Chapter 16 Text

and F atoms is much higher than it is for the other hydrogen halides, as shown
Table 8.4. As a result HF is a weak acid, whereas all the other hydrogen halides
are strong acids in water.
A third factor that affects the ease with which a hydrogen atom ionizes from
fJX is the stability of the conjugate base, X". In general, the greater the stability
of the conjugate base, the stronger is the acid. The strength of an acid is often a
combination of all three factors: the polarity of the H—Xbond, the strength of
the H—X bond, and the stability of the conjugate base, X~.
Binary Acids
In general, the H—X bond strength is the most important factor determining
acid strength among the binary acids (those containing hydrogen and just one
other element) in which X is in the same group in the periodic table. The strength
of an H—X bond tends to decrease as the element X increases in size. As a result,
the bond strength decreases, and the acidity increases down a group. Thus, HC1
is a stronger acid than HF, and H2S is a stronger acid than HjO.
Bond strengths change less moving across a row in the periodic table than
they do down a group. As a result, bond polarity is the major factor determining
acidity for binary acids in the same row. Thus, acidity increases as the electronegativity
of the element X increases, as it generally does moving from left to
right in a row. For example, the acidity of the second-row elements varies in the
following order: CH4 asic
gan
ands
_
-first
Many common acids, such as sulfuric acid, contain one or more O—H bonds:
TT (~i C f~\__ T-T
" V " I
jj
Acids in which OH groups and possibly additional oxygen atoms are bound to
a central atom are called oxyacids. The OH group is also present in bases. What
factors determine whether an OH group will behave as a base or as an acid?
•Period.2
-•feriod'3';
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