Precipitation Reactions of Hydrated Ions


When examining the hydrolysis reactions of metal cations and oxoanions, possible interactions between the cations and anions in solution were ignored. However, some cation and anion pairs form insoluble salts.

A salt is:

  • soluble if it dissolves in water to give a solution with a concentration of at least 0.1M at room temperature
  • insoluble if the concentration of an aqueous solution is less than 0.001 M at room temperature
  • slightly soluble when the solution falls between these extremes


Measuring the solubilities of different salts reveals a number of patterns which are often referred to as "solubility rules". Students are encouraged to memorize these rules which can be employed to determine if a given salt will dissolve in water or precipitate from solution.




Text book listings of these rules do not cover all the possible cations and cover only a few anions.

Even chemical handbooks such as the CRC Handbook of Chemistry and Physics, which contains an extensive tabulation of the physical constants of inorganic compounds, cover only a fraction of the known inorganic salts.
















Our goal will be to develop a general scheme for predicting the solubility of a wide variety of inorganic salts.


yMm+ (aq) + mXy-MyXm(s) + zH2O


Thermodynamically, whether precipitation occurs or not depends on the Gibbs Free Energy change, DGo.


DGo = DHo - TDS



Thermodynamics Review
  • Some reactions are spontaneous because they give off energy in the form of heat

    DH < 0


  • Some reactions are spontaneous because they lead to an increase in disorder
    • DH > 0


  • Gibbs free energy reflects the balance between these forces
  • G is the amount of energy available to do work (at constant pressure)
    • DG > 0 non spontaneous (soluble)
    DG < 0 spontaneous (precipitation occurs)


To analyze the thermodynamics of precipitation reactions, both enthalpy and entropy changes must be considered.

Rule #1. Acidic cations and basic anions give insoluble salts.
  • Weakly, moderately, strongly, very strongly acidic cations

  • weakly, moderately, strongly,very strong basic anions




This generalized rule includes two of the traditional "solubility rules":
  • all carbonates are insoluble except those of the Group 1A elements and NH4+
  • all hydroxides are insoluble except those of the Group 1A elements, Sr2+, and Ba2+
This generalized rule
  • adds the salts of a number of additional oxo anions, such as phosphate, arsenate, silicate and borate


  • predicts the behavior of cations not listed in the rules, i.e. TlOH,
    Eu(OH)2 and Tl2CO3 are all soluble


Rule #2. Nonacidic cations and nonbasic anions give moderately insoluble salts.
  • There aren't many nonacidic cations and nonbasic anions




Rule #3. Cross-combinations give soluble salts.






The rule that all nitrates are soluble falls into this category.

Rule #4.The solubility of feebly basic anions such as sulfate are difficult to predict.

  • The tendency of these salts to precipitate or not precipitate varies with the relative magnitude of the opposing entropy and enthalpy terms
  • Many feebly acidic cations and feebly basic anions give insoluble salts


The solubilities on insoluble salts of basic anions are enhanced in strongly acidic solution.
  • the basic anion tends to react more completely with the strong acid H3O+ than with the less acidic metal cation
The equilibrium generated from competition between H3O+ and a metal cation is enhanced if the oxo anion decomposes (Le Chatlier's Principle).
  • carbonates => CO2
  • sulfites => SO2


PROBLEMS
Classify each of the following salts as soluble or insoluble in water and in strongly acidic solutions.
  • Ag2SeO4
  • K3PO4
  • ZnSO4
  • Th3(PO4)4
Solution:
  1. Evaluate Z2/r and electronegativity to classify acidity of cations



  2. Calculate pKb to classify basicity of anions



  3. Apply the generalized solubility rules



  4. What happens on acidification?