Page 162 - Modern Analytical Chemistry

1400-CH06 9/9/99 7:40 AM Page 145

Chapter 6 Equilibrium Chemistry 145

Equilibrium constants for complexation reactions involving solids are defined
by combining appropriate K sp and K f expressions. For example, the solubility of
AgCl increases in the presence of excess chloride as the result of the following com-
plexation reaction
–
–
AgCl(s)+Cl (aq) t AgCl 2 (aq) 6.21
This reaction can be separated into three reactions for which equilibrium constants
are known—the solubility of AgCl, described by its K sp
+
–
AgCl(s) t Ag (aq)+Cl (aq)
–
and the stepwise formation of AgCl 2 , described by K 1 and K 2
–
+
Ag (aq)+Cl (aq) t AgCl(aq)
–
–
AgCl(aq)+Cl (aq) t AgCl 2 (aq)
The equilibrium constant for reaction 6.21, therefore, is equal to K sp ´K 1 ´K 2 .
EXAMPLE 6.4

Determine the value of the equilibrium constant for the reaction
PbCl 2 (s) t PbCl 2 (aq)

SOLUTION
This reaction can be broken down into three reactions. The first of these
reactions is the solubility of PbCl 2 , described by its K sp
2+
–
PbCl 2 (s) t Pb (aq) + 2Cl (aq)
and the second and third are the stepwise formation of PbCl 2 (aq), described by
K 1 and K 2
2+
+
–
Pb (aq)+Cl (aq) t PbCl (aq)
–
+
PbCl (aq)+Cl (aq) t PbCl 2 (aq)
Using values for K sp , K 1 , and K 2 from Appendices 3A and 3C, we find the
equilibrium constant to be
–5
K = K sp ´K 1 ´K 2 = (1.7 ´10 )(38.9)(1.62) = 1.1 ´10 –3

6D. 4 Oxidation–Reduction Reactions
In a complexation reaction, a Lewis base donates a pair of electrons to a Lewis acid.
In an oxidation–reduction reaction, also known as a redox reaction, electrons are redox reaction
not shared, but are transferred from one reactant to another. As a result of this elec- An electron-transfer reaction.
tron transfer, some of the elements involved in the reaction undergo a change in ox-
idation state. Those species experiencing an increase in their oxidation state are oxi-
dized, while those experiencing a decrease in their oxidation state are reduced. For
example, in the following redox reaction between Fe 3+ and oxalic acid, H 2 C 2 O 4 ,
iron is reduced since its oxidation state changes from +3 to +2.
+
3+
2+
2Fe (aq)+H 2 C 2 O 4 (aq)+2H 2 O(l) t 2Fe (aq) + 2CO 2 (g)+2H 3 O (aq) 6.22