Chemistry · Ch 11 — Adsorption and Colloids
Factors Affecting Adsorption of Gases on Solids
Factors Affecting Adsorption of Gases on Solids
All solids adsorb gases to some extent, and five factors together determine how much. (1) Nature of the adsorbate (the gas): gases with a high critical temperature liquefy more readily (see Chapter 10) and are correspondingly adsorbed to a larger extent; easily-liquefiable gases such as SO2, Cl2 and NH3 are adsorbed far more than gases like N2, O2 and H2 which are hard to liquefy. Table 11.3 (attached to this section) tabulates this directly: as critical temperature rises from N2 (126 K) through HCl, NH3, Cl2 up to SO2 (430 K), the volume of gas adsorbed on the same adsorbent rises from 8 cm3 to 380 cm3. (2) Nature of the adsorbent: substances offering a large surface area per unit mass -- silica gel and charcoal, for instance, because of their porous structure -- are effective adsorbents and can take up appreciable volumes of gas. (3) Surface area of the adsorbent: since adsorption is inherently a surface phenomenon, the extent of adsorption increases as the adsorbent's surface area increases; finely divided substances, rough surfaces and colloidal substances are all good adsorbents precisely because they present more surface area per unit mass. (4) Temperature: adsorption is exothermic, so by Le Chatelier's principle (Chapter 12) it is favoured at low temperature -- the amount of gas adsorbed is inversely related to temperature. Fig 11.2 (attached to this section) plots the volume of N2 adsorbed per unit mass of adsorbent against pressure at several different temperatures, and shows that at any fixed pressure the amount adsorbed falls as temperature rises. (5) Pressure of the gas: at a fixed tempe …
Data supporting Factor 1 (nature of the adsorbate): as a gas's critical temperature rises, so does the volume of it adsorbed on the same mass of adsorbent, because a gas closer to its own liquefaction point is held more strongly at a solid surface.
Gas | Critical temperature (K) | Volume adsorbed (cm3)
N2 | 126 | 8 …
What this figure shows. A plot of the volume of N2 gas adsorbed per unit mass of adsorbent (y-axis) against the equilibrium pressure of the gas (x-axis), drawn as a family of curves for several different temperatures (the printed curve labels read approximately 195 K, 244 K and 273 K, though the surrounding running text separately states the range as '193 K to 273 K' -- this platform flags the small numeric mismatch between the two as a likely OCR/printing artefact in the source rather than resolving it to one exact figure, since both point to the same qualitative conclusion). Each curve rises with pressure, and at any fixed pressure the curves for higher temperatures sit below the curves for lower temperatures, i.e. the amount of N2 adsorbed per unit mass of adsorbent decreases as temperature increases at constan …