Give three points of difference between physisorption and chemisorption.

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Q. Give three points of difference between physisorption and chemisorption.

CBSECBSE Class XII Board 2020Subjective· 3mImportance★★★★★est

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Concept understanding — Physisorption and Chemisorption

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.

Note

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.

Watch out

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 $E_a$ , just like any chemical reaction. This means:

At very low temperatures, chemisorption may be impossible because molecules lack the energy to overcome the barrier. …

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Physisorption and chemisorption differ fundamentally in the nature of the bonding force (weak van der Waals vs. strong chemical bonds), the enthalpy change (low vs. high), and the reversibility (easily reversible vs. often irreversible). These differences arise from whether the adsorbate molecules are merely held by intermolecular forces or actually form chemical bonds with the adsorbent surface.

The key to understanding the difference lies in what holds the adsorbate to the surface. In physisorption, it is the same weak, universal force that makes gases condense into liquids — van der Waals forces. In chemisorption, it is a true chemical bond, like the one that holds atoms together in a molecule. This single distinction cascades into every other property.

Let’s break down the three most important points of contrast.

Nature of the bonding force.

Physisorption involves van der Waals forces — weak, long-range, and non-specific. Any gas will physisorb on any solid to some extent, just as any gas will condense if cooled enough. Chemisorption involves chemical bond formation (covalent, ionic, or coordinate) — strong, short-range, and highly specific. A gas will chemisorb only on a surface that can form a bond with it. For example, oxygen chemisorbs on tungsten but not on gold.
Enthalpy of adsorption (heat released).

Because the forces are weak, physisorption releases a small amount of heat — typically in the range of 20–40 kJ/mol. This is comparable to the enthalpy of condensation. Chemisorption, forming a real bond, releases much more heat — typically 80–400 kJ/mol — comparable to the enthalpy of a chemical reaction. This is a direct, measurable consequence of bond strength.
Reversibility and activation energy.

Physisorption is easily reversible. Since no bond is broken, simply reducing the pressure or raising the temperature a little desorbs the gas. It requires no activation energy — it happens as soon as the molecule touches the surface. Chemisorption is often irreversible or requires high temperatures to reverse. It frequently has an activation energy barrier (like any chemical reaction), so it may be slow at low temperatures and only proceed at a measurable rate when the surface is heated. …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CBSE 2025Set D1 markMCQ
$Q. Which of the following adsorptions forms multimolecular layer?$
(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. …
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CBSE 2021Set A1 markMCQ
$Q. How many layers are involved in chemical adsorption?$
(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.

…

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CBSE 2021Set A1 markMCQ
$Q. In physical adsorption the approximate heat energy evolved in kJ/mol is$
(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. …
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CBSE 2020Set ANNUAL1 mark
$Q. Why does physisorption decrease with rise of temperature ?$
›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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Q.Give three points of difference between physisorption and chemisorption.

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