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Strength of Acids and Bases

STRENGTH OF ACIDS AND BASES

When an acid is dissolved in water, it dissociates into ions and produces hydronium ions. The characteristic properties of acids are due to hydronium ions.

Dissociation of some acids in aqueous solution, to form H3o+ ions is shown as follows:

HCl + H2O à H3O + + Cl

H2SO4 + 2H2O  à 2H3O+ + SO4

HNO3 + H2O à H3O+ + NO- 3

 

STRONG AND WEAK ACIDS

When an acid such as hydrochloric acid (HCI) dissolves in water, almost all of it dissociates. Such acids are known as strong acids.

On the other hand, when acetic acid dissolves in water only a small fraction of the  molecules ionizes. Such acids are called a weak acids.

Some common strong acids and weak acids are listed in Table 24.1


Table 24.1. Strong Acids and Weak Acids

           Strong Acids

 

1. Hydrochloric acid                          HCI

2. Trioxonitrate(V) acid                     HNO3

3. Tetraoxosulphate(VI) acid             H2SO4

4. Hydrobromic acid                           HBr

                       Weak Acids

 

1. Ethanoic acid (Acetic acid)           CH3COOH

2. Tetraoxophosphate(V) acid               H3PO4

3. Trioxoborate(III) acid                      H3BO3

4. Trioxocarbonate(IV) acid                H2CO3

 
The words strong and weak used in. this way do not refer to the concentration of the acid but refer to the degree of dissociation of the acid.

When solution of some acid is mixed with solution of same base, the H+ (aq) ions from acid combine with OH( aq) from base to form undissociated water (H2O) molecules. Thus, during neutralization of an acid with a base the following reaction takes place:

STRONG AND WEAK BASES

A base when dissolved in water undergoes dissociation to produce hydroxyl ions (OH-).

The characteristic properties of bases are due to OH(aq) ions.

A base such as NaOH or KOH which is completely dissociated in aqueous solution is called a strong base. Hydroxides of group-1 metals are strong bases. Ba(OH)2 is also a strong base as it is completely ionized in aqueous solution. On the other hand, a base which is weakly dissociated in aqueous solution is called a weak base.

For example, Mg(OH)2 and NH3 are weak bases. Strengths of acid and bases can be compared quantitatively in terms of their ionization constants Ka and Kb. ·

This has been discussed in detail in Unit 25.