Acids like perchloric acid (HCIO4), hydrochloric acid (HCl), hydrobromic acid (HBr), hydroiodic acid (HI), trioxonitrate(V) acid (HNO3) and tetraoxosulphate(VI) acid H2SO4) are termed strong because they are almost completely dissociated into their constituent ions in an aqueous medium, thereby acting as strong proton (H+) donors. Similarly, strong bases like sodium hydroxide (NaOH), potassium hydroxide (KOH), caesium hydroxide (CaOH) and barium hydroxide Ba(OH)2 almost completely dissociate into ions in an aqueous medium giving hydroxyl ions, OH- According to Arrhenius concept they are strong bases as they are able to completely dissociate and produce OH – in the medium.
The strength of acids or bases is experimentally measured by determining its ionization constants or dissociation constants.
PARTIAL IONIATION OF WEAK ACIDS AND BASES
Strung acids ionize almost completely into hydrogen ion and the corresponding anion in water and, therefore, the molar concentration of hydrogen ions in the solution is same as that of acid itself. But weak acids are not completely ionized. The relative strengths of weak acids can be compared in terms of their ionization constants. For example, the ionization equilibrium of an acid HA may be represented as:
Applying law of chemical equilibrium
K = [H3O +] [A -] / [HA] [ H2O]
Since concentration of water is very large and remains almost constant in solution, it can be combined with K to give another constant Ka.
K[H2O] = [A -] / [HA]
Ka = [H3O +] [A -] / [HA]
Here Ka is called ionization constant of acid.
The value of ionization constant gives an idea about the relative strength of the acid. Larger the value of Ka , higher is the concentration of H30+, and stronger is the acid. If dissociation constants of two acids are known, their relative strengths can be predicted. For example,
KCH3COOH = [ H3O+] [CH3COO -]
= 1.8 x 10 -5 at 298 k
KPH = [ H3O+] [ F -]
= 6.7 x 10 -4 at 298 K
Since KHF > KCH3COOH , therefore, HF is stronger acid than CH3COOH.
The value of dissociation constants of some of the monoprotic acids have been given in Table 25.1.
Table 25.1. The Ionization Constants of Some Common Weak Acids at 298 K
Chloroethanoic acid, ClCH2COOH
Nitrous acid, HN02
Hydrofluoric acid, HF
Formic acid, HCOOH
Benzoic acid, C6H5COOH
Ethanoic acid, CH3COOH
Hypochlorous acid, HOC!
Ammonium ion, NH4 +
Hydrocyanic acid, HCN
| Ionization Constant, Ka
1.4 x 10-3
4.5 x 10-4
3.5 x 10-4
1.8 x 10-4
6.5 x 10-5
1.8 x 10-5
3.0 x 10-8
s.6 x w-10
4.9 x w - lo
1.3 x w-10
2.0 x 10- 16
Calculation of [ H3O +] and Degree of Ionization
From the knowledge of Ka it is possible to calculate hydronium ion concentration and degree of ionization of a weak acid. As an example, let us take the case of acetic acid. The following equation represents the ionization of acetic acid in aqueous solution:
Suppose C moles of CH3COOH are dissolved per litre of solution and let a be the degree of ionization of CHFOOH, then at equilibrium the concentration of various species would be as follows:[CH3 COOH] = C(1-a) [CH3 COO -] = Ca
Therefore , ka = (Ca ) (Ca)/ C (l-a) = Ca2 / (l-a)
Since for weak acid a is very small as compared to 1, a in the denominator can be neglected. the expression of Ka then becomes
Ka = Ca2
Knowing the value of Ka it is possible to calculate the degree of ionization of the acid at any particular concentration C
From the degree of ionization, hydronium ion concentration can be calculated as:[H3O+] = Ca
Ionization Constants of Bases in Water
The relative strengths of bases are also compared m terms of their dissociation or ionization constants. The ionization constant Kb for a weak base B can be represented as follows:
Smaller the value of ionization constant for a base, weaker is the base.
If C is the molar concentration of base and a is its degree of dissociation, then by similar calculation (as in case of acids) we can derive the relations,
Table 25.2. The Ionization Constants of Some Bases at 298 K
Base Ionization Constant, Kb
Ammonia, NH3 1.8 x 10-5
Methylamine, CH3NH2 4.6 x 10-4
Dimethyle, (CH3) 2NH 5.4 x 10-4
Trimethylamine, (CH3)3N 6.5 x 10-5
Aniline, C6H5NH2 4.3 x 10-10
Pyridine, C5H5N 1.8 x 10-9
The degree of ionization of a weak acid or a weak base is suppressed (reduced) by common ion effect.
An acid on ionization gives hydrogen ion and the corresponding anion. A strong acid is completely ionized while a weak acid is only partially ionized. In case of weak acids the ions formed are in equilibrium with the unionized molecules of the acid. The fraction of the total number of molecules of the acid dissolved that ionizes is known as degree of ionization of the acid.
The degree of ionization of the acid decreases if some strong electrolyte that can provide any of the product ions is added to the solution of the acid. For example, consider the ionization of acetic acid.
If sodium acetate is added to the above solution, the concentration of acetate ions increases and the above equilibrium shifts in backward direction according to Le-Chatelier’s principle. Thus, addition of acetate ions reduces the degree of ionization of acetic acid. Similarly, if H+ ions are provided from some source, such as dilute hydrochloric acid, the degree of ionization of the weak acid is suppressed.
Similarly, the degree of ionization of a weak base is suppressed by common ion effect. A base on ionization gives hydroxyl ions (OH-) and the corresponding cation (M+). Addition of some strong electrolyte which provides either of the ions (OH- or M+) reduces the degree of ionization of the weak base in accordance with Le-Chatelier’s principle. For example, the degree of ionization of ammonia is suppressed by addition of ammonium chloride or sodium hydroxide.