Page 166 - Modern Analytical Chemistry

1400-CH06 9/9/99 7:40 AM Page 149

Chapter 6 Equilibrium Chemistry 149

The effect on the solubility of AgCl of adding AgNO 3 is obvious,* but what is the ef-
+
fect of adding a ligand that forms a stable, soluble complex with Ag ? Ammonia, for
+
example, reacts with Ag as follows
+
+
Ag (aq) + 2NH 3 (aq) t Ag(NH 3 ) 2 (aq) 6.28
+
+
Adding ammonia decreases the concentration of Ag as the Ag(NH 3 ) 2 complex
+
forms. In turn, decreasing the concentration of Ag increases the solubility of AgCl
as reaction 6.27 reestablishes its equilibrium position. Adding together reactions
6.27 and 6.28 clarifies the effect of ammonia on the solubility of AgCl, by showing
that ammonia is a reactant.
–
+
AgCl(s) + 2NH 3 (aq) t Ag(NH 3 ) 2 (aq)+Cl (aq) 6.29

EXAMPLE 6.6

What is the effect on the solubility of AgCl if HNO 3 is added to the equilibrium
solution defined by reaction 6.29?
SOLUTION

Nitric acid is a strong acid that reacts with ammonia as shown here
+
–
HNO 3 (aq)+NH 3 (aq) t NH 4 (aq)+NO 3 (aq)
Adding nitric acid lowers the concentration of ammonia. Decreasing
ammonia’s concentration causes reaction 6.29 to move from products to
reactants, decreasing the solubility of AgCl.

Increasing or decreasing the partial pressure of a gas is the same as increasing
†
or decreasing its concentration. The effect on a reaction’s equilibrium position can
be analyzed as described in the preceding example for aqueous solutes. Since the
concentration of a gas depends on its partial pressure, and not on the total pressure
of the system, adding or removing an inert gas has no effect on the equilibrium po-
sition of a gas-phase reaction.
Most reactions involve reactants and products that are dispersed in a solvent.
If the amount of solvent is changed, either by diluting or concentrating the solu-
tion, the concentrations of all reactants and products either decrease or increase.
The effect of these changes in concentration is not as intuitively obvious as when
the concentration of a single reactant or product is changed. As an example, let’s
consider how dilution affects the equilibrium position for the formation of the
aqueous silver-amine complex (reaction 6.28). The equilibrium constant for this
reaction is
[Ag (NH 3 2 +
) ] eq
b 2 = 6.30
2
[Ag + ] [NH 3 ] eq
eq
*Adding AgNO 3 decreases the solubility of AgCl.
†The relationship between pressure and concentration can be deduced from the ideal gas law. Starting with PV = nRT, we
solve for the molar concentration
n P
Molar concentration = =
V RT
Of course, this assumes an ideal gas (which is usually a reasonable assumption under normal laboratory conditions).