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Chemisorption (or chemical adsorption) is adsorption in which
the forces involved are valence forces of the same kind as those operating in
the formation of chemical compounds. The problem of distinguishing between
chemisorption and physisorption is basically the same as that of
distinguishing between chemical and physical interaction in general. No
absolutely sharp distinction can be made and intermediate cases exist, for
example, adsorption involving strong hydrogen bonds or weak charge transfer.
Some features which are useful in recognizing chemisorption include:
- (a)
- the phenomenon is characterized by chemical specificity;
- (b)
- changes in the electronic state may be detectable by suitable physical
means (e.g. u.v., infrared or microwave spectroscopy, electrical conductivity,
magnetic susceptibility);
- (c)
- the chemical nature of the adsorptive(s) may be altered by surface
dissociation or reaction in such a way that on desorption the original species
cannot be recovered; in this sense chemisorption may not be reversible;
- (d)
- the energy of chemisorption is of the same order of magnitude as the
energy change in a chemical reaction between a solid and a fluid: thus
chemisorption, like chemical reactions in general, may be exothermic or
endothermic and the magnitudes of the energy changes may range from very small
to very large;
- (e)
- the elementary step in chemisorption often involves an activation energy;
- (f)
- where the activation energy for adsorption is large (activated
adsorption), true equilibrium may be achieved slowly or in practice not at
all. For example in the adsorption of gases by solids the observed extent of
adsorption, at a constant gas pressure after a fixed time, may in certain
ranges of temperature increase with rise in temperature. In addition, where
the activation energy for desorption is large, removal of the chemisorbed
species from the surface may be possible only under extreme conditions of
temperature or high vacuum, or by some suitable chemical treatment of the
surface;
- (g)
- since the adsorbed molecules are linked to the surface by valence bonds,
they will usually occupy certain adsorption sites on the surface and
only one layer of chemisorbed molecules is formed (monolayer adsorption).
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