Chapter 16 Text

The formation of hydronium ions is one of the complex features of the interaction
of the H+ ion with liquid water. In fact, the H3O+ ion can form hydrogen
bonds to additional H2O molecules to generate larger clusters of hydrated hydrogen
ions, such as H5O2+ and H9O4+ (Figure 16.1 ^).
Chemists use H+(aq) and H3O+(flg) interchangeably to represent the same
thing—namely the hydrated proton that is responsible for the characteristic properties
of aqueous solutions of acids. We often use the H+(fl(f) ion for simplicity and
convenience, as we did in Equation 16.1. The H3O+(flg) ion, however, more closely
represents reality.
proton-Transfer Reactions
VVhen we closely examine the reaction that occurs "when HC1 dissolves in water,
we find that the HC1 molecule actually transfers an H+ ion (a proton) to a water
molecule as depicted in Figure 16.2 T. Thus, we can represent the reaction as
occurring between an HC1 molecule and a water molecule to form hydronium
and chloride ions:
HC%) + H20{/) * H3O>g) + CT(aq) [16.3]
Bransted and Lowry proposed defining acids and bases in terms of their
ability to transfer protons. According to their definition, an acid is a substance
(molecule or ion) that can donate a proton to another substance. Likewise, a base is a
substance that can accept a proton. Thus, when HC1 dissolves in water (Equation
16.3), HC1 acts as a Brensted-Lowry acid (it donates a proton to H2O), and F^O
acts as a Br0nsted-Lowry base (it accepts a proton from HC1).
Because the emphasis in the Bransted-Lowry concept is on proton transfer,
the concept also applies to reactions that do not occur in aqueous solution. In
the reaction between HC1 and NH3, for example, a proton is transferred from the
acid HC1 to the base NH3: >
16.2 Bransted-Lowry Acids and Bases 615
:C1—H
H
H
— H
H
I
H—N—H
H
[16.4]
This reaction can occur in the gas phase. The hazy film that forms on the windows
of general chemistry laboratories and on glassware in the |ab is largely solid
NH4C1 formed by the gas-phase reaction of HC1 and NH3 (Figure 16.3 T).
(b)
A. Figure 16.1 Lewis structures and
molecular models for H5O2+ and H9O4+.
There is good experimental evidence for
the existence of both these species.
A. Figure 16.2 When a proton is
transferred from HC! to H2O, HCI acts as
the Br0nsted-Lowry acid and H2O acts
as the Br0nsted-Lowry base.
A Figure 16.3 The HC\(g) escaping
from concentrated hydrochloric acid and
the NHs^) escaping from aqueous
ammonia (here labeled ammonium
hydroxide) combine to form a white fog
of NH4CI(s).