CHM 51 Chapter 16 – Aqueous Ionic Equilibrium
16.5 – Solubility Equilibria and the Solubility Product Constant
• All ionic compounds dissolve in water to some degree.
– However, many compounds have such low solubility in water that we classify them as insoluble.
• We can apply the concepts of equilibrium to salts dissolving, and use the equilibrium constant for the process to measure relative solubilities in water.
• The equilibrium constant for the dissociation of a solid salt into its aqueous ions is called the solubility product, Ksp.
• For an ionic solid MnXm, the dissociation reaction is
• MnXm(s) nMm+(aq) + mXn−(aq).
• The solubility product would be
• Ksp = [Mm+]n[Xn−]m.
Example
Ksp =
Ksp is not the molar solubility but we can use the value of Ksp to determine the molar solubility of solute by setting up an ICE table similarto the example below
Example: Calculate the molar solubility of PbCl2 in pure water
The molar solubility of Ag2SO4 in pure water is 1.2 x 10-5M. Calculate Ksp
Ksp and Relative solubility
• Molar solubility is related to Ksp.
• But you cannot always compare solubilities of compounds by comparing their Ksp.
To compare Ksp, the compounds must have the same dissociation stoichiometry
Compound / Ksp / SolubilityMg(OH)2 / 2.06 x 10-13 / 3.72 x 10-5M
CaF2 / 1.46 x 10-10 / 3.32 x 10-4M
FeCO3 / 3.07 x 10-11 / 5.54 x 10-6M
Mg(OH)2(s) Mg2+(aq) + 2-OH(aq)
CaF(s) Ca2+(aq) + 2F-(aq)
FeCO3(s) Fe2+(aq) + 2 CO32-
The effect of a common ion on solubity
The solubility of a slightly soluble ionic compound is decreased by the presence of a common ion in the solution (Le Chatelier’s principle)
Example: What is the solubility of CaF2 in a solution that is 0.100 M NaF?
CaF(s) Ca2+(aq) + 2F-(aq)
What is the molar solubility of CaF2 in a solution containing 0.100 M NaF?
The Effect of pH on Sobulility
• For insoluble ionic hydroxides, the higher the pH, the lower the solubility of the ionic hydroxide.
– And the lower the pH, the higher the solubility
– Higher pH = increased [OH−]
Mg(OH)2(aq) Mg2+(aq) + 2 -OH(aq)
• For insoluble ionic compounds that contain anions of weak acids, the lower the pH, the higher the solubility.
M2(CO3)n(s) 2 Mn+(aq) + nCO32−(aq)
H3O+(aq) + CO32− (aq) HCO3− (aq) + H2O(l)
16.6 Precipitation
• Precipitation will occur when the concentrations of the ions exceed the solubility of the ionic compound.
• If we compare the reaction quotient, Q, for the current solution concentrations to the value of Ksp, we can determine if precipitation will occur.
– Q = Ksp, the solution is saturated, no precipitation.
– Q < Ksp, the solution is unsaturated, no precipitation.
– Q > Ksp, the solution would be above saturation, the salt above saturation will precipitate.
• Some solutions with Q > Ksp will not precipitate unless disturbed; these are called supersaturated solutions.
A solution containing lead (II) nitrate is mixed with one containing sodium bromide to form a solution that is 0.0150 M in Pb(NO3)2 and 0.00350M in NaBr. Will precipitate form in newly mixed solution?
Selective Precipitation
• A solution containing several different cations can often be separated by addition of a reagent that will form an insoluble salt with one of the ions, but not the others.
• A successful reagent can precipitate with more than one of the cations, as long as their Ksp values are significantly different.
16.7 Qualittative Chemical Analysis
• An analytical scheme that utilizes selective precipitation to identify the ions present in a solution is called a qualitative analysis scheme.
– Wet chemistry
• A sample containing several ions is subjected to the addition of several precipitating agents.
• Addition of each reagent causes one of the ions present to precipitate out.
Group 1 cations – Insoluble chlorides
• Group one cations are Ag+, Pb2+, and Hg22+.
• All these cations form compounds with Cl− that are insoluble in water.
– As long as the concentration is large enough
– PbCl2 may be borderline
• Molar solubility of PbCl2 = 1.43 × 10−2 M
• Precipitated by the addition of HCl
Group 2 cations – Acid-insoluble sulfides
• Group two cations are Cd2+, Cu2+, Bi3+, Sn4+, As3+, Pb2+, Sb3+, and Hg2+.
• All these cations form compounds with HS− and S2− that are insoluble in water at low pH.
• Precipitated by the addition of H2S in HCl.
Group 3 cations – Base-insoluble sulfides and hydroxide
• Group three cations are Fe2+, Co2+, Zn2+, Mn2+, Ni2+ precipitated as sulfides, as well as Cr3+, Fe3+, and Al3+ precipitated as hydroxides.
• All these cations form compounds with S2− that are insoluble in water at high pH.
• Precipitated by the addition of H2S in NaOH.
Group 4 – Insoluble phosphate
• Group four cations are Mg2+, Ca2+, Ba2+.
• All these cations form compounds with PO43− that are insoluble in water at high pH.
• Precipitated by the addition of (NH4)2HPO4.
Group 5 – Alkaline metal ions and NH+
• Group five cations are Na+, K+, NH4+.
• All these cations form compounds that are soluble in water—they do not precipitate.
• They are identified by the color of their flame.
Dang7