Page 89 - Instant notes

Fundamentals of acids and bases 75

reaction between an acid and a base (an acid-base reaction) involves the exchange of a
proton, for example:
−
+
HCl+NH 3→Cl +NH 4
acid base
These definitions apply under all conditions, but the most important acid-base systems
use water as a solvent. In this case, equilibria are set up in water, which for HA (a general
acid) is:

and for B (a general base) is:

In the first equilibrium, the H 2O molecule acts as a base, accepts a proton and forms the
+
hydronium ion, H 3O , which is the hydrated form of the proton in solution. In the
−
second, the H 2O molecule acts as an acid and forms the hydroxide ion, OH . The base
that results from the transfer of a proton from the acid is called the conjugate base of the
−
−
acid. Therefore, A is the conjugate base of HA and OH is the conjugate base of H 2O.
Similarly, the acid that results from the acceptance of a proton by the base is called the
+
+
conjugate acid of the base. This means that BH is the conjugate acid of B and H 3O is
the conjugate acid of H 2O.
The autoprotolysis constant

Since water can act as both acid and base, pure water itself ionizes into hydronium and
hydroxide ions in the autoprotolysis equilibrium,

+
+
The hydronium ion, H 3O , is often (inaccurately) represented as a proton, H , but this is
merely equivalent to removing H 2O from both sides of the equation, giving:

+
+
and so H (aq)and H 3O (aq) should be considered to be equivalent.
This is a dynamic equilibrium, which means that protons are continually exchanged
between neighboring water molecules. The equilibrium constant (the autoprotolysis
constant or water dissociation constant) is given by:

since water is a pure liquid and has an activity of unity (see Topic C1). In fact, this
equation does not just apply to pure water, but relates the activities of the hydronium and
hydroxide ions in all aqueous solutions. The value of this equilibrium constant is
1.00×10 −14 at 298 K (25°C). This means that for pure water, the concentration of each ion