Page 101 - MODERN ELECTROCHEMISTRY

44 CHAPTER 2

Fig. 2.10. Schematic diagram to
show that in liquid water there are
networks of associated water mole-
cules and also a certain fraction of
free, unassociated water molecules.

atoms, such as Cl, F, O, and S when they are in solution. There is nothing mysterious
about the energy behind H bonding; it derives from the positive charge on the proton
and the negative one on for example.
–1
However, H bonding (the value of the bond strength is small, only 10–40 kJ mol )
does affect the properties of water and is responsible for water’s anomalously high
boiling point. If one extrapolates the boiling points of the hydrides of the elements in
group VI of the Periodic Table to the expected value for the hydride of O, it turns out
to be ~ 215 K. The fact that it is actually 158 K higher than that is undoubtedly because
individual water molecules are not free to evaporate as the temperature is increased.
Many of them bond to each other through the H bonds. The thermal stability of water
has had an important effect on the structure of the earth, for if there were no H bonds,
the seas would never have formed (they would have remained in the vapor phase) and
it is doubtful if life would have begun.
Structural research on water, which originated in a classic paper by Bernal and
Fowler, has shown that under most conditions liquid water is best described as a rather
broken-down, slightly expanded (Table 2.3) form of the ice lattice (Fig. 2.8). Thus,
X-ray and other techniques indicate that in water there is a considerable degree of
short-range order that is characteristic of the tetrahedral bonding in ice. Thus, liquid
water partly retains the tetrahedral bonding and resulting network structure charac-
teristic of crystalline ice. In addition to the water molecules that are part of the network,
some structurally free, nonassociated water molecules can be present in interstitial
regions of the network (Fig. 2.10). When a network water molecule breaks its hydrogen
bonds with the network, it can move as an interstitial water molecule that can rotate
freely. The classification of the water molecules into network water and free (or
interstitial) water is not a static one. It is dynamic. As argued in a classic paper by
Frank and Wen, clusters of water molecules cooperate to form networks and at the
same time the networks can break down. A water molecule may be free in an interstitial