
125 Surface Chemistry
balanced but on the surface the particles are not surrounded by atoms
or molecules of their kind on all sides, and hence they possess unbalanced
or residual attractive forces. These forces of the adsorbent are responsible
for attracting the adsorbate particles on its surface.The extent of
adsorption increases with the increase of surface area per unit mass of
the adsorbent at a given temperature and pressure.
Another important factor featuring adsorption is the heat of
adsorption. During adsorption, there is always a decrease in residual
forces of the surface, i.e., there is decrease in surface energy which
appears as heat. Adsorption, therefore, is invariably an exothermic
process. In other words, ∆H of adsorption is always negative. When a
gas is adsorbed, the freedom of movement of its molecules become
restricted. This amounts to decrease in the entropy of the gas after
adsorption, i.e., ∆S is negative. Adsorption is thus accompanied by
decrease in enthalpy as well as decrease in entropy of the system. For
a process to be spontaneous, the thermodynamic requirement is that,
at constant temperature and pressure, ∆G must be negative, i.e., there
is a decrease in Gibbs energy. On the basis of equation, ∆G = ∆H – T∆S,
∆G can be negative if ∆H has sufficiently high negative value as – T∆S
is positive. Thus, in an adsorption process, which is spontaneous, a
combination of these two factors makes ∆G negative. As the adsorption
proceeds, ∆H becomes less and less negative ultimately ∆H becomes
equal to T∆S and ∆G becomes zero. At this state equilibrium is attained.
There are mainly two types of adsorption of gases on solids.
If accumulation of gas on the surface of a solid occurs on account of
weak van der Waals’ forces, the adsorption is termed as physical
adsorption or physisorption. When the gas molecules or atoms are
held to the solid surface by chemical bonds, the adsorption is termed
chemical adsorption or chemisorption. The chemical bonds may be
covalent or ionic in nature. Chemisorption involves a high energy of
activation and is, therefore, often referred to as activated adsorption.
Sometimes these two processes occur simultaneously and it is not
easy to ascertain the type of adsorption. A physical adsorption at low
temperature may pass into chemisorption as the temperature is
increased. For example, dihydrogen is first adsorbed on nickel by van
der Waals’ forces. Molecules of hydrogen then dissociate to form hydrogen
atoms which are held on the surface by chemisorption.
Some of the important characteristics of both types of adsorption
are described below:
Characteristics of physisorption
(i) Lack of specificity: A given surface of an adsorbent does not show any
preference for a particular gas as the van der Waals’ forces are universal.
(ii) Nature of adsorbate: The amount of gas adsorbed by a solid
depends on the nature of gas. In general, easily liquefiable gases
(i.e., with higher critical temperatures) are readily adsorbed as van
der Waals’ forces are stronger near the critical temperatures. Thus,
1g of activated charcoal adsorbs more sulphur dioxide (critical
temperature 630K), than methane (critical temperature 190K) which
is still more than 4.5 mL of dihydrogen (critical temperature 33K).
5.1.3 Types of
Adsorption