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pH of Salts Solutions (Hydrolysis)

Dr. M. Sasvári: Chemistry Lectures. pH of Salts Solutions (Hydrolysis). Further reading: Ebbing, Gen. Chem: Acid-base properties of salts solutions: Hydrolysis Prediction of weather a salt solution is acidic, basic or neutral The pH of salts solutions Solubility equilibria

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pH of Salts Solutions (Hydrolysis)

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  1. Dr. M. Sasvári: Chemistry Lectures pH of Salts Solutions (Hydrolysis) • Further reading: Ebbing, Gen. Chem: • Acid-base properties of salts solutions: Hydrolysis • Prediction of weather a salt solution is acidic, basic or neutral • The pH of salts solutions • Solubility equilibria • The solubility product constant • Solubility and the common ion effect • Precipitation calculations Chem Eq 5

  2. CH3COONa Acetate ion Sodium ion ANION CATION NH4Cl Chloride ion Ammonium ion ANION CATION • Salts are strong electrolytes Complete ionization in water: CH3COO-+ Na+ Sodium acetate NH4++ Cl - Ammonium chloride Reaction with water: Hydrolysis Chem Eq 5

  3. Acetate ion is a conjugated base - pKb H H + + O A HA O H Basic pH Anionhydrolysis Chem Eq 5

  4. CH3COO-+ H2O Acetate ion Acetic acid conjugated base ( acid (HA) • Anionhydrolysis Acetate ion is a conjugated base (reacts with water, takes a proton) CH3COOH+ OH- Hydrolysis of an anion: Reaction of an anion with water to produce the weak acid and a hydroxide ion. Salt solutions containing a hydrolyzing anion and a non-hydrolyzing cation (i.e. the salt is made of a weak acid and of a strong base) have BASIC pH. Chem Eq 5

  5. H H O O H H H Ammonium ion is a conjugated acid pKa + + + NH NH 4 3 Cationhydrolysis Acidic pH Chem Eq 5

  6. Ammonium ion Ammonia conjugated acid (B+ base (B) • Cationhydrolysis Ammonium ion is a conjugated base (reacts with water, takes a proton) NH4++ H2O NH3+ H+ Hydrolysis of a cation: Reaction of an cation with water to produce the weak base and a hydrogen ion. Salt solutions containing a hydrolyzing cation and a non-hydrolyzing anion (i.e. the salt is made of a weak base and of a strong acid) have ACIDIC pH. Chem Eq 5

  7. Na acetate Acetic acid Weak acid Conjugated base pKa= 4.7 pKb = 9.3 pKb-lg c pKa-lg c pOH= pH= 2 2 • Calculations Calculation of pH of a conjugated base is exactly the same as calculating pH of a weak base (see before) Chem Eq 5

  8. Ammonia Ammonium chloride Conjugated acid Weak base pKa= 9.3 pKb = 4.7 pKb-lg c pKa-lg c pOH= pH= 2 2 • Calculations Calculation of pH of a conjugated acid is exactly the same as calculating pH of a weak acid (see before) Chem Eq 5

  9. Ammonium ion: Cation hydrolysis pKa=9.3 NH4+ + H2O NH3+ H3O+ conjugated acid [B+] base (B) Acetate ion: Anion hydrolysis pKb=9.3 CH3COO - + H2O CH3COOH+ OH- H2O conjugated base [A-] acid (HA) • Cation and anion hydrolysis Example: Ammonium acetate (CH3COONH4 ) If pKa ~pKb pH is neutral If pKa > pKb pH is BASIC Estimation of pH: ACIDIC If pKa < pKb pH is Chem Eq 5

  10. Acidic Salts Acidic salts have a dissociable H+ • Examples: • NaHSO4 (acidic pH) • NaHCO3 (basic pH) • NaH2PO4 (acidic pH) • Na2HPO4 (basic pH) Chem Eq 5

  11. HSO4- Na+ + NaHSO4 Sulfuric acid is a diprotic acid pKa<2 HSO4- SO42- + H+ H2SO4 H+ + HSO4- pKa<2 HSO4- SO42- + H+ NaHSO4 is a strong acid Sodium hydrogen sulfate Sodium ion Hydrogensulfate ion Sulfate ion Complete ionization NaHSO4 Na+ + H+ + SO42- Strong acid (~HCl) Chem Eq 5

  12. NaHCO3 pKa HCO3- Carbonic acid is a diprotic acid pKa1=6.4 H2CO3 H+ + HCO3- pKa2=10.3 HCO3- CO32- + H+ pKb HCO3- pKb1=14-6.4=7.6 + H2O NaHCO3 is basic Na+ + HCO3- Sodium hydrogen carbonate Hydrogencarbonate (bicarbonate) ion 2nd Ionization: 2=10.3 CO32- + H+ If pKa>pKb pH is BASIC Hydrolysis: 1=7.6 H2CO3 + OH- Chem Eq 5

  13. Phosphoric acid is a triprotic acid pKa1=2.1 H3PO4 H+ + H2PO4- pKa2=7.2 H2PO4- HPO42 -+ H+ pKa2=12.3 HPO42 - PO43 -+ H+ NaH2PO4 pKa H2PO4- HPO42 - + H+ pKb H2PO4- + H2O NaH2PO4 is acidic Na+ + H2PO4- Sodium dihydrogen phosphate Dihydrogenphosphate ion 2nd Ionization: 2=7.2 If pKa<pKb pH is pKb1=14-2.1 = 11.9 Hydrolysis: ACIDIC 1=11.9 H3PO4+ OH- Chem Eq 5

  14. Phosphoric acid is a triprotic acid pKa1=2.1 H3PO4 H+ + H2PO4- pKa2=7.2 H2PO4- HPO42 -+ H+ pKa2=12.3 HPO42 - PO43 -+ H+ Na2HPO4 pKa HPO42- pKb HPO42- + H2O H2PO4 - + OH- • Na2HPO4 is basic 2Na+ + HPO42- Sodium dihydrogen phosphate Hydrogenphosphate ion 3rd Ionization: 3=12.3 PO43 - + H+ If pKa> pKb pH is Hydrolysis: pKb2=14-7.2=6.8 BASIC 2=6.8 Chem Eq 5

  15. Solubility of Salts Chem Eq 5

  16. AgCl(solid) Ag+(aq) +Cl -(aq) [Ag+ ](aq) [Cl-] (aq) K = AgCl(solid) Saturated solution The solubility product: Ksp= Ag+Cl- precipitate Solubility of Salts Saturated solutions: A salt solution is in equilibrium with its solid form: Heterogeneous equilibrium as the solid AgCl concentration has no changes: Chem Eq 5

  17. Conditions for precipitation Actual ion concentrations: The ion product: Q = Ag+*Cl- Q < Ksp  no precipitation Q = Ksp  equilibrium Q > Ksp  precipitation Chem Eq 5

  18. Solubility rules Solubility: (molar concentration of saturated solution) Water soluble salts: Alkali metal cations ( I A: Li+, Na+, K+) All nitrates (NO3-), all acetates Most of the chlorides (Cl-) except AgCl, Hg2Cl2 and PbCl2 Most of the sulfates (SO42-) except CaSO4, BaSO4, PbSO4, Hg2SO4 Mainly water insoluble: S2- All sulfides, except I A and II A elements and (NH4)2S CO32- All carbonates except IA and NH4+ salts PO43- All phosphates except IA and NH4+ salts OH- All hydroxides except IA Chem Eq 5

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