Physisorption vs Chemisorption: Differences Explained

Adsorption comes in two distinct types. Physisorption (physical adsorption) holds molecules to a surface with weak van der Waals forces, the same gentle attraction that lets a gecko cling to a wall. Chemisorption (chemical adsorption) holds them with true chemical bonds formed by sharing or transferring electrons between the adsorbate and the surface. Though both pile molecules onto surfaces, they differ greatly in strength, specificity, and behaviour.

Telling them apart matters because catalysis, gas sensors, and surface coatings depend on which type dominates. Catalysis needs chemisorption, but too strong a bond poisons the surface. JEE and NEET love comparison tables of the two types.

Key Concepts

Enthalpy of adsorption is the clearest numeric difference: physisorption releases only about 20 to 40 kJ mol-1, similar to the heat of liquefaction, while chemisorption releases 40 to 400 kJ mol-1, comparable to chemical bond energies. Physisorption is non-specific: any gas physisorbs on any cold surface. Chemisorption is highly specific: only certain gases chemisorb on certain surfaces, for example hydrogen on nickel or oxygen on platinum. Physisorption builds multilayers because van der Waals forces act at a distance, while chemisorption forms only a monolayer since it needs direct contact with surface atoms. Physisorption is reversible and needs no activation energy; chemisorption is often irreversible and may need activation energy, like a chemical reaction.

How It Works

In physisorption, the process is: Step 1: gas molecules drift near the cold surface. Step 2: weak van der Waals attractions pull them in, releasing a small amount of heat. Step 3: additional layers pile on top of the first because the forces reach beyond one molecular diameter. In chemisorption: Step 1: molecules strike specific active sites on the surface. Step 2: electron sharing or transfer forms a genuine surface bond, releasing large heat. Step 3: no second layer forms because the second layer cannot touch the surface atoms directly. Raising the temperature easily reverses physisorption, but chemisorbed layers often need very high temperatures to desorb.

Worked Example

Suppose 0.50 mol of a gas is adsorbed on a metal surface. If it physisorbs with ΔH ≈ -25 kJ mol-1, the heat released is 0.50 × 25 = 12.5 kJ. If instead it chemisorbs with ΔH ≈ -200 kJ mol-1, the heat released is 0.50 × 200 = 100 kJ. Dividing, 100/12.5 = 8, so chemisorption releases eight times more heat for the same amount of gas. Scientists use exactly this heat difference to distinguish the types: tens of kJ mol-1 signals physisorption, hundreds signals chemisorption.

Common Misconceptions

• Stronger adsorption is always better for catalysis. Not true. The Sabatier principle says the bond must be just right: too weak and reactants will not stick; too strong and products never leave, poisoning the catalyst.
• Physisorption is favoured at high temperature. The opposite is true. Being exothermic and reversible, physisorption is strongest at low temperatures and fades as heating drives desorption.
• Chemisorption can form multilayers. It cannot. Chemical bonds require direct contact with surface atoms, so only a single molecular layer forms.

Real-World Applications

Catalytic converters rely on chemisorption of carbon monoxide and nitrogen oxides on platinum and rhodium. Hydrogenation of vegetable oils uses hydrogen chemisorbed on nickel. Gas sensors detect toxic gases through chemisorption-induced electrical changes. Physisorption drives gas storage in porous materials and BET surface-area measurements.

Physisorption is weak, reversible, non-specific, and multilayered; chemisorption is strong, often irreversible, specific, and monolayer-only. Keep the enthalpy ranges, 20-40 versus 40-400 kJ mol-1, in mind.