John A. Schreifels

1
Chapter 18

• Solubility and Complex-Ion Equilibria

2
Overview

• Solubility Equilibria
• Solubility Product Constant
• Solubility and Common Ion Effect
• Precipitation Calculations
• Effect of pH on Solubility
• Complex-Ion Equilibria
• Complex Ion Formation
• Complex Ions and Solubility
• Application of Solubility Equilibria
• Qualitative analysis of metal ions

3
Solubility Equilibria

• Solubility of a solid treated as with other
  equilibria. Solution is saturated. No more
  solid will dissolve since dynamic equilibrium.
• AgCl(s) ? Ag(aq) Cl?(aq) Ksp AgCl?
• Solid not included in the equilibrium expression.
• MyXz(s) ? yMp(aq)zX?q(aq) KspMpyX?qz
• where Ksp solubility product.
• E.g. determine the equilibrium expression for
  each PbCl2, Ag2SO4,Al(OH)3.
• Ksp can be determined if the solubility is known.
• E.g. Determine Ksp for silver chromate (Ag2CrO4)
  if its solubility in water is 0.0290 g/L at 25?C.
• Determine molar solubility.
• Determine Ksp.
• E.g. 2 Determine Ksp of CaF2 if its solubility
  is 2.20x10?4M.

4
SOLUBILITY FROM Ksp

• Can be determined by using stoichiometry to
  express all quantities in terms of one variable-
  solubility, x. i.e. for the reaction. Use
  equilibrium table to write concentration of each
  in terms of the compound dissolving
• E.g. determine the solubility of PbCl2 if its
  Ksp 1.2x10?5
• PbCl2(s) ? Pb2(aq) 2Cl?(aq)
• E.g.1 Determine solubility of AgCl if its Ksp
  1.8x10?10M2.
• E.g.2 Determine solubility of Ag2CO3 if its Ksp
  8.1x10?12M3.
• E.g.3 Determine solubility of Fe(OH)3 if its Ksp
  4x10?38M4

5
Factors that Affect Solubility

• The commonIon effect (Remember LeChateliers
  Principle)
• E.g. Determine solubility of PbCl2 (Ksp
  1.2x10?5)in 0.100M NaCl.
• Write equilibrium table in terms of x and Cl?

a commonion reduces the solubility of the
compound.

• Assume that Cl?NaCl gtgtx
• Solve for x.
• E.g. determine the solubility of CaF2 in a
  solution of CaCl2. Ksp 3.9x10?11.

6
Precipitation of Ionic Compounds

• Starting with two solutions, Qsp used to predict
  precipitation and even the extent of it.
• Precipitation reverse of dissolution
• Precipitation occurs when Qsp gt Ksp until Qsp
  Ksp
• If Qsp lt Ksp, precipitation wont occur.
• E.g. determine if precipitation occurs after
  mixing 50.00 mL 3.00x10?3 M BaCl2 and 50.00 mL
  3.00x10?3 M Na2CO3.
• Solution
• CBaCl2 1.50x10?3 M CNa2CO3 1.50x10?3 M
• Qsp 1.50x10?3 M?1.50x10?3 M 2.25x10?6
• Qsp gt1.1x10?10. Ksp? precipitation.
• E.g. 2 determine equilibrium concentration of
  each after precipitation occurs.
• Solution
• assume complete precipitation occurs
• set up equilibrium table and solve for
  equilibrium concentration of barium and carbonate
  ion concentrations.

7
Precipitation of Ionic Compounds

• Eg. 3 determine the fraction of Ba2 that has
  precipitated.
• Solution
• Use the amount remaining in solution (results of
  E.g. 2) divided by starting concentration to
  determine the fraction of barium that is left in
  solution.
• Subtract from above.
• E.g.4 determine the Br? concentration when AgCl
  starts to precipitate if the initial
  concentration of bromide and chloride are 0.100
  M. Ksp(AgBr) 5.0x10?13 Ksp(AgCl) 1.8x10?10.

8
Factors that Affect Solubility-pH

• pH of the Solution LeChateliers Principle
  again.
• E.g. determine the solubility of CaF2 at a pH of
  2.00. Ksp 3.9x10?11. Ka(HF) 6.6x10?4.
• Strategy
• Determine the ratio of F? and HF from the pH
  and Ka.
• Write an expression for solubility in terms of Ka
  and pH and
• Substitute into solubility equation to determine
  the solubility.
• Solution
• Ksp 3.9x10?11 x?F?2 (pH changes the amount
  of free Fluoride.)
• Let x solubility. Then 2x F? HF
• From equilibrium equation
• 2x F?(11/0.066) 16.15F? or
• F? 2x/16.15 0.124x
• 3.9x10?11 x?(0.0124x)2
• x 1.36x10?3 M vs. 2.13x10?4 M (normal
  solubility)

9
Separation of Ions By Selective Precipitation

• Metal ions with very different Ksp can be
  separated.
• Divalent metal ions are often separated using
  solubility variations for the metal sulfides.
• Solution is saturated with H2S at 0.100 M pH
  adjusted to keep one component soluble and the
  other insoluble.
• H2S is diprotic acid the overall reaction to get
  to sulfide is
• Combine with solubility equilibrium reaction to
  get the overall equilibrium expression and
  constant.
• E.g. determine the solubility of 0.00500 M Zn2
  in 0.100 M H2S at pH 1. Ksp 1.10x10?21.

10
Complex Ions

• Formation of Complex Ions (Coordination
  Complexation ) an ion formed from a metal ion
  with a Lewis base attached to it by a coordinate
  covalent bond.
• Ag(aq) 2NH3(aq) ? Ag(NH3)2(aq) Kf 1.7x107
• Large equilibrium constant indicates that free
  metal is completely converted to the complex.
• Eg. What is the concentration of the silver
  amine complex above in a solution that is
  originally 0.100 M Ag and 1.00 M NH3?
• E.g. determine the Ag (free silver
  concentration) in 0.100 M AgNO3 that is also 1.00
  M NaCN.

11
Factors that Affect Solubility Complexation

• Free metal ion concentration in solution is
  reduced when complexing agent added to it
• Free metal ion concentration needed in solubility
  expression.

• E.g. determine if precipitation will occur in a
  solution containing 0.010 M AgNO3 and 0.0100 M
  Nal in 1.00 M NaCN. Recall Kf 5.6x1018
• Agl(s)?Ag l? Ksp 8.5.x10?17
• Strategy
• Determine the free metal concentration in the
  solution.
• Use free metal concentration with iodide
  concentration to get Qsp
• If Qsp lt Ksp, no precipitation
• If Qsp gt Ksp, precipitation
• If Qsp Ksp, precipitation is starting.

12
Solubility with Complexing Agent

• E.g. Determine the solubility of AgI in 1.00 M
  NaCN. Recall Kf 5.6x1018
• Agl(s)?Ag l? Ksp 8.5.x10?17
• Strategy
• Combine to equilibria equations to find a single
  equation describing the equilibrium.

the presence of a complexing agent increases the
solubility

• Setup equilibrium table and solve.