Beside ionization degree, acid-base strength can be defined by equilibrium constant, which are acid equilibrium constant (Ka) for acid and base equilibrium constant (Kb) for base. Ka and Kb for each solution are different based on their ionization reaction. The following that shown Ka and Kb.
Acid solution | Ka |
HNO3 | Very high |
HClO4 | 1.0 x 1010 |
HI | 1.0 x 1010 |
H2CO3 | 4.3 x 10-7(Ka1) 5.6 x 10-11(Ka2) |
According to the above table, strong acid have very high Ka value (≥10), whereas weak acid have very low Ka value. It can be explained by ionization reaction and chemical equilibrium. Strong acids undergo ionization reaction completely, whereas weak acids undergo partly ionization reaction. Ionization reaction of strong acid is irreversible reaction, whereas that of weak acid is reversible reaction which forms chemical equilibrium.
Generally, the ionization of a weak acid can be represented as follow:
HA (aq) H+ (aq) + A-(aq)
The acid equilibrium constant for the reaction above is:
Ka =
Relation acid equilibrium constant with ionization degree as follow:
α =
Generally, the ionization of a weak base can be represented as follow:
LOH(aq) L+ (aq) + OH-(aq)
The base equilibrium constant for the reaction above is
Kb =
Relation base equilibrium constant with ionization degree as follow:
α =
