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Salt, Hydrolysis of Salts

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1 Salt, Hydrolysis of Salts

2 SALT: Neutralization product of acid base reactions.
Classification by their origin: Arrhenius-type acid + base: NaOH + HCl = NaCl + H2O Brønsted-type base + acid: NH3 + HCl = NH4Cl Lewis-type acid + base reaction: AgCl + 2NH3 = [Ag(NH3)2]Cl

3 Classification by their composition:
Normal (neutral) salts: Salts as results of a stoichiometric neutralization reaction: 2NaOH + H2SO4 = Na2SO4 + 2H2O 3KOH + H3PO4 = K3PO4 + 3H2O

4 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

5 Classification by their composition:
Base salts are products of a partial neutralization of a polyvalent (polyacidic) base. Bi(OH)3 + HNO3 = Bi(OH)2NO3 + H2O 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

6 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+ + Al SO42- or, e.g.: (NH4)2Fe(SO4)2 (Mohr’s salt) When dissolved in water: (NH4)2Fe(SO4)2 = 2NH4+ + Fe SO42-

7 Salt with a complex anion:
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-

8 Hydrolysis of Salts Hydrolysis: an acid-base reaction between
water 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-

9 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-

10 Qualitative Aspects No 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-

11 Qualitative Aspects Cation-hydrolysis: Salts of strong acids and weak bases are acidic in solution. Dissolution: NH4Cl  NH4+ + Cl- Hydrolysis: NH4+ + H2O  NH3 + H3O+ 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-

12 Quantitative Aspects Two equilibria coexist in aqueous solutions of
hydrolyzable salts. Self ionization of water: H2O  H+ + OH- Kw = [H+] [OH-] Hydrolysis of a cation (C+) 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+]

13 Quantitative Aspects Hydrolysis of a cation (C+) 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


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