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Chapter 6 Equilibrium Chemistry 139

EXAMPLE 6.1
Calculate the equilibrium constant for the reaction

2A+B t C+3D

given the following information
Rxn 1: A + B t D K 1 = 0.40
Rxn 2: A + E t C+D+F K 2 = 0.10
Rxn 3: C + E t B K 3 = 2.0
Rxn 4: F + C t D+B K 4 = 5.0
SOLUTION
The overall reaction is given as
Rxn 1 + Rxn 2 – Rxn 3 + Rxn 4

If Rxn 3 is reversed, giving
Rxn t C + E K 5 = 1 = 1 = .
5
050
B
:
K 3 20
.
then the overall reaction is
Rxn 1 + Rxn 2 + Rxn 5 + Rxn 4
and the overall equilibrium constant is
K overall = K 1 ´K 2 ´K 5 ´K 4 = 0.40 ´0.10 ´0.50 ´5.0 = 0.10

6D Equilibrium Constants for Chemical Reactions

Several types of reactions are commonly used in analytical procedures, either in
preparing samples for analysis or during the analysis itself. The most important of
these are precipitation reactions, acid–base reactions, complexation reactions, and
oxidation–reduction reactions. In this section we review these reactions and their
equilibrium constant expressions.

6D.1 Precipitation Reactions
A precipitation reaction occurs when two or more soluble species combine to form
an insoluble product that we call a precipitate. The most common precipitation re- precipitate
action is a metathesis reaction, in which two soluble ionic compounds exchange An insoluble solid that forms when two
parts. When a solution of lead nitrate is added to a solution of potassium chloride, or more soluble reagents are combined.
for example, a precipitate of lead chloride forms. We usually write the balanced re-
action as a net ionic equation, in which only the precipitate and those ions involved
in the reaction are included. Thus, the precipitation of PbCl 2 is written as
–
Pb (aq) + 2Cl (aq) t PbCl 2 (s)
2+
In the equilibrium treatment of precipitation, however, the reverse reaction de-
scribing the dissolution of the precipitate is more frequently encountered.
PbCl 2 (s) t Pb (aq) + 2Cl (aq)
2+
–