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]+ + ClSalt 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+ + CNHydrolysis: CN- + H2O  HCN + OH-
Qualitative Aspects
3.
Cation-hydrolysis: Salts of strong acids and weak
bases are acidic in solution.
Dissolution: NH4Cl  NH4+ + ClHydrolysis: 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+ + CNCation-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-
2.
Kw = [H+] [OH-]
Hydrolysis of a cation (C+) or an anion (A-)
a) C+ + H2O  COH + H+
K h(cation)
=
[H+] =
[COH] [H+]
[COH] Kw
=
[C+]
[C+] [OH-]
Kh[C+]
and pH = - log
Kw
= K
b
Kh[C+]
Quantitative Aspects
2.
Hydrolysis of a cation (C+) or an anion (A-)
b)
A- + H2O  HA + OH-
K h(anion) =
[OH-] =
[HA] [OH-]
=
[A-]
Kh[A-]
[HA] Kw
[A-] [H+]
Kw
= K
a
and pH = 14- pOH