Surface Chemistry

Lesson

Part 1 — adsorption: why molecules stick to surfaces. (This chapter is for formative assessment; it is not tested in the board exam.)

In this lesson you will

Distinguish adsorption from absorption and name the adsorbate and adsorbent
Compare physisorption and chemisorption
Explain how the nature of the gas, surface area, pressure and temperature affect adsorption of gases on solids
Describe applications of adsorption

Table of everyday examples of adsorption and colloids
Where you meet it	What is happening
Silica gel packet in a shoe box	Water vapour adsorbed on silica gel
Charcoal in a water filter	Colour and smell molecules adsorbed
Milk, fog, ink	Colloids
Smoke settling in a chimney precipitator	Charged colloidal particles neutralised

Concept

Adsorption and absorption

Adsorption is the accumulation of molecules at the surface of a solid or liquid, so their concentration there is higher than in the bulk. The substance adsorbed is the adsorbate; the surface is the adsorbent. In absorption the substance spreads throughout the bulk. Removing an adsorbed substance is desorption; when both happen together we say sorption.

Table comparing adsorption and absorption
Feature	Adsorption	Absorption
Where	Surface only	Throughout the bulk
Concentration	Higher at the surface than inside	Uniform
Rate	Fast at first, then slows	Steady
Example	Water vapour on silica gel	Water vapour in anhydrous CaCl₂

Concept

Why adsorption is exothermic

Adsorption lowers the residual surface forces, so it releases heat: $\Delta H$ is negative. The adsorbed molecules lose freedom of movement, so $\Delta S$ is also negative. For adsorption to be spontaneous ( $\Delta G < 0$ ), the heat released must outweigh the entropy loss, i.e. $|\Delta H| > T|\Delta S|$ . As adsorption proceeds, $|\Delta H|$ falls; when $\Delta H = T\Delta S$ , equilibrium is reached.

\Delta G = \Delta H - T\Delta S < 0 \quad\text{with } \Delta H < 0,\ \Delta S < 0

Table of the signs of enthalpy, entropy and Gibbs energy change for adsorption
Quantity	Sign for adsorption	Reason
ΔH	Negative	Attractions form; heat released
ΔS	Negative	Molecules lose freedom on the surface
ΔG	Negative (spontaneous)	\|ΔH\| > T\|ΔS\|

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