Salt, Hydrolysis of Salts
SALT: Neutralization product of acid base reactions.Classification by their origin:
Arrhenius-type acid + base:NaOH + HCl = NaCl + H2O
Brnsted-type base + acid:NH3 + HCl = NH4Cl
Lewis-type acid + base reaction:AgCl + 2NH3 = [Ag(NH3)2]Cl
Classification by their composition:
Normal (neutral) salts: Salts as results of a stoichiometric neutralization reaction:
2NaOH + H2SO4 = Na2SO4 + 2H2O
3KOH + H3PO4 = K3PO4 + 3H2O
Classification by their composition:
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:
Base salts are products of a partial neutralization of a polyvalent (polyacidic) base.
Bi(OH)3 + HNO3 = Bi(OH)2NO3 + H2OMixed 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
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 (Mohrs salt)
When dissolved in water:
(NH4)2Fe(SO4)2 = 2NH4+ + Fe2+ + 2SO42-
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) chloridewhen 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 SaltsHydrolysis: an acid-base reaction betweenwater and the ion(s) of the dissolved salt.
Preliminary consideration:
Water is neutral and amphoteric:
H2O H+ + OH-
Dissolved salts exist in ionized form:
NaCl Na+ + Cl-
If any of the ions in solution has acid-base character, it will affect the self-ionization equilibrium of the solvent.
Cations of strong bases have no acid-base character while those of weak bases are acidic.
K+ + H2O = N. R.NH4+ + H2O NH3 + H3O+
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 AspectsNo hydrolysis: Salts of strong acids and strong bases are neutral in solution.(NaCl, K2SO4, CaCl2.)
Anion-hydrolysis: Salts of weak acids and strong bases are basic in solution.
Dissolution: KCN K+ + CN-Hydrolysis: CN- + H2O HCN + OH-
Qualitative AspectsCation-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-
Cation-hydrolysis: Salts of strong acids and weak bases are acidic in solution.
Dissolution: NH4Cl NH4+ + Cl-Hydrolysis: NH4+ + H2O NH3 + H3O+
Quantitative AspectsTwo equilibria coexist in aqueous solutions of hydrolyzable salts.
Self ionization of water:
H2O H+ + OH-Kw = [H+] [OH-]
Quantitative Aspects