Environmental Engineering Reference
In-Depth Information
repulsive forces in liquid water are of low magnitudes. The surface tension of water
(72 mN/m), an indication of the forces at the surface of the liquid, is also unusually
high. The most anomalous property is its heat capacity. For a compound of such low
molecular weight it is abnormally high. It is also interesting that the heat capacity
reduces to about half when water is frozen or when it is boiled. It is this particular
property of liquid water that helps maintain the ocean as a vast storehouse of energy.
Although the P - V - T relationship for water is abnormal giving rise to the above-
mentioned anomalies, its transport properties such as diffusion constant and viscosity
are similar to those of most molecular liquids.
We, therefore, conclude that peculiar intermolecular forces are at play to give water
its anomalous properties. To gain an understanding, we need to focus on the structure
of water and the ice lattice. Figure 3.8 shows the structure of an ice lattice wherein
the central water molecule is tetrahedrally linked to four other water molecules. The
O-H intramolecular bond distance is 0.10 nm whereas the intermolecular O-H bond
distance is only 0.176 nm. At first glance, it may seem that the O-H bond is also
covalent in nature. It is indeed larger than the true covalent O-H bond distance of
0.1 nm, but it is smaller than the combined van der Waals radii of the two molecules
(0.26 nm). Bonds that have these intermediate characters are called hydrogen bonds .
For many years it was thought that the H-bond had a predominant covalent nature.
However, more recently it has been convincingly shown that it is an electrostatic
interaction. The H atom in water is covalently linked to the parent O atom, but enters
into an electrostatic linkage with the neighboring O atom.Thus H acts as a mediator in
bondingbetweentwoelectronegativeOatomsandisrepresentedasO-H-O.Although
not covalent, the H-bonds show some characteristics of weak covalent bonds; for
example,theyhavebondenergiesof10-40 kJ/molandaredirectionalinnature.Actual
covalentbondshaveenergiesoftheorderof500 kJ/molandweakvanderWaalsbonds
FIGURE 3.8 A solute molecule within the tetrahedral cage of water molecules.
 
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