WHAT IS DISSOCIATION OF WEAK ACIDS AND BASESS ?

WHAT IS DISSOCIATION OF WEAK ACIDS AND BASESS ?

Dissociation of a Weak Acid

          Consider the dissociation of a weak monobasic acid HA in water, represented by the equation

                      HA+H₂O ⇋ H₃O⁺ + A⁻

Applying the law of chemical equilibrium, the equilibrium constant Kc is given by the expression

                              [H₃O⁺][A⁻]

               Kc  =  ----------------------       .........(1)

                                  [HA] × k

          The square brackets, as usual, represent concentrations in moles per litre of the entities enclosed therein.

          Since water is present in large excess in dilute solutions, its concentration may be taken as constant, Say, k. Further, since the symbol  H₃O⁺  simply indicates that hydrogen ion is hydrated, it may be replaced by  H⁺ , for simplicity. The above equation may then be put as

                        [H⁺][A⁻]

               Kc = ----------------------                .............(2)

                               [HA] × k

assuming that the activity coefficients of the species involved are equal to unity each.

          Since the product of the two constants Kc and k is equal to another constant, say, Ka Eq. 2 may be written as

                   Ka =  [H⁺][A⁻] / [HA]