salt, hydrolysis of salts
TRANSCRIPT
Salt, Hydrolysis of SaltsSalt, 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+ + 2SO4
2-
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 SaltsHydrolysis 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.
4.4. CationsCations of strong bases have no acid-base character while those of weak bases are acidic.
K+ + H2O = N. R.
NH4+ + H2O NH3 + H3O+
5.5. AnionsAnions of strong acids have no acid-base character while those of weak asids are bases.
SO42- + H2O = N. R.
CN- + H2O HCN + OH-
Qualitative AspectsQualitative Aspects
1.1. No hydrolysis:No hydrolysis: Salts of strong acids and strong bases are neutralneutral in solution.
(NaCl, K2SO4, CaCl2….)
2.2. Anion-hydrolysis:Anion-hydrolysis: Salts of weak acids and strong bases are basicbasic in solution.
Dissolution: KCN K+ + CN-
Hydrolysis: CN- + H2O HCN + OH-
Qualitative AspectsQualitative Aspects
4.4. Cation-anion hydrolysis: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-
3.3. Cation-hydrolysis:Cation-hydrolysis: Salts of strong acids and weak bases are acidicacidic in solution.
Dissolution: NH4Cl NH4+ + Cl-
Hydrolysis: NH4+ + H2O NH3 + H3O+
Quantitative AspectsQuantitative AspectsTwo equilibria coexist in aqueous solutions of Two equilibria coexist in aqueous solutions of
hydrolyzable salts.hydrolyzable salts.
1.1. Self ionization of water:Self ionization of water:
H2O H+ + OH- Kw = [H+] [OH-]
2.2. Hydrolysis of a cation (CHydrolysis of a cation (C++) or an anion (A) or an anion (A--))
a) C+ + H2O COH + H+
K h(cation) =[COH] [H+]
[C+]
[COH] Kw
[C+] [OH-] ==Kw
Kb
[H+] = Kh[C+] and pH = - log Kh[C+]
Quantitative AspectsQuantitative Aspects
2.2. Hydrolysis of a cation (CHydrolysis of a cation (C++) or an anion (A) or an anion (A--))
b) A- + H2O HA + OH-
[OH-] = Kh[A-] and pH = 14- pOH
K h(anion) =[HA] [OH-]
[A-]
[HA] Kw
[A-] [H+] ==Kw
Ka