Physisorption vs Chemisorption

Physisorption is driven by London dispersion forces, dipole–dipole interactions, and induced-dipole forces — collectively called van der Waals forces. These are electrostatic in origin but much weaker than covalent bonds (typically 0.1–10 kJ mol⁻¹ per contact). Chemisorption involves the overlap of electron orbitals between adsorbate and adsorbent surface atoms, forming bonds with energies of 40–400 kJ mol⁻¹. For example, hydrogen gas adsorbing on nickel dissociates: H₂ + 2 Ni(surface) → 2 Ni–H, creating genuine Ni–H sigma bonds.

Adsorption is always exothermic (ΔH_ads < 0) because surface molecules have unsatisfied valencies that are partially satisfied on adsorption, lowering enthalpy. For physisorption, |ΔH_ads| is typically 5–40 kJ mol⁻¹ — comparable to enthalpies of condensation. For chemisorption, |ΔH_ads| ranges from 40 kJ mol⁻¹ up to 400 kJ mol⁻¹, similar to bond dissociation energies. The magnitude of ΔH_ads is therefore a direct diagnostic: if calorimetric measurement yields > 40 kJ mol⁻¹, chemisorption is occurring.

Because physisorption involves only weak forces, it is readily reversible: reducing the partial pressure of the adsorbate or increasing temperature restores the original adsorbent surface with no chemical change. This reversibility underpins physical adsorption-based gas separation and BET surface area measurements. Chemisorption is generally irreversible under mild conditions; restoring the surface requires breaking chemical bonds, which demands high temperatures or reactive chemical treatments. For instance, CO chemisorbed on Pt catalyst surfaces can only be removed by oxidation (CO + ½O₂ → CO₂) or very high-temperature reduction.

Physisorption is non-specific: virtually any gas will physisorb on any solid at sufficiently low temperature, because van der Waals forces act universally. The rate of physisorption has essentially zero activation energy — molecules adsorb as soon as they reach the surface. Chemisorption is highly specific: only certain adsorbate–adsorbent combinations can form the required electronic overlap. It may also require an activation energy (E_a > 0) — called activated adsorption — because bonds within the adsorbate molecule (e.g., H–H in H₂) must begin to stretch before the surface bond can form. This explains why chemisorption often increases with temperature in a range where physisorption decreases.

Chemisorption produces only a monolayer of adsorbate, since the chemical bond is localized between the adsorbate and the top-most surface atoms; once every surface site is occupied, no further chemisorption can occur. Physisorption, involving long-range forces, can produce multilayer adsorption — molecules adsorb on top of already-adsorbed layers. The BET (Brunauer–Emmett–Teller) theory models exactly this multilayer physisorption to determine adsorbent surface areas, which is a standard industrial technique.

Physisorption decreases monotonically as temperature rises, because thermal energy overcomes the weak van der Waals attraction, desorbing molecules. Chemisorption shows a more complex behaviour: at very low temperatures the process may be kinetically hindered (activation energy not overcome), so increasing temperature initially increases the extent of chemisorption; at very high temperatures, the equilibrium shifts toward desorption. A plot of amount adsorbed vs. temperature for a system like H₂ on Ni therefore shows a maximum — a hallmark of chemisorption with activated adsorption.

• \Delta G_{ads} — Gibbs free energy of adsorption (kJ mol⁻¹): Must be negative for spontaneous adsorption. Becomes less negative (less favourable) at higher temperatures because adsorption restricts molecular freedom, making ΔS_ads negative.
• \Delta H_{ads} — Enthalpy of adsorption (kJ mol⁻¹): Always negative (exothermic). Physisorption: −5 to −40 kJ mol⁻¹; Chemisorption: −40 to −400 kJ mol⁻¹.
• T — Absolute temperature (K): Higher temperatures increasingly disfavour adsorption due to the entropy penalty.
• \Delta S_{ads} — Entropy of adsorption (J mol⁻¹ K⁻¹): Always negative: adsorbate molecules lose translational and rotational freedom when confined to the surface.

Problem. A student measures the enthalpy of adsorption of nitrogen gas (N₂) on two different solids: Solid A gives ΔH_ads = −15 kJ mol⁻¹ and Solid B gives ΔH_ads = −180 kJ mol⁻¹. (a) Classify the adsorption on each solid. (b) Predict which adsorption will be reversible by simply reducing N₂ pressure at room temperature. (c) On Solid B, would you expect mono- or multilayer adsorption?

Answer. Solid A: physisorption (reversible, multilayer possible). Solid B: chemisorption (irreversible under mild conditions, monolayer only).

• Langmuir Adsorption Isotherm — The Langmuir isotherm quantitatively models monolayer chemisorption, directly applying the distinction between physisorption and chemisorption to derive an equation for fractional surface coverage as a function of pressure.
• Freundlich Adsorption Isotherm — The Freundlich isotherm is an empirical model that often describes physisorption on heterogeneous surfaces and introduces the concept of adsorption capacity and intensity constants.
• Heterogeneous Catalysis — Industrial catalysts work by chemisorbing reactants onto a solid surface, lowering activation energy. Understanding chemisorption is essential to explain why catalysts are specific and how they become poisoned.
• Van der Waals Forces — The intermolecular forces underlying physisorption — London dispersion, dipole–dipole, and induced-dipole interactions — are the same van der Waals forces that determine boiling points and physical states of substances.
• BET Surface Area Analysis — The BET method exploits reversible multilayer physisorption of N₂ at 77 K to measure the specific surface area of porous solids — a direct industrial application of physisorption principles.