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Salt, Hydrolysis of Salts

Salt, Hydrolysis of Salts. SALT: Neutralization product of acid base reactions. Classification by their origin: Arrhenius-type acid + base: NaOH + HCl = NaCl + H 2 O Br ø nsted-type base + acid: NH 3 + HCl = NH 4 Cl. Lewis-type acid + base reaction:

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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+ + Al3+ + 2SO42- or, e.g.: (NH4)2Fe(SO4)2(Mohr’s salt) When dissolved in water: (NH4)2Fe(SO4)2 = 2NH4+ + Fe2+ + 2SO42-

  7. Complex saltsare 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. • Cationsof strong bases have no acid-base character while those of weak bases are acidic. K+ + H2O = N. R. NH4+ + H2O  NH3 + H3O+ • Anionsof 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. Hydrolysis of a cation (C+) or an anion (A-) a) C+ + H2O  COH + H+ Kw [COH] [H+] [COH] Kw K h(cation) = = = Kb [C+] [C+] [OH-] [H+] = Kh[C+] and pH = - log Kh[C+] Quantitative Aspects Two equilibria coexist in aqueous solutions of hydrolyzable salts. • Self ionization of water: H2O  H+ + OH- Kw = [H+] [OH-]

  13. Hydrolysis of a cation (C+) or an anion (A-) b)A- + H2O  HA + OH- Kw [HA] [OH-] [HA] Kw K h(anion) = = = Ka [A-] [A-] [H+] [OH-] = Kh[A-] and pH = 14- pOH Quantitative Aspects

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