Page 105 - Basic physical chemistry for the atmospheric sciences

Acids and bases IJ I

ested in the concentration of coj - (aq) in the solution, we would have
q
to consider Reaction (5 . 1 6 ) , since this is the only source of Coj - (a ) .
Values for the successive acid-dissociation constants for several
polyprotic acids are given in Appendix IV.

5.8 Hydrolysis
A reaction between liquid water and an ionic species in the aqueous
solution is called a hydrolysis reaction. For example, Reaction (5 . 1 6)
is a hydrolysis reaction. Another example is

(5 . 1 7 )
The equilibrium constant for such a reaction is called the hydrolysis
constant and is given the symbol Kh . Th u s , the hydrolysis constant for
Reaction (5 . 1 6 ) is

[Coj - ( aq)][H30 + (aq)]
Kh
[HC03 (aq)]
Some important aspects of hydrolysis are illustrated by the following
example. When the salt sodium acetate (NaC2H302) is added to water,
the pH rises above 7. This means that the concentration of o H - (aq)
in the solution increases relative to that of H30 + ( aq). The reactions
involved indicate why this is so

HC2H302(aq) + H20(1) � C2H30 2 (aq) + Hp + ( aq) ( 5 . 1 9)

C2H 3 0 2 (aq) + H20(1) � HC2H 3 0 2 (aq) + OH - (aq) (5 .20)
which, on addition, yields the net reaction
NaC2H302(s) + 3H20(1) �
Na ( aq) + 20H - (aq) + H3 0 + (aq) + HC2H302(aq) (5. 2 1 )
+
)
Thus, two oH - (aq) are produced for every one H 0 3 + ( aq .
i
The hydrolysis constant fo r the reaction of sodium acetate n water
i s , from reaction (5.20)
[HC2H30 2 (aq)][OH - (aq)]
K (5.22)
[C2H 3 0 2 - ( aq)]
h
We could think of Reaction (5 . 2 0) as the net reaction resulting from
t h e sum of the following two reactions