The Foundations of Chemistry

The ionization of hydrogen fluoride, a weakacid, is similar, but it occurs to only a slight

extent, so we use a double arrow to indicate that it is reversible.

Various measurements (electrical
conductivity, freezing point depression,
etc.) indicate that HF is only slightly
ionized in water.

376 CHAPTER 10: Reactions in Aqueous Solutions I: Acids, Bases, and Salts

Problem-Solving Tip:Conjugate Acid-Base Pairs

We can describe Brønsted–Lowry acid–base reactions in terms of conjugate acid–base

pairs.These are two species that differ by a proton. In the preceding equation, HF (acid 2 )

and F(base 2 ) are one conjugate acid–base pair, and H 2 O (base 1 ) and H 3 O
(acid 1 ) are

the other pair. The members of each conjugate pair are designated by the same numer-

ical subscript. In the forward reaction, HF and H 2 O act as acid and base, respectively. In

the reverse reaction, H 3 O
acts as the acid, or proton donor, and Facts as the base, or

proton acceptor.

+ –

base 1 acid 2 acid 1 base 2

H 2 O(
) + HF(aq) H 3 O+ + F–(aq)

O

H

H :

:

+

H+transfer

H :

:

:F: :

:

O + :F:

H

H :

HH

+tr

ans

fer

HF F–

–H+

+H+

H 2 OOH–

–H+

+H+

H 3 O+ H 2 O

–H+

+H+

Conjugate acid Conjugate base

• H

+,remo

veH

+,decreasec

harge

by 1

+H+

,addH+

,increasecharge

by^1

When the weakacid, HF, dissolves in water, the HF molecules give up some H
ions

that can be accepted by either of two bases, For H 2 O. The fact that HF is only slightly

ionized tells us that Fis a stronger base than H 2 O. When the strongacid, HCl, dissolves

in water, the HCl molecules give up H
ions that can be accepted by either of two bases,

Clor H 2 O. The fact that HCl is completely ionized in dilute aqueous solution tells us

Fis a stronger base than H 2 O. H 2 O
is a stronger base than Cl. Fis
therefore a stronger base than Cl.

It makes no difference which conjugate
acid–base pair, HF and For H 3 O

and H 2 O, is assigned the subscripts 1
and 2.

The double arrow is used to indicate
that the reaction occurs in both the
forward and the reverse directions.
We use rectangles to indicate one
conjugate acid–base pair and ovals to
indicate the other pair.