PHYSICAL CHEMISTRY IN BRIEF

CHAP. 13: PHYSICAL CHEMISTRY OF SURFACES [CONTENTS] 452

The basic differences between physical adsorption and chemisorption are shown in the fol-
lowing table.

Physical adsorption Chemisorption

Specificity unspecific specific

Number of adsorbed layers > 1 1

Adsorption heat in kJ mol−^1 5–50 50–800

Rate of adsorption high relatively low

Desorption by decreasing pressure by increasing temperature

13.2.4 Quantitative description of the adsorption isotherm in pure

gases

Quantitative description of adsorption is most often performed using an adsorption isotherm
equation. For pure gases, the following isotherms are used in simple cases:
a) EmpiricalFreundlich isotherm

a=α pβ, (13.37)

whereα,βare temperature-dependent parameters. It usually applies thatβ <1. This equation
provides a good description of both physical adsorption and chemisorption at medium pressures.
The following limits follow from this equation

plim→ 0

(

∂a

∂p

)

T

=∞ [forβ <0], plim→∞a=∞, (13.38)

b)Langmuir isotherm

a=amax

b p

1 +b p

, (13.39)

whereamax,bare parameters, andamaxhas the physical meaning of the maximum amount of
adsorbate atp→∞.