | Physisorption |
|---|
| Force | van der Waals (weak) |
| Specificity | Non-specific — occurs on any surface |
| Reversibility | Reversible |
| Enthalpy change | Low (20–40 kJ/mol) |
| Activation energy | None — occurs instantly |
| Layers formed | Multilayers possible |
Chemisorption: The Chemical Grip
Chemisorption involves the formation of a chemical bond (covalent or ionic) between the adsorbate and the adsorbent surface. This is a true chemical reaction — electrons are shared or transferred, and the adsorbate molecule often gets dissociated into atoms or fragments.
Because a chemical bond is formed, chemisorption is highly specific. Hydrogen will chemisorb on nickel but not on gold; oxygen will chemisorb on platinum but not on glass. The process is often irreversible — you cannot simply pump away the adsorbed gas; you may need to heat the surface to extreme temperatures or chemically react it off.
The enthalpy of chemisorption is large, typically 80–400 kJ/mol — comparable to the energy of a covalent bond.
A common mistake: students think chemisorption is always slow. In fact, it can be fast if no activation energy is needed (e.g., oxygen on clean tungsten). But many chemisorption processes do have an activation barrier, making them slow at low temperatures — just like any chemical reaction.
Key characteristics at a glance:
| Property | Chemisorption |
|---|
| Force | Chemical bond (covalent/ionic) |
| Specificity | Highly specific |
| Reversibility | Often irreversible |
| Enthalpy change | High (80–400 kJ/mol) |
| Activation energy | Often present — increases with temperature |
| Layers formed | Monolayer only (bonds saturate) |
The Critical Difference: Activation Energy
This is the single most important distinction for exams.
Physisorption has zero activation energy — it happens the instant a molecule touches the surface. Chemisorption often requires an activation energy Ea, just like any chemical reaction. This means:
- At very low temperatures, chemisorption may be impossible because molecules lack the energy to overcome the barrier. …