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