PHYSICAL CHEMISTRY IN BRIEF

CHAP. 13: PHYSICAL CHEMISTRY OF SURFACES [CONTENTS] 451

Obr.13.6:Adsorption isosteres

13.2.2 Adsorption heat

The course of the isosteres allows for the determination of theadsorption heat(enthalpy),
which is given by the relation
(
∂lnpi
∂T

)

ai

=−

∆adsH

RT^2

=−

Hm(a,i)−Hm(g),i

RT^2

, (13.36)

whereHm(a,)i is the molar enthalpy of the adsorbed gas, andHm(g),i is the molar enthalpy of the
gas phase.

13.2.3 Physical adsorption and chemisorption

There are two possible types of bond between the adsorbent and adsorbate molecules:
a) Mutual interactions such as those between molecules in the liquid phase, the van der
Waals forces, prevail between the adsorbent and adsorbate molecules. This phenomenon is
referred to asphysical adsorption.
b) A very strong, almost chemical bond exists between the adsorbent and adsorbate atoms.
This phenomenon is referred to aschemisorption.