Page 100 - Basic physical chemistry for the atmospheric sciences

86 Basic physical chemistry

5.2 The nature of u + (aq)
In Section 4 . 5 we pointed out that water is an excellent solvent for
ionic compounds because of the strong attractive forces exerted by
the water molecules. Thus, each ion in aqueous solution may be
attached to several (four to six) water molecules. We refer to this as
2
hydration. Since the H + ion (or proton) is very small, it should be
hydrated to a much greater extent than other ions. Consequently,
"free" protons are unlikely in aqueous solutions. For example, the
hydrated proton H30 + (consisting of one proton and a water mole­
cule - called the hydronium ion) is very stable, and is more likely to
exist in aqueous solutions than is H + . Four water molecules attached
H
to a proton (H90 ; or + · 4H20) might be even more stable. Unfortu­
nately, the preferred form of the positive ion in aqueous solutions is
not known. Therefore, if we wish to emphasize its likely hydrated
form, we will indicate it by H30+ (aq); other times we will indicate it
as H + ( aq). The notation "aq" itself can serve to remind us that all ions
in water are extensively hydrated, and that the exact form of the ion
may not be known.

5.3 The Br0nsted-Lowry theory; conjugate acid-base pairs
In light of the above comments on the nature of + (aq), instead of
H
representing the dissociation of HCl in water by Reaction (4. 1 8 ), we
might better represent it by

HCl(aq) + H20(1) � H30 + (aq) + C l -(aq) (5. 5)
From this viewpoint, we could consider an acid as a substance that
can donate a proton to another molecule (rather than as a substance
that releases free protons).
The Arrhenius theory views all bases as substances that produce
OH -(aq) ions. However, acid-base type reactions can occur in non­
aqueous solvents, in which OH - (aq) cannot be present because there
are no oxygen atoms in the system. For example, HCl reacts with
pure liquid ammonia

HCl(aq) + NH3(1) � NH;(aq) + Cl - (aq) (5.6)
Since NH3 has eliminated the acid HCl, we could consider NH3 as
a base.
These problems with the Arrhenius theory led J . Br�nsted and T.