Page 99 - Basic physical chemistry for the atmospheric sciences

Acids and bases

)
hydrobromic acid (HBr , hydroiodic acid (HI), and perchloric acid
(HC104) . Based on the discussion so far, we might hypothesize that
acids have the following propt;rties in common: they contain hydro­
ge , they dissolve in water to produce ions that conduct electricity
n
(i. e . , they are electrolytes), and one of the ions they release is + (aq).
H
Some common bases are sodium hydroxide (NaOH), potassium
hydroxide (KOH), magnesium hydroxide (Mg(OH)z), calcium oxide
(CaO), sodium carbonate (Na2C03), and ammonia (NH3). We could
postulate that, like acid , a common property of bases is their ability
s
to dissolve in water to produce ions. Also, since bases counteract
acids, we might conclude that one of the ions they produce can remove
the hydrogen ion. For Na H , KOH, and Mg(OH)z this ion is clearly
O
OH - (aq). But what is it for Na2C03 and NH3? To answer this question
let us consider what happens when ammonia dissolves in water. It
reacts with water molecules to form ammonium ions, H !(aq),
N
I
(5. )
The ammonia also reacts with the H + (aq) i n water

NHiaq) + H + (aq) � NH ! (aq) (5. 2 )
We see that Reaction ( 5 . 1 ) releases OH -(a ) , which can remove
q
H + (aq). For Na 2 C03(s) the reactions in water are
(5.3)
followed by

(5.4)
which also releases OH ( aq). Therefore , we might describe a base as
-
a substance that produces (either directly or indirectly) OH - ( aq) ions .
The above description o f acids and bases, in which H + (aq) and
oH - (aq) ions are viewed as responsible for acidic and basic proper­
ties, respectively, and different acidic (or electrolytic) strengths are
attributed to varying degrees of ionic dissociation, was developed by
the Swedish chemist S. Arrhenius between 1 8 80 and 1 8 90. While very
u s eful, this theory has some problem . The first problem has to do
s
s
with the nature of the positive-charge carrier in aqueous solution ; the
second problem is that some substances can act as base , even though
s
t h ey do not release oH -(aq) ions. We will now consider both of
m
t h ese proble s .