Physisorption and Chemisorption

Introduction

Nitrogen may attach weakly to a cold solid, while hydrogen can dissociate and bind to a clean metal surface. Both are adsorption, yet their energies and consequences differ. The terms physisorption and chemisorption distinguish the dominant interaction, helping explain catalyst activity, gas storage and surface measurements.

Core explanation

Physisorption is dominated by relatively weak intermolecular attractions such as dispersion forces and, in suitable systems, dipolar interactions. Molecules commonly retain their chemical identity. The process can occur on many parts of a surface and sometimes in multiple layers, especially below a gas's condensation temperature. Because individual interactions are comparatively weak, adsorption and desorption can often be reversed by changing pressure or temperature.

Chemisorption involves the formation of chemical bonds between adsorbate and surface atoms. It may require rearrangement or dissociation of the incoming molecule, as when hydrogen forms adsorbed H atoms on a suitable metal. It usually occupies a finite population of sites, with approximately one species or fragment per site in a simple model. Bond formation is often stronger than physisorption, but exact enthalpy ranges overlap; there is no universal numerical cutoff that classifies every surface event.

Physical adsorption is generally favored at lower temperature because uptake commonly releases heat and warming promotes desorption. Chemisorption can require an activation barrier, so its rate may initially increase with temperature even while its equilibrium coverage eventually decreases. Thus the statement “heating always decreases adsorption” is too crude: equilibrium amount and approach rate are different questions. A catalyst may need enough thermal energy to cross barriers but not so much that the key reactant leaves every site.

Chemisorption is often specific because bond formation depends on the adsorbate and surface electronic structures. Physisorption is less specific but still selective through pore dimensions, polar interactions and molecular size. A porous solid can show both kinds of uptake simultaneously; a molecule may first enter a pore by diffusion and physically adsorb before reacting at an active site. The boundary between the categories can be gradual rather than absolute.

Evidence may include adsorption enthalpy, ease of desorption, spectroscopic bond changes and whether the molecule dissociates. One observation by itself is rarely decisive. Strongly adsorbed species may also block a catalyst, so “stronger adsorption” is not automatically better. Effective catalysis often requires reactants to attach sufficiently to react and products to leave.

Step-by-step reasoning

1. Ask whether the adsorbate keeps its molecular identity. 2. Look for evidence of new surface bonds or dissociation. 3. Compare reversibility and temperature dependence while separating equilibrium from rate. 4. State that real surfaces may show mixed interactions and avoid assigning a category from a single arbitrary energy threshold.

Visual explanation

Draw one molecule held just above a surface by several faint dotted attractions. Beside it draw a molecule split into two fragments attached by solid lines to metal atoms. The first sketch represents physical attraction; the second represents a surface chemical reaction and stronger site specificity.

Real-world analogy

A magnet lightly holding a note against a board resembles reversible physical attachment. Glue bonding the note to the board resembles chemical attachment requiring more work to remove. Neither comparison predicts molecular energies exactly; it illustrates why the means of attachment matters more than merely saying “stuck.”

Real-world example

Activated carbon can physically adsorb many organic vapors in its pores, which makes it useful in filters. A platinum surface can chemisorb hydrogen, splitting H₂ into adsorbed H atoms that can participate in hydrogenation. The two materials differ in both interaction and purpose.

Why?

Why can moderate heating sometimes increase the observed amount of chemisorbed material during a short experiment? If bond formation has an activation barrier, warming increases the rate of crossing it. At longer times, higher temperature may still reduce equilibrium coverage for an exothermic adsorption process.

Common misconception

“Physical means weak enough to ignore.” Individually weak interactions can produce substantial uptake over an enormous pore area, and multiple interactions can stabilize a molecule. Physical adsorption is technologically important even without a new covalent bond.

Worked example

Two gases contact a clean surface. Gas A adsorbs readily at 77 K and mostly leaves on gentle warming; spectra show intact molecules. Gas B adsorbs after an activation delay and spectra reveal surface-bonded fragments. Classify the dominant processes. A fits physisorption; B fits dissociative chemisorption. The observations are stronger evidence together than temperature alone.

Quick check

1. Which process can produce multilayers most naturally? Answer: Physisorption, because later molecules can attract earlier adsorbed molecules. 2. Does chemisorption always occur faster at low temperature? Answer: No. An activation barrier can slow its initial rate strongly at low temperature.

Exam focus

Compare interaction type, specificity, layers, reversibility and temperature effects. Qualify broad trends because neither class has a universal heat cutoff. In catalyst questions, explain the need for both adsorption and desorption rather than assuming maximum binding strength gives maximum activity.

Advanced insight

A potential-energy curve can contain a shallow physisorbed well separated by a barrier from a deeper chemisorbed well. A molecule can occupy either state depending on temperature and time. This energy-landscape picture reconciles seemingly conflicting observations of weak initial uptake and slower bond-forming uptake on one surface.

Summary

Physisorption is dominated by intermolecular attraction and often supports reversible, multilayer uptake. Chemisorption forms surface bonds and can dissociate molecules at specific sites. Temperature affects both equilibrium and kinetics, which must be considered separately.

Practice questions

1. A gas leaves a surface when pressure is reduced, and spectroscopy shows intact molecules. Suggest the dominant adsorption type. Answer: Physisorption is likely, though more evidence would strengthen the assignment. 2. Why may a strongly bound reaction product inhibit a catalyst? Answer: It can occupy active sites and prevent fresh reactants from adsorbing until it desorbs. 3. A molecule attaches only after a measurable activation delay and splits into atoms. What is indicated? Answer: Dissociative chemisorption is indicated by activation and surface-bonded fragments.