Q.Which of the following is incorrect for physisorption?

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.

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:

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

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.

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.

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

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.

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.