Solubility

1
Solubility Simultaneous EquilibriaPart II
Effect of pH, Complex Ion Formation Selective
Precipitation

• Dr. C. Yau
• Fall 2009

Based on Brady Senese 5/eChap. 17 Sec 3
1
2
Effect of pH on Solubility
Consider the following equilibrium CaCO3 (s)
Ca2 (aq) CO32- (aq)
Why would the pH affect the solubility? Could
addition of H3O affect the solubility? Would you
expect it to affect Ca2 or CO32-? CO32- (aq)
H3O (aq) HCO3- (aq) H2O (l) HCO3-
(aq) H3O (aq) H2CO3 (aq) H2O (l)
If the cmpd contains the anion of a weak acid,
addition of H3O (from a strong acid) increases
it solubility.
2
3

• Addition of acid to mineral sample is a field
  test to identify presence of carbonates.
• What would one look for as a positive test for
  carbonates?
• CO32- (aq) H3O (aq) HCO3- (aq)
  H2O (l)
• HCO3- (aq) H3O (aq) H2CO3 (aq)
  H2O (l)

If the addition of acid to a mineral sample
produces effervescence, the sample contains
carbonates.
4
Effect of Adding Strong Acid

• Write balanced eqns to explain whether addition
  of a strong acid would affect the solubility of
  the following compounds
• a) lead(II) bromide
• b) copper(II) hydroxide
• c) iron(II) sulfide
• Write equations to explain the effect.

5
Selective Precipitation

• Consider the Ksp of AgCl and Ag2CrO4
• Ksp Solubility
• AgCl 1.8x10-10 1.3x10-5 M
• Ag2CrO4 2.6x10-12 8.7x10-5 M
• If we add Ag to a soln containing
• 0.10 M Cl- and 0.10 M CrO42-, what will happen?
• Ans. Ag2CrO4 is more soluble. AgCl will
  precipitate out first, leaving CrO42- in solution.

6

• CrO42- and Cl- can be separated by adding Ag.
• Ag will precipitate out the Cl- as AgCl (s).
• The mixture is centrifuged to spin down the white
  AgCl ppt.
• The supernatant soln containing the CrO42- is
  then pipetted out.

7
Complex Ion Formation

• Metal ions can combine with anions or neutral
  molecules to form complex ions
• Cu2 4 H2O Cu(H2O)42
• Fe2 6 CN? Fe(CN)64?
• The H2O and CN? in the above reactions are called
  ligands. They are acting as Lewis bases. The
  metal cations are acting as Lewis acids.

8
Aqueous Metal Ions Are Complex Ions

• Water acts as a ligand, the Lewis base that forms
  a coordinate covalent bond with the metal.
• In the solvation of ionic compounds, ions are
  dissolved in water through ion-dipole
  interactions
• Complex ions are soluble, hence complexation is a
  means of dissolving some solids.

9
Writing Complex Ion Formation Eqns

• Write the equation for the complex ion formation
  of the aluminum ion with 4 hydroxide as ligands.
• Al3 (aq) 4 OH- (aq)
• Write the equation for the complex ion formation
  of the tin(IV) ion with 6 fluoride ions as
  ligands.
• Write the equation for the complex ion formation
  of the mercury(II) ion with 4 chloride ions as
  ligands.
• Practice p.720 17.62

10
Complexation Kinst

• Complexes are governed by the instability
  constant, Kinst
• Ag(NH3)2 Ag
  2NH3
• When we reverse an equation, we invert K, thus
  Kform1/Kinst.
• Ag 2NH3 Ag(NH3)2

11
Complex Ion Formation Increases Solubility of a
Salt

• Consider the dissolution of AgBr(s) and what
  happens when NH3 is added
• AgBr (s) Ag (aq) Br- (aq)
• Ag(NH3)2

AgBr becomes more soluble.
12
Classical Schemefor Identifying "Group I Cations"

• Ksp Solubility
• Hg2Cl2 1.2x10-18 M 6.7x10-7
• AgCl 1.8x10-10 M 1.3x10-5
• PbCl2 1.7x10-5 M 1.6x10-2
• Given a soln possibly containing some combination
  of Hg22, Ag and Pb2 ions, you are to determine
  which one or ones are present.

13
SolubilityHg2Cl2 6.7x10-7 MAgCl 1.3x10-5 M
PbCl2 1.6x10-2 M (soluble when hot)
Write a flowchart for this analysis.

• To unknown soln, add excess amt of 6MHCl.
• If a ppt forms, one or more of the cations are
  present.
• Heat the mixture to boiling and remove the
  supernatant. To the supernatant, add K2CrO4. If a
  yellow ppt forms, Pb2 is present.
• To the ppt, add 6 M NH3. If a ppt remains, Hg22
  is present.
• To the supernatant, neutralize the NH3 with HCl.
  If a white ppt forms, Ag is present.