The Foundations of Chemistry

that Clis a weaker base than H 2 O. Thus, the weaker acid, HF, has the stronger conju-
gate base, F. The stronger acid, HCl, has the weaker conjugate base, Cl. We can
generalize:

The stronger the acid, the weaker is its conjugate base; the weaker the acid, the

stronger is its conjugate base.

“Strong” and “weak,” like many other adjectives, are used in a relative sense. We do not
mean to imply that the fluoride ion, F, is a strong base compared with species such as
the hydroxide ion, OH. We mean that relative to the anions of strong acids,which are very
weak bases, Fis a much stronger base.
Ammonia acts as a weak Brønsted–Lowry base, and water acts as an acid in the ioniza-
tion of aqueous ammonia.

Be careful to avoid confusing solubility

in waterand extent of ionization.They

are not necessarily related. Ammonia is

very solublein water (15 mol/L at

25°C). In 0.10 Msolution, NH 3 is

only 1.3% ionized and 98.7%

nonionized.

10-4 The Brønsted–Lowry Theory 377

As we see in the reverse reaction, ammonium ion, NH 4
, is the conjugate acid of NH 3.
The hydroxide ion, OH, is the conjugate base of water. In three dimensions, the molec-
ular structures are

Water acts as an acid (H
donor) in its reaction with NH 3 , whereas it acts as a base
(H
acceptor) in its reactions with HCl and with HF.

Whether water acts as an acid or as a base depends on the other species present.

N

H

H

H :

base 1 acid 2 acid 1 base 2

NH 3 (aq) + H 2 O(
) NH 4 +(aq) + OH–(aq)

+

+ O

H

:

:

:

H+transfer

H+O

H

:

:

N :

H

H

H

H+transfer

H

N + O N +

H

+

O

Trigonal pyramidal molecule Angular molecule Tetrahedral ion Linear ion

H

H

H

H

H

H

H

H

H

H+transfer

H+transfer