An Introduction to Environmental Chemistry

The Chemistry of Continental Solids 69

Box 4.1 Properties of water and hydrogen bonds

The water molecule H 2 O is triangular in
shape, with each hydrogen (H) bonded to the
oxygen (O) as shown in Fig. 1. The shape
results from the geometry of electron orbits
involved in the bonding. Oxygen has a much
higher electronegativity (Box 4.2) than
hydrogen and pulls the bonding electrons
toward itself and away from the hydrogen
atom. The oxygen thus carries a partial
negative charge (usually expressed as d-), and
each hydrogen a partial positive charge (d+),
creating a dipole (i.e. electrical charges of
equal magnitude and opposite sign a small
distance apart). At any time a small
proportion of water molecules dissociate
completely to give H+and OH-ions.

eqn. 1

for which the equilibrium constant is:

eqn. 2

The activity of pure water is by convention
unity (1), so equation 2 simplifies to:

eqn. 3

The polar nature of the water molecule
allows the ions of individual water molecules

Kw==aH aOH+--.10^14 mol l^2 -^2

Kw==aH aOHaH O mol l

+-. --

2

1014 2 2

HO H 2 ()lª()+aq+OH()-aq

to interact with their neighbours. The small

hydrogen atom can approach and interact

with the oxygen of a neighbouring molecule

particularly effectively. The interaction

between the hydrogen atom, with its partial

positive charge, and oxygen atoms of

neighbouring water molecules with partial

negative charges, is particularly strong—by

the standards of intermolecular interactions

• though weaker than within covalent
  bonding. This type of interaction is called
  hydrogen bonding.
  The molecules in liquid water are less
  randomly arranged than in most liquids
  because of hydrogen bonds. The polarity of
  the bonds makes water an effective solvent
  for ions; the water molecules are attracted
  to the ion by electrostatic force to form a
  cluster around it. Moreover, ionic-bonded
  compounds, with charge separation between
  component ions, are easily decomposed by
  the force of attraction of the water dipole.
  Hydrogen bonding gives water a relatively
  high viscosity and heat capacity in
  comparison with other solvents. Hydrogen
  bonds also allow water to exist as a liquid
  over a large temperature range. Since most
  biological transport systems are liquid, this
  latter property is fundamental to supporting
  life.

H d+

d+^ H

d–

O

Fig. 1The water molecule.