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Salt_ Hydrolysis of Salts

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					Salt, Hydrolysis of Salts
SALT:         Neutralization product of acid base
              reactions.

Classification by their origin:

1.   Arrhenius-type acid + base:
       NaOH + HCl = NaCl + H2O

2.   Brønsted-type base + acid:
       NH3 + HCl = NH4Cl

3.   Lewis-type acid + base reaction:
       AgCl + 2NH3 = [Ag(NH3)2]Cl
Classification by their composition:

a)   Normal (neutral) salts:
     Salts as results of a stoichiometric neutralization
     reaction:

       2NaOH + H2SO4 = Na2SO4 + 2H2O

       3KOH + H3PO4 = K3PO4 + 3H2O
Classification by their composition:


b)   Acid salts:
     Salts formed by an incomplete neutralization of a
     polybasic acid.

       KOH + H3PO4 = KH2PO4 + H2O

       2KOH + H3PO4 = K2HPO4 + 2H2O

       NaOH + H2CO3 = NaHCO3 + H2O
     Classification by their composition:

     c)   Base salts
          are products of a partial neutralization of a polyvalent
          (polyacidic) base.

            Bi(OH)3 + HNO3 = Bi(OH)2NO3 + H2O
d)    Mixed salts
      Salts formed in a reaction of a polyacidic base with two
      different acids (One cation, two different anions)

          Ca(OH)2 + HCl + HOCl = CaCl(OCl) + 2H2O
e)   Double salts: Composed of two different cations and one
     kind of anion

       K2SO4 + Al2(SO4)3 = 2KAl(SO4)2 (alum)

     When dissolved, they dissociate into all of their ionic
     components:

       KAl(SO4)2 = K+ + Al3+ + 2SO42-

       or, e.g.: (NH4)2Fe(SO4)2 (Mohr’s salt)

     When dissolved in water:

     (NH4)2Fe(SO4)2 = 2NH4+ + Fe2+ + 2SO42-
f)   Complex salts are coordination compounds composed of
     a non-dissociable complex ion and a dissociable
     counterion

       Salt with a complex cation:
       [Ag(NH3)2]Cl    diamminesilver(l) chloride

       when dissolved:
       [Ag(NH3)2]Cl = [Ag(NH3)2]+ + Cl-


       Salt with a complex anion:
       Na3[Ag(S2O3)2] Sodium dithiosulphatoargentate (I)


       In aqueous solution:
        Na3[Ag(S2O3)2] = 3Na+ + [Ag(S2O3)2]3-
Hydrolysis of Salts

Hydrolysis: an acid-base reaction between
water and the ion(s) of the dissolved salt.


Preliminary consideration:

1.   Water is neutral and amphoteric:

     H2O  H+ + OH-

2.   Dissolved salts exist in ionized form:

     NaCl  Na+ + Cl-
3.   If any of the ions in solution has acid-base character, it
     will affect the self-ionization equilibrium of the solvent.


4.   Cations of strong bases have no acid-base character
     while those of weak bases are acidic.

       K+ + H2O = N. R.
       NH4+ + H2O  NH3 + H3O+


5.   Anions of strong acids have no acid-base character
     while those of weak asids are bases.

       SO42- + H2O = N. R.
       CN- + H2O  HCN + OH-
Qualitative Aspects

1.   No hydrolysis: Salts of strong acids and strong bases
     are neutral in solution.
       (NaCl, K2SO4, CaCl2….)



2.   Anion-hydrolysis: Salts of weak acids and strong bases
     are basic in solution.

       Dissolution: KCN  K+ + CN-
       Hydrolysis: CN- + H2O  HCN + OH-
Qualitative Aspects
3.   Cation-hydrolysis: Salts of strong acids and weak
     bases are acidic in solution.

       Dissolution: NH4Cl  NH4+ + Cl-
       Hydrolysis: NH4+ + H2O  NH3 + H3O+

4.   Cation-anion hydrolysis: Salts of weak acids and weak
     bases can be acidic, basic or neutral in solution, owing
     to the hydrolysis of both ions. The reaction depends on
     relative acid-base strengths.

        Dissolution: NH4CN  NH4+ + CN-
        Cation-Hydrolysis: NH4+ + H2O  NH3 + H3O+
        Anion-hydrolysis: CN- + H2O  HCN + OH-
Quantitative Aspects
Two equilibria coexist in aqueous solutions of
hydrolyzable salts.

1.   Self ionization of water:

         H2O  H+ + OH-           Kw = [H+] [OH-]


2.   Hydrolysis of a cation (C+) or an anion (A-)

        a) C+ + H2O  COH + H+

                           [COH] [H+]    [COH] Kw        Kw
          K h(cation)                 =                = K
                              [C+]      [C+] [OH-]         b
          =

          [H+] =        Kh[C+]     and pH = - log    Kh[C+]
Quantitative Aspects

2.   Hydrolysis of a cation (C+) or an anion (A-)

       b)     A- + H2O  HA + OH-




                            [HA] [OH-]     [HA] Kw       Kw
            K h(anion) =               =               = K
                               [A-]        [A-] [H+]       a




            [OH-] =        Kh[A-]    and pH = 14- pOH

				
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posted:2/29/2012
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