Q.Write two differences between physisorption and chemisorption. OR Explain why the solid catalyst is used in a finely divided form in case of heterogeneous catalysis.

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. …

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:

$\Delta G=\Delta H-T\Delta S$ …

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 anhydro …

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. …

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

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. …