Q.Describe thermodynamical changes when a gas is adsorbed on a solid?
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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. …
When a gas is adsorbed on a solid it is spontaneous (ΔG < 0); the gas loses freedom so entropy decreases (ΔS < 0), and since ΔG = ΔH − TΔS, ΔH must be negative — adsorption is exothermic. …
Adsorption of a gas: ΔH < 0, ΔS < 0, ΔG < 0 (becoming 0 at equilibrium).
When a gas is adsorbed on a solid surface:
- Free energy: the process occurs on its own, so ΔG is negative.
- Entropy: gas molecules moving freely in 3-D become restricted on the 2-D surface, so their randomness (entropy) decreases → ΔS is negative.
- Enthalpy: from ΔG = ΔH − TΔS, with ΔG negative and (−TΔS) positive, ΔH must be sufficiently negative — i.e. adsorption is exothermic (heat of adsorption is released). …
- CBSE 2025Set D1 markMCQQ.Which of the following adsorptions forms multimolecular layer?(a) Physical adsorption(b) Chemical adsorption(c) Both (A) and (B)(d) None of these
›Reveal solutionSolution
Physical adsorption (physisorption) forms multimolecular layers; chemisorption forms only a monolayer.
Adsorption is classified into two types by the nature of the attractive force:
- Physical adsorption (physisorption): the adsorbate is held by weak van der Waals forces. These forces are non-specific and long-range, so once the first layer of molecules is adsorbed, further layers can pile up on top. Hence physisorption gives a multimolecular (multilayer) film, especially at low temperature and high pressure. …
- CBSE 2021Set A1 markMCQQ.How many layers are involved in chemical adsorption?(a) One(b) Two(c) Many(d) Zero
›Reveal solutionSolution
Chemical adsorption (chemisorption) is unilayer — only one layer forms.
In chemisorption the adsorbate is held to the surface by strong chemical (covalent) bonds. Because chemical bonding can only occur directly with the surface atoms, chemisorption stops after a single layer (monolayer) is formed — it cannot build up further layers.
…
- CBSE 2021Set A1 markMCQQ.In physical adsorption the approximate heat energy evolved in kJ/mol is(a) 20-40(b) 40-60(c) 60-80(d) 40-400
›Reveal solutionSolution
Physisorption is due to weak van der Waals forces, releasing a small heat of about 20-40 kJ/mol.
In physical adsorption (physisorption), the adsorbate molecules are held to the adsorbent surface by weak van der Waals forces. Because these forces are weak, only a small amount of energy is released.
- Physical adsorption: enthalpy of adsorption ≈ 20-40 kJ/mol. …
- CBSE 2020Set ANNUAL1 markQ.Why does physisorption decrease with rise of temperature ?
›Reveal solutionSolution
Since physisorption releases heat and relies only on weak attractions, warming the system pushes the equilibrium back towards the gas phase.
Physisorption (physical adsorption) is an exothermic process held together only by weak van der Waals forces between the adsorbate and the adsorbent surface. By Le Chatelier's principle, since heat is a "product" of an exothermic process, raising the temperature shifts the adsorption–desorption equilibrium towards desorption (away from the adsorbed state). Additionally, at higher temperature, adsorbed molecules gain more kinetic energy, making it easier for them to overcome the we …
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