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The effect of Complex Ion Formation on Solubility

    Complex ions are formed when an anion (Lewis base) is reacted with a metal cation (Lewis acid). Of all the elements, the transition metal ions tend to create complex ions. This tendency is due to the fact that transition metals have at least one oxidation state. Lewis acids will react with molecules that are acting as Lewis bases, also known as electron donors.

    1. 1. The Common Ion Effect and Le Châtelier's Principle
    2. 2. Ksp Limitations
    3. 3. The Ion Product
    4. 4. Definitions
    5. 5. Outside links
    6. 6. References
    7. 7. Problems
    8. 8. Contributors

    The Common Ion Effect and Le Châtelier's Principle

    As the amount of a ion product increases, the equilibrium shifts to form more of the original molecule in the reactants. For example: In this video, a few drops of HCl is added to saturated NaCl solution. The result is that sodium chloride crystals appear!

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    (Video courtesy of bsummer86).

    Ksp Limitations

    Generally, the concept of Ksp is limited to soluble solutes which are only insignificantly soluble. In other words, when a soluble solute is considerably soluble, the idea of ion activities is utilized. The Diverse Ion Effect, also known as the Salt Effect, states that even when it might appear that an ion is not participating in the overall equilibria, that ion's interaction with other ions is still significant.

    Incomplete Dissociation: Remember, we cannot assume all the ions in solution dissociated! Instead, what are known as Ion Pairs are formed. Molecules of solute are now in the solution, and these molecules are more prevalent as the charges on the ions increase.

    The Ion Product

    This is known as Q.

    • When Q is less than the Ksp, precipitation cannot occur.
    • When Q is greater than the Ksp, precipitation should occur.
    • When Q is equal to the Ksp, the solution is saturated.

    Definitions

    • complex ion- polyatomic cation or anion composed of a central metal ion to which other groups (molecules or ions) called ligands are bonded.
    • coordination compounds- substances containing complex ions.
    • formation constant, Kf- the equilibrium constant describing the formation of a complex ion from a central ion and its attached groups.

     

    Complex Ion

    Equilibrium Reaction

    Kf

    [Co(NH3)6]3+

    Co3+ + 6NH3 ↔ [Co(NH3)6]3+

    4.5 x 10^33

    [Cu(NH3)4]2+

    Cu2+ + 4NH3 ↔ [Cu(NH3)4]2+

    1.1 x 10^13

    [Fe(CN)6]4-

    Fe2+ + 6CN- ↔ [Fe(CN)6]4-

    1 x 10^37

     

    [Fe(CN)6]3-

    Fe3+ + 6CN- ↔ [Fe(CN)6]3-

    1 x 10^42

    [PbCl3]-

    Pb2+ + 3Cl- ↔ [PbCl3]-

    2.4 x 10^1

    [Ag(NH3)2]+

    Ag+ + 2NH3 ↔ [Ag(NH3)2]+

    1.6 x 10^7

    [Ag(CN)2]-

    Ag+ + 2CN- ↔ [Ag(CN)2]-

    5.6 x 10^18

    [Ag(S2O3)2]3-

    Ag+ + 2S2O3 2- ↔ [Ag(S2O3)2]2+

    1.7 x 10^13

    [Zn(NH3)4]2+

    Zn2+ + 4NH3 ↔ [Zn(NH3)4]2+

    4.1 x 10^8

    [Zn(CN)4]2-

    Zn2+ + 4CN- ↔ [Zn(CN)4]2-

    1 x 10^18

    [Zn(OH)4]2-

    Zn2+ + 4OH- ↔ [Zn(OH)4]2-

    4.6 x 10^17

     

    How to predict reactions involving complex ions

    How to discover if a precipitate will be present in a complex ion solution

    References

    1. Petrucci, Ralph H., William S. Harwood,F. Geoffrey Herring, Jeffry D. Madura. General Chemistry: Principles and Modern Applications-9th Edition. New Jersey: Pearson Education, 2007. Chapter 18: Solubility and Complex-Ion Equilibria (pp.761-770)
    2. Houk, Clifford C., Richard Post. Chemistry Concepts and Problems: A Self Teaching Guide- Second Edition. New York: John Wiley & Sons, Inc, 1996. Chapter 12: Chemical Equilibrium (pp.250)

    Problems

    AgCl(s) ↔ Ag+(aq) + Cl-(aq)

    An extra Cl- ion is added to the above equilibrium.  Would adding more Cl-  ion to the equilibrium cause an increas or a decrease in 1) the formation of AgCl(s), 2) the concentration of Ag+?

    ANSWER: 1) increase 2) decrease

    __________________________________________________________________________________

    The solubility of AgCl is 1.2x10-5 mol/L.

    AgCl(s) <--> Ag+(aq) + Cl-(aq)

    Ksp= [Ag+][Cl-]= (1.2x10-5 mol/L) x (1.2x10-5 mol/L)

    If the Cl- concentration increased, what must happen to the Ag+ concentration?HINT: Remember, the Ksp is a constant and does not change!

    ANSWER: The Ag+ concentration must decrease.

    Contributors

    • Belinda Breen (UCD)

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