Differentiate physisorption and chemisorption.

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Q. Differentiate physisorption and chemisorption.

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Concept understanding — Adsorption — Physisorption and Chemisorption

A solid surface attracts contacting gas or liquid species because atoms sitting right at the surface — unlike atoms buried in the bulk — have unsatisfied ('residual') valency, and this attraction happens strictly at the surface, which is what marks adsorption out as a surface phenomenon, distinct from absorption, where the absorbed substance spreads uniformly through the whole bulk of the absorbing material. The solid that does the attracting is the adsorbent; the substance attracted and held is the adsorbate; the surface of separation where the adsorbate's concentration builds up is the interface; and removing the adsorbed substance again is desorption. Where adsorption and absorption both occur together, the combined process is called sorption (M.C. Bain), and the specific case of gases taken up within a metal (e.g. hydrogen in palladium) is called occlusion (T. Graham).

Adsorption can occur at any interfacial surface — gas–solid, liquid–solid, liquid–liquid, solid–solid or gas–liquid — and is always spontaneous, so $\Delta G < 0$ , reaching equilibrium exactly when $\Delta G = 0$ . Because molecules moving from a free, disordered phase into an ordered adsorbed layer necessarily lose randomness, $\Delta S < 0$ for adsorption; since $\Delta G = \Delta H - T\Delta S$ and $-T\Delta S$ is therefore positive, $\Delta H$ must be sufficiently negative for $\Delta G$ to stay negative overall — so adsorption is always exothermic, a conclusion that follows directly from the entropy decrease. Adsorption is also characteristically fast, much faster than the comparatively slow, diffusion-limited process of absorption.

Based on the nature of the forces involved, adsorption splits into two kinds. Chemisorption holds the adsorbate by genuine chemical bonds, releases a large heat of adsorption (40–400 kJ/mole), is slow, highly specific to the particular adsorbent–adsorbate pair, involves real electron transfer, forms only a single monolayer, occurs only at fixed 'active centre' sites (so it depends on surface area), and needs appreciable activation energy to form an activated complex. With rising temperature it first increases (supplying the activation energy needed) and then decreases (desorption). Physisorption, by contrast, holds the adsorbate only by weak van der Waals forces, dipole–dipole interactions and dispersion forces, releases much less heat (of the order of 40 kJ/mole), is instantaneous and non-specific, involves no electron transfer, can build up a multilayer, occurs over the whole surface rather than fixed sites, needs essentially no activation energy, decreases monotonically as temperature rises, and — unlike chemisorption's amount, which pressure cannot increase much — genuinely increases in extent as pressure rises.

The extent of adsorption depends on four qualitative factors: the surface area of the adsorbent (higher area → more adsorbed, since adsorption is a surface effect); the nature of the adsorbate (easily-liquefiable gases like $\text{SO}_2$ , $\text{NH}_3$ , HCl, $\text{CO}_2$ , which have strong van der Waals attraction and high critical temperature, adsorb readily, while 'permanent' gases like $\text{H}_2$ , $\text{N}_2$ , $\text{O}_2$ , with low critical temperature, adsorb only slowly); temperature (chemisorption rises then falls, physisorption falls steadily); and pressure (chemisorption's rate rises with pressure but its total amount is capped by the fixed number of active sites, while physisorption's extent genuinely rises with pressure). These same trends show up directly in the adsorption isobar (x/m vs T at constant P): physisorption's isobar falls steadily, while chemisorption's isobar rises to a maximum (surface activation) and then falls (desorption as kinetic energy grows).

Step 1. The two types of adsorption are distinguished by the NATURE OF THE FORCE holding the adsorbate to the surface: physisorption uses weak van der Waals forces, dipole–dipole interactions and dispersion forces, while chemisorption uses genuine chemical bond formation.

Step 2. This difference in bonding strength directly explains the heat of adsorption: physisorption releases only ~40 kJ/mole, while chemisorption releases 40–400 kJ/mole.

Step 3. It also explains their speed and specificity: physisorption is instantaneous and non-specific (occurring with almost any adsorbent–adsorbate pair), while chemisorption is slow and highly specific to particular pairs, since a genuine chemical bond can only form between chemically compatible species.

Step 4. Structurally, physisorption can build up a multilayer of adsorbate, occurring over the whole surface, while chemisorption forms only a single monolayer, restricted to fixed 'active centre' sites — so chemisorption depends strongly on surface area while physisorption is less sensitive to it in this specific sense.

Step 5. With temperature, physisorption decreases steadily as temperature rises, while chemisorption first increases (as heat supplies the activation energy for bond formation) and then decreases (as desorption sets in). Finally, chemisorption genuinely involves electron transfer between adsorbent and adsorbate, while physisorption involves none, and chemisorption requires an activation energy to form an activated complex while physisorption's activation energy is essentially insignificant.

✓Final answer

Physisorption (weak physical forces, low heat, reversible, non-specific, multilayer, decreases with temperature) and chemisorption (chemical bonds, high heat, highly specific, monolayer, rises-then-falls with temperature, involves electron transfer) differ across virtually every measurable property, as tabulated in Table 10.1 of the chapter.

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

CBSE 2024Set ANNUAL1 mark
$Q. Define chemical adsorption.$
›Reveal solutionSolution
Chemisorption binds adsorbate to adsorbent by real chemical bonds, unlike physisorption's weak van der Waals forces.

Adsorption is the accumulation of a substance (adsorbate) at the surface of another substance (adsorbent). It is of two types — physical adsorption (physisorption) and chemical adsorption (chemisorption).

Chemical adsorption (chemisorption): the adsorbate molecules are held to the adsorbent surface by strong chemical forces (covalent or ionic bonding), similar in strength to a true chemical bond.

Characteristics:

Heat of adsorption is high, typically 80-240 kJ/mol (comparable to a chemical reaction).

It is highly specific — occurs only between an adsorbate and adsorbent that can form a chemical bond.

It generally requires activation energy, so it increases with temperature initially (unlike physisorption).

It forms only a monolayer on the surface and is largely irreversible.

Example: oxygen adsorbed on tungsten forms a layer of tungsten oxide (chemical bonding), and H2 chemisorbed on a Ni surface during catalytic hydrogenation.

✓Final answer
$Chemisorption (chemical adsorption) is the adhesion of adsorbate molecules to an adsorbent surface via strong chemical bonds, with a high heat of adsorption (80-240 kJ/mol), high specificity, and (usually) monolayer, irreversible character.$
CBSE 2022Set ANNUAL1 markMCQ
$Q. In the equilibrium state adsorption process follows the condition$
(a) dH > 0
(b) dH = T.dS
(c) dH > T.dS
(d) dH < T.dS OR The rate of adsorption increases in the solution of adsorbent when
(a) the quantity of the adsorbent in the solution increases
(b) the area of the surface of the adsorbent in the solution decreases
(c) the temperature of the solution increases
(d) the quantity of the adsorbent in the solution decreases
›Reveal solutionSolution

At equilibrium, the Gibbs free energy change for the adsorption process is zero, which forces $\Delta H = T\Delta S$ .

Reasoning: Adsorption is generally a spontaneous, exothermic process ( $\Delta H<0$ ) accompanied by a decrease in entropy ( $\Delta S<0$ , since the adsorbate loses translational freedom on binding to the surface). The Gibbs free energy relation is:

$\Delta G=\Delta H-T\Delta S$

For the process to be spontaneous, $\Delta G<0$ (requiring $|\Delta H|>|T\Delta S|$ in magnitude, since both terms are negative here). As adsorption proceeds, $\Delta S$ becomes progressively less negative in magnitude (or the system approaches a balance) until, at the equilibrium state (dynamic equilibrium between adsorption and desorption), $\Delta G=0$ . Setting $\Delta G=0$ in the equation above gives:

$0=\Delta H-T\Delta S \implies \Delta H=T\Delta S$

✓Final answer
$\Delta H=T\Delta S$
CBSE 2021Set TERM11 markMCQ
$Q. The term "sorption" stands for :$
(a) absorption
(b) adsorption
(c) Both absorption and adsorption
(d) None of these
›Reveal solutionSolution

Sorption is the general term used when it isn't clear (or both processes occur together) whether uptake is by absorption or adsorption.

Adsorption is a surface phenomenon — the substance gets concentrated only at the surface, not inside the bulk. Absorption is a bulk phenomenon — the substance is uniformly distributed throughout the bulk of the solid/liquid. When both processes occur simultaneously (e.g., water vapour taken up by anhydrous CaCl2 — partly adsorbed on the surface and partly absorbed into the bulk), the phenomenon is called sorption.

✓Final answer
$Sorption stands for both absorption and adsorption (option c).$
CBSE 2019Set ANNUAL1 markMCQ
$Q. The physical adsorption is due to$
(a) strong coulombic forces
(b) Vander waals' forces
(c) hydrogen bonding
(d) covalent bond formation
›Reveal solutionSolution

Physisorption arises purely from weak van der Waals attractions between the adsorbate molecules and the adsorbent surface — option (b).

Physical adsorption does not involve any chemical bond formation; the adsorbate molecules are held to the adsorbent's surface only by weak, non-specific van der Waals forces (dispersion/dipole forces), similar to the forces responsible for the liquefaction of gases. Because these forces are weak, physisorption has a low enthalpy of adsorption (typically 20–40 kJ/mol), is readily reversible, and adsorbed molecules can build up in multiple layers. This is unlike chemisorption, which involves the formation of strong chemical bonds (ionic/covalent-type) between adsorbate and adsorbent.

✓Final answer
$(b) Van der Waals' forces — the weak, non-specific attractive forces responsible for physical adsorption.$

CBSE 2018Set ANNUAL1 mark

Q. Differentiate between chemisorption and physisorption.

›Reveal solution

The two types of adsorption differ in the strength/nature of the forces binding adsorbate to adsorbent.

Feature	Physisorption	Chemisorption
Force involved	Weak van der Waals forces	Strong chemical bond (covalent-like) forces
Enthalpy of adsorption	Low (20–40 kJ/mol)	High (80–240 kJ/mol)
Specificity	Not specific — occurs on any surface	Highly specific — occurs only between suitable adsorbate–adsorbent pairs
Reversibility	Reversible	Generally irreversible
Layer formed	Can form multimolecular layers	Forms only a monomolecular layer
Effect of temperature	Decreases with rise in temperature	First increases then decreases with temperature (needs activation energy)
Activation energy	Low/none needed	Often needs significant activation energy

Both phenomena occur at a surface (adsorbent) where adsorbate particles accumulate, but physisorption is essentially a physical, non-specific 'condensation'-like effect, while chemisorption involves actual electron sharing/transfer forming new chemical bonds.

✓Final answer

Chemisorption = strong chemical bonding, monolayer, mostly irreversible, specific. Physisorption = weak van der Waals forces, multilayer, reversible, non-specific.

CBSE 2018Set ANNUAL1 markMCQ
$Q. For chemisorption, which is wrong ?$
(a) it forms multimolecular layers on adsorbate
(b) irreversible
(c) surface compounds are formed
(d) it requires activation energy
›Reveal solutionSolution
Chemisorption involves chemical bond formation between adsorbate and adsorbent, so it is confined to a single molecular layer, unlike physisorption which can form multiple layers.

Characteristics of chemisorption:

It forms only a monomolecular layer on the adsorbent surface (never multimolecular) — statement (a) is therefore WRONG.

It is generally irreversible because chemical bonds are formed between adsorbate and adsorbent — statement (b) is correct.

Surface compounds are formed due to chemical interaction — statement (c) is correct.

It requires activation energy (it is 'activated adsorption') — statement (d) is correct.

Multimolecular (multilayer) adsorption is instead a hallmark of physisorption (physical adsorption held together by weak van der Waals forces), not chemisorption.

✓Final answer
$a) it forms multimolecular layers on adsorbate$

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Q.Differentiate physisorption and chemisorption.

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