To give a more complete description of
reactions, we indicate the physical
states of reactants and products: (g) for
gases, () for liquids, and (s) for solids.
The notation (aq) following ions
indicates that they are hydrated in
aqueous solution; that is, they interact
with water molecules in solution. The
complete ionization of a strong
electrolyte is indicated by a single
arrow (n).Acetic acid is the most familiar organic
acid.Our stomachs have linings that are
much more resistant to attack by acids
than are our other tissues.The carboxylate group XCOOH is4-2 Aqueous Solutions: An Introduction 131Because strong acids ionize completely or very nearly completely in dilute solutions,
their solutions contain predominantly ions rather than acid molecules. Consider the ioniza-
tion of hydrochloric acid. Pure hydrogen chloride, HCl, is a molecular compound that is
a gas at room temperature and atmospheric pressure. When it dissolves in water, it reacts
nearly 100% to produce a solution that contains hydrogen ions and chloride ions:
H 2 O
HCl(g)8888nH(aq)Cl(aq) (to completion)Similar equations can be written for all strong acids.
Weak acidsionize only slightly (usually less than 5%) in dilute aqueous solution. Some
common weak acids are listed in Appendix F. Several of them and their anions are given
in Table 4-6.
The equation for the ionization of acetic acid, CH 3 COOH, in water is typical of weak
acids:
CH 3 COOH(aq) 34 H(aq)CH 3 COO(aq) (reversible)The double arrow 34 generally signifies that the reaction occurs in bothdirections and
that the forward reaction does not go to completion. All of us are familiar with solutions
of acetic acid. Vinegar is 5% acetic acid by mass. Our use of oil and vinegar as a salad
dressing suggests that acetic acid is a weak acid; we could not safely drink a 5% solution
of any strong acid. To be specific, acetic acid is 0.5% ionized (and 99.5% nonionized) in
5% solution.
A multitude of organic acids occur in living systems. Organic acids contain the carboxy-
late grouping of atoms, XCOOH. Most common organic acids are weak. They can ionize
slightly by breaking the OXH bond, as shown on the following page for acetic acid:
TABLE 4-6 Some Common Weak Acids and Their AnionsCommon Weak Acids Anions of These Weak AcidsFormula Name Formula NameHF* hydrofluoric acid F fluoride ion
CH 3 COOH acetic acid CH 3 COO acetate ion
HCN hydrocyanic acid CN cyanide ion
HNO 2 † nitrous acid NO 2 nitrite ion
H HCO^3 hydrogen carbonate ion
2 CO 3
† carbonic acid
CO 32 carbonate ionH HSO^3 hydrogen sulfite ion
2 SO 3
† sulfurous acid
SO 32 sulfite ion
H 2 PO 4 dihydrogen phosphate ion
H 3 PO 4 phosphoric acid HPO 42 hydrogen phosphate ion
PO 43 phosphate ion
H(COO) 2 hydrogen oxalate ion
(COOH) 2 oxalic acid (COO)
2
2 oxalate ion*HF is a weak acid, whereas HCl, HBr, and HI are strong acids.
†Free acid molecules exist only in dilute aqueous solution or not at all. Many salts of these acids are common, stable
compounds, however.
Citrus fruits contain citric acid, so
their juices are acidic. This is shown
here by the color changes on the
indicator paper. Acids taste sour.The names given here correspond to
the aqueous solutions.OHCO