Salt Hydrolysis of Salts SALT Neutralization product of

Salt, Hydrolysis of Salts

SALT: Neutralization product of acid base reactions. Classification by their origin: 1. Arrhenius-type acid + base: Na. OH + HCl = Na. Cl + H 2 O 2. Brønsted-type base + acid: NH 3 + HCl = NH 4 Cl 3. Lewis-type acid + base reaction: Ag. Cl + 2 NH 3 = [Ag(NH 3)2]Cl

Classification by their composition: a) Normal (neutral) salts: Salts as results of a stoichiometric neutralization reaction: 2 Na. OH + H 2 SO 4 = Na 2 SO 4 + 2 H 2 O 3 KOH + H 3 PO 4 = K 3 PO 4 + 3 H 2 O

Classification by their composition: b) Acid salts: Salts formed by an incomplete neutralization of a polybasic acid. KOH + H 3 PO 4 = KH 2 PO 4 + H 2 O 2 KOH + H 3 PO 4 = K 2 HPO 4 + 2 H 2 O Na. OH + H 2 CO 3 = Na. HCO 3 + H 2 O

Classification by their composition: c) Base salts are products of a partial neutralization of a polyvalent (polyacidic) base. Bi(OH)3 + HNO 3 = Bi(OH)2 NO 3 + H 2 O 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 = Ca. Cl(OCl) + 2 H 2 O

e) Double salts: Composed of two different cations and one kind of anion K 2 SO 4 + Al 2(SO 4)3 = 2 KAl(SO 4)2 (alum) When dissolved, they dissociate into all of their ionic components: KAl(SO 4)2 = K+ + Al 3+ + 2 SO 42 or, e. g. : (NH 4)2 Fe(SO 4)2 (Mohr’s salt) When dissolved in water: (NH 4)2 Fe(SO 4)2 = 2 NH 4+ + Fe 2+ + 2 SO 42 -

f) Complex salts are coordination compounds composed of a non-dissociable complex ion and a dissociable counterion Salt with a complex cation: [Ag(NH 3)2]Cl diamminesilver(l) chloride when dissolved: [Ag(NH 3)2]Cl = [Ag(NH 3)2]+ + Cl. Salt with a complex anion: Na 3[Ag(S 2 O 3)2] Sodium dithiosulphatoargentate (I) In aqueous solution: Na 3[Ag(S 2 O 3)2] = 3 Na+ + [Ag(S 2 O 3)2]3 -

Hydrolysis 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: H 2 O H+ + OH- 2. Dissolved salts exist in ionized form: Na. Cl 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. Cations of strong bases have no acid-base character while those of weak bases are acidic. K+ + H 2 O = N. R. NH 4+ + H 2 O NH 3 + H 3 O+ 5. Anions of strong acids have no acid-base character while those of weak asids are bases. SO 42 - + H 2 O = N. R. CN- + H 2 O HCN + OH-

Qualitative Aspects 1. No hydrolysis: Salts of strong acids and strong bases are neutral in solution. (Na. Cl, K 2 SO 4, Ca. Cl 2…. ) 2. Anion-hydrolysis: Salts of weak acids and strong bases are basic in solution. Dissolution: KCN K+ + CNHydrolysis: CN- + H 2 O HCN + OH-

Qualitative Aspects 3. Cation-hydrolysis: Salts of strong acids and weak bases are acidic in solution. Dissolution: NH 4 Cl NH 4+ + Cl. Hydrolysis: NH 4+ + H 2 O NH 3 + H 3 O+ 4. 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: NH 4 CN NH 4+ + CNCation-Hydrolysis: NH 4+ + H 2 O NH 3 + H 3 O+ Anion-hydrolysis: CN- + H 2 O HCN + OH-

Quantitative Aspects Two equilibria coexist in aqueous solutions of hydrolyzable salts. 1. Self ionization of water: H 2 O H+ + OH- 2. Kw = [H+] [OH-] Hydrolysis of a cation (C+) or an anion (A-) a) C+ + H 2 O COH + H+ K h(cation) = [H+] = [COH] [H+] [COH] Kw = [C+] [OH-] Kh[C+] and p. H = - log Kw = K b Kh[C+]

Quantitative Aspects 2. Hydrolysis of a cation (C+) or an anion (A-) b) A- + H 2 O HA + OH- K h(anion) = [OH-] = [HA] [OH-] = [A-] Kh[A-] [HA] Kw [A-] [H+] Kw = K a and p. H = 14 - p. OH