BaSO4(s) Ba2+(aq) + SO42−(aq) Solubility Products Consider the equilibrium that exists in a saturated solution of BaSO4 in water:
Solubility Products The equilibrium constant expression for this equilibrium is Ksp = [Ba2+] [SO42−] where the equilibrium constant, Ksp, is called the solubility product.
Solubility Products • Ksp is not the same as solubility. • Solubility is generally expressed as the mass of solute dissolved in 1 L (g/L) or 100 mL (g/mL) of solution, or in mol/L (M).
BaSO4(s) Ba2+(aq) + SO42−(aq) Factors Affecting Solubility • The Common-Ion Effect • If one of the ions in a solution equilibrium is already dissolved in the solution, the equilibrium will shift to the left and the solubility of the salt will decrease.
Practice Calculate the molar solubility of CaF2 at 25oC in a solution that is a) 0.010 M Ca(NO3)2, b) 0.010 M NaF Ksp = 3.9x 10-11 (from Appendix D)
PRACTICE EXERCISE The value for Ksp for manganese(II) hydroxide, Mn(OH)2 , is 1.6 10–13. Calculate the molar solubility of Mn(OH)2 in a solution that contains 0.020 MNaOH. Answer: 4.0 10–10 M
Factors Affecting Solubility • pH • If a substance has a basic anion (CO32-), PO43-, CN-, S2-), it will be more soluble in an acidic solution. • Substances with acidic cations (like metal hydroxides) are more soluble in basic solutions.
Factors Affecting Solubility Mg(OH)2(s) Mg2+(aq) + 2OH-(aq) Ksp = 1.8 x 10-11; sat soltn; pH 10.52; [Mg2+] = 1.7 x 10-4 M Suppose solid Mg(OH)2 is equilibriated with a solution buffered at pH 9.0 The pOH is 5.0,so [OH-] = 1.0 x 10-5
Factors Affecting Solubility Mg(OH)2(s) Mg2+(aq) + 2OH-(aq) [OH-] = 1.0 x 10-5 Insert [OH-] into the solubility product Ksp = [Mg2+][OH-]2 = 1.8 x 10-11 = [Mg2+](1.0 x 10-5)2 = 1.8 x 10-11 [Mg2+] = 0.18 M Mg(OH)2 will dissolve until [Mg2+] = 0.18M
Factors Affecting Solubility • The solubility of almost any ionic compound is affected by a sufficiently acidic or basic solution • The affects are very noticeable when one or both of the ions are at least moderately acidic or basic • Metal hydroxides, like Mg(OH)2, are prime examples
Factors Affecting Solubility • Other salts with basic anions, such as CO32-, PO43-, CN-, and S2- behave similarly • The solubulity of slightly soluble salts containing basic ions increases as [H+] increases and pH is lowered. • Salts with anions negligible basicity (anions of strong acids) are unaffected by pH
SAMPLE EXERCISE Predicting the Effect of Acid on Solubility Which of the following substances will be more soluble in acidic solution than in basic solution: (a) Ni(OH)2(s), (b) CaCO3(s), (c) BaF2(s), (d) AgCl(s)?
SAMPLE EXERCISE Predicting the Effect of Acid on Solubility Solve: (a) Ni(OH)2(s) will be more soluble in acidic solution because of the basicity of OH– ; the H+ ion reacts with the OH– ion, forming water.
SAMPLE EXERCISE Predicting the Effect of Acid on Solubility (b) Similarly, CaCO3(s) dissolves in acid solutions because CO32– is a basic anion. The reaction between CO32– and H+ occurs in a stepwise fashion, first forming HCO3–. H2CO3 forms in appreciable amounts only when the concentration of H+ is sufficiently high.
SAMPLE EXERCISE continued (c) The solubility of BaF2 is also enhanced by lowering the pH, because F– is a basic anion. (d) The solubility of AgCl is unaffected by changes in pH because Cl– is the anion of a strong acid and therefore has negligible basicity.
Factors Affecting Solubility Complex Ions • Metal ions can act as Lewis acids (e- pair acceptors) and form complex ions with Lewis bases (e- pair donors) in the solvent.
Factors Affecting Solubility • Lewis bases, other than water, can also interact with metal ions, particularly transition metals. • The formation of complex ions can affect the solubility of a metal salt
Factors Affecting Solubility AgCl Ksp = 1.8 x 10-10 will dissolve in NH3, a Lewis base
Factors Affecting Solubility AgCl(s) Ag+(aq) + Cl- (aq) Ag+(aq) + 2NH3(aq) Ag(NH3)2+(aq) AgCl(s) + 2NH3(aq) Ag(NH3)2+(aq) + Cl- (aq) NH3 causes AgCl(s) to dissolve, moving the rxn to the right
Factors Affecting Solubility • For a Lewis base, like NH3, to increase solubility of a metal salt, it must interact greater with the metal ion than water does • Assembly of a metal-ion & Lewis base is called a complex ion
[Ag(NH3)2+] Kf = = 1.7 x 107 [Ag+][NH3]2 Factors Affecting Solubility • The stability of a complex ion in an aqueous solution is determined by the size of the equilibrium constant for its formation from the hydrated metal ion
Factors Affecting Solubility • Amphoterism • Amphoteric metal oxides and hydroxides are soluble in strong acid or base, because they can act either as acids or bases. • Examples of such cations are Al3+, Zn2+, and Sn2+.
Will a Precipitate Form? • In a solution, • If Q = Ksp the system is at equilibrium and the solution is saturated. • If Q < Ksp more solid will dissolve until Q = Ksp • If Q > Ksp the salt will precipitate until Q = Ksp
SAMPLE EXERCISE 17.15 Predicting whether a Precipitate will Form Will a precipitate form when 0.10 L of 8.0 x 10-3 M Pb(NO3)2 is added to 0.40 L of 5.0 x 10-3 M Na2SO4? The Ksp for PbSO4 = 6.3 x 10-7 Q = 6.4 x 10-6; it will precipitate
PRACTICE EXERCISE Will a precipitate form when 0.050 L of 2.0 x10-2 M NaF is mixed with 0.010 L of 1.0 x 10-2 M Ca(NO3)2? The Ksp of CaF2 is 3.9 x 10-11 Q = 4.6 x 10-7 it will precipitate
Selective Precipitation of Ions One can use differences in solubilities of salts to separate ions in a mixture.