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Strength of Acids and Bases
The strength of acids and bases can be determined by the concentration of H O (or) OH3
+ -
produced per mole of the substance dissolved in H O2 . 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
acid
+ H2O
base 2
H3O+
acid 2
+ A-
base 11
The equilibrium constant for the above ionisation is given by the following expression
K= [H O ][A ] [HA][H O]
3 + -
2
…..(8.1)
We can omit the concentration of H O2 in the above expression since it is present in large excess and essentially unchanged.
K = [H O ][A ]
[HA]a 3
+ -
…..(8.2)
Here, Ka 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 Ka value (K for HCl at 25 C is 2×10 )
a
o 6 Acids such as formic acid (K =1.8 10 at 25 Ca
-4 o× ) , acetic acid (1.8 10 at 25 C-5 o× ) etc.. are partially ionised 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 H O + Cl acid 1 base 2 acid 2 base 1
3 + -
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 H O3
+ . 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.
XII U8-Ionic equilibrium.indd 6 2/19/2020 5:11:32 PM
HClO4
HCl H SO
HNO3
H O
HNO
HF CH COOH
NH
-OH H2
Strong acids
Weak acids
Very weak acids
- 4
-
- 4
- 3
2 -
2 -
- 3
- 2
2-
-
ClO
Cl HSO
NO H O
NO
F CH COO
NH
O H
Very weak bases
Weak bases
Strong bases
2 4
3 +
2
3
3