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E-5
Part III. Interactive Equilibria
Interactive equilibria involve a series of reaction equilibria that have one or more species in common. Most
equilibria in natural systems are interactive. In a system of interactive equilibria, a change in the equilibrium
position of one reaction affects the equilibrium position of the other reactions. All of the reactions shift to come
to new equilibrium positions, so that the reaction quotient (Q) of each reaction is again equal to K for that
reaction. The stress created by the addition (or removal) of species is relieved by a shift in the position of
equilibrium. To see how this interaction works, let us consider how the oxalic acid (H2C2O
4
) equilibria might
interact with a solution containing acetic acid (CH3COOH) and acetate (CH3COO
-
). We have previously
discussed the acetic acid equilibrium:
CH3COOH
(aq)
+ H2O
(l)
CH3COO
-
(aq)
+ H3O
+
(aq)
(1)
Oxalic acid is a weak diprotic acid. The following two reactions describe the ionization of oxalic acid in water:
H2C2O
4
(aq)
+ H2O
(l)
HC2O
4
-
(aq)
+ H3O
+
(aq)
(5)
HC2O
4
-
(aq)
+ H2O
(l)
C2O
4
2-
(aq)
+ H3O
+
(aq)
(6)
The above equilibria can be combined to give:
H2C2O
4
(aq)
+ 2 H2O
(l)
©2O
4
2-
(aq)
+ 2 H3O
+
(aq)
(7)
The equilibrium constant, K', for the combined reactions is
K' =
[H3O
+
]²[C2O
4
2-
]
[H2C2O
4
]
Let us assume that both acetic acid and oxalic acid have been dissolved in the same aqueous solution, and that
the concentrations of all species have reached values such that both the
K
a
of acetic acid and the K' of oxalic acid are satisfied. Therefore, the system is at equilibrium.
Now, suppose some acetate (CH3COO
-
) is added to the solution. The value of the reaction quotient, Q, for
acetic acid now exceeds the value of the equilibrium constant, K
a
, therefore the system shifts to restore the
equilibrium. Le Châtelier's Principle will also predict that the added acetate will cause a shift in the equilibrium
position of the acetic acid system in order to restore the value of Q to the K
a
value for acetic acid.
Therefore, the net effect of adding acetate ions to the acetic acid-oxalic acid system at equilibrium is to lower
the [H3O
+
]. Lowering [H3O
+
] will in turn make the value of Q for the oxalic acid system less than its K' value
(see equation (7)). Consequently, a shift in the equilibrium position of reaction (7) must occur in order to
restore the K' for oxalic acid. Thus, the oxalate ion concentration, [C2O
4
2-
], will increase and oxalic acid
concentration, [H2C2O
4
], will decrease to restore the value of Q for the oxalic acid system to the K' value. The
net effect is to alter the concentrations of all species in such a way, that both the K
a
for acetic acid and the K' for
oxalic acid are again satisfied.
