Page 98 - Basic physical chemistry for the atmospheric sciences

84 Basic physical chemistry

Solution. Hydrochloric acid is a gas under normal conditions, but it
is a strong electrolyte that dissolves in water to form equal numbers of
H + (aq) and Cl ( aq). Therefore, when 0.02 mole of HCI dissolve in 1
-
L of water, 0.02 mole of + (aq) and 0.02 mole of ci -(aq) will form
H
[we can neglect the small additional number of H + (aq) produced by
+
the dissociation of the water]. Hence, [H ( aq)] = 0 . 0 2 M . Since we
can apply Eq. (4.22) to the solution
1 . 00 x 1 0 - 14
---- = 5 X 1 0 - 13 M
0 . 0 2
Comparing the concentration of OH - (aq) calculated i n the above
exercise with that calculated for pure water in Exercise 4 . 8 , we see
that by adding 0.02 mole of HCI to 1 L of water, we have lowered the
oH - (aq) concentration from 1 0 - 7 M to 5 x 1 0 - 1 3 M ! But the H + (aq)
concentration has increased from 1 0 - 7 M to 0 . 0 2 M . Clearly, the
solution is now very acidic. Thu s , HCI is both a strong electrolyte and
a strong acid, because when a small amount of it is added to water it
l
produces a strongly acidic solution. Similar y , a substance that dis­
-
solves in water to produce relatively high concentrations of OH ( aq)
ions (compared to those in pure water), such as NaOH(s), will raise
the concentration of OH -(aq) relative to H + (aq) by a tremendous
amount (see Exercise 5 . 8 ) . Such materials are called bases, because
when added to water they form basic solutions. 1 It follows from the
above definitions that acids and bases have opposite (or opposing)
tendencies. Thus, bases react with acids to neutralize their acidity.
For example, two neutralizing reactions are

HCI + NaOH � H20 + NaCl
H2S04 + C ao� H20 + CaS04

In both cases, an acid reacts with a base to produce water and a third
class of substance called a salt (e. . , NaCl and CaS04).
g
The acidity (or alkalinity, as the case may be) of water is very
important because H + (aq) and OH - (aq) ions play crucial roles in
many reactions in aqueous solution . For example, the acidity (or
s
e
alkalinity) determines the ability of water to sustain fish and plant lif ;
i t also determines the solubility of many materials n water.
i
In addition to the acids we have already mentioned, some other
common acids are sulfuric acid (H 2 S04 , nitric acid (HN0 ) , formic
)
3
)
)
acid (HCOOH , phosphoric acid (H3P04 , hydrogen fluoride ( H F ) ,