Strength of Acids and Bases

Strength of Acids and Bases

The strength of acids and bases can be determined by the concentration of H₃O⁺ (or) OH^-produced per mole of the substance dissolved in H₂O . Generally we classify the acids / bases either as strong or weak. A strong acid is the one that is almost completely dissociated in water while a weak acid is only partially dissociated in water.

Let us quantitatively define the strength of an acid (HA) by considering the following general equilibrium.

HA + H₂O ↔ H₃O⁺ + A^-

acid 1 + base 2 ↔ acid 2 + base 1

The equilibrium constant for the above ionisation is given by the following expression

We can omit the concentration of H₂O in the above expression since it is present in large excess and essentially unchanged.

Here, K_a is called the ionisation constant or dissociation constant of the acid. It measures the strength of an acid. Acids such as HCl,HNO₃ etc... are almost completely ionised and hence they have high K_a value (K_a for HCl at 25^o C is 2×10⁶ ) Acids such as formic acid (K_a =1.8 ×10^-4at 25^oC) , acetic acid (1.8 ×10^-5at 25^oC) etc.. are partially ionized in solution and in such cases, there is an equilibrium between the unionised acid molecules and their dissociated ions. Generally, acids with K_a value greater than ten are considered as strong acids and less than one are considered as weak acids.

Let us consider the dissociation of HCl in aqueous solution,

HCl + H - OH ↔ H₃O⁺ + Cl^-

acid 1 + base 2 ↔ acid 2 + base 1

As discussed earlier, due to the complete dissociation, the equilibrium lies almost 100% to the right. i.e., the Cl^- ion has only a negligible tendency to accept a proton form H₃O⁺ . It means that the conjugate base of a strong acid is a weak base and vice versa.

The following table illustrates the relative strength of conjugate acid – base pairs.