Solubility Equilibria

Solubility EquilibriaReview: Solubility RulesSalts are generally more soluble in HOT waterAlkali Metal salts are very soluble in water. Ammonium salts are very soluble in water.Salts containing the nitrate ion, NO3-, are very soluble in water.Most salts of Cl-, Br- and I- are very soluble in water - exceptions are salts containing Ag+ and Pb2+.Dissolving a salt...

At first, the only process occurring is the dissolving of the salt - the dissociation of the salt into its ions.

However, soon the ions floating in the water begin to collide with the salt crystal and are pulled back in to the salt. (precipitation)

Eventually the rate of dissociation is equal to the rate of precipitation.

The solution is now saturated. It has reached equilibrium.

There is no change in amount of solid precipitate at the bottom of the beaker.Concentration of the solution is constant.The rate at which the salt is dissolving into solution equals the rate of precipitation.

Saturated Solution5Example: BaSO4, in waterWhen barium sulfate dissolves, it dissociates into ions. When the solution is saturated, the following equilibrium exists:

BaSO4 (s) Ba2+(aq) + SO42-(aq)

Since this is an equilibrium, we can write an equilibrium expression for the reaction:

K = [Ba2+][SO42-]

Since K is always calculated by just multiplying concentrations, it is called a solubility product constant, Ksp

ExampleFor each salt below, write a balanced equation showing its dissociation in water.Then write the Ksp expression for the salt.

Iron (III) hydroxide, Fe(OH)3Nickel sulfide, NiSSilver chromate, Ag2CrO4Zinc carbonate, ZnCO3Calcium fluoride, CaF2Some Ksp Values

Note: These are experimentally determined, and maybe slightly different on a different Ksp table.Solubility vs KspSolubilityKspQuantity of solute that dissolves to form a saturated solutiong/L or mol/L Changes with changes of concentrationEquilibrium constant for equilibrium between an ionic solid and its saturated solutionUnitlessOne value for a given solute at any specific temperatureSolubility and KspGenerally, it is fair to say that salts with very small solubility product constants (Ksp) are only sparingly soluble in water.When comparing the solubilities of two salts, however, you can sometimes simply compare the relative sizes of their Ksp values.This works only if the salts have the same formula type!

ExampleCuI has Ksp = 5.0 x 10-12 and CaSO4 has Ksp = 6.1 x 10-5. Which is more soluble in water?

Calcium sulfate is more soluble.

But be careful...

ExampleA saturated solution of silver chromate, Ag2CrO4, has [Ag+] = 1.3 x 10-4 M. What is the Ksp for Ag2CrO4?ExampleThe Ksp of NiCO3 is 1.4 x 10-7 at 25C. Calculate its molar solubility. What mass of NiCO3 is needed to prepare 500 mL of saturated solution?

Calculate the pH of a saturated solution of silver hydroxide, AgOH. Ksp = 2.0 x 10-8

ExampleThe Common Ion Effect on SolubilityThe solubility of MgF2 in pure water is 2.6 x 10-4 mol/L. What happens to the solubility if we dissolve the MgF2 in a solution of NaF, instead of pure water?

The Common Ion Effect on SolubilityThe presence of a common ion in a solution will lower the solubility of a salt.

LeChateliers Principle:

The addition of the common ion will shift the solubility equilibrium backwards. This means that there is more solid salt in the solution and therefore the solubility is lower!

ExampleCalculate the solubility of MgF2 (Ksp = 7.4 x 10-11) in a solution of 0.080 M NaF.Criteria for Precipitation or DissolutionSolubility equilibria can be achieved starting from either sideWhen precipitating from the ions, one can calculate for Q (ion product) at any moment in precipitation processIf Q > Ksp, precipitation occurs until Q=KspIf Q = Ksp, equilibrium (saturated solution)If Q < Ksp, solid dissolves until Q = KspExampleWill a precipitate form if 10.0 mL of 0.010 M AgNO3 and 10.0 mL of 0.00010 M NaCl are mixed? Assume final volume of solution is 20.0 mL.Ksp AgCl = 1.7 x 10-10ExampleAssume that 0.10 M NaF solution is added slowly, with stirring, to a solution that contains 0.10 M Cr(NO3)3 and 0.10 M Ca(NO3)2. Which salt precipitates first?Ksp, CaF2 = 5.3 x 10-9Ksp, CrF3 = 6.6 x 10-11ExampleThe solubility product for gadolinium hydroxide, Gd(OH)3 is 1.8 x 10-23. If a solution is 0.010 M in Gd3+ ion and the pH of the solution is slowly increased, at what pH will Gd(OH)3 begin to precipitate?Solubility and pHSolubility of any substance whose anion is basic will be affected by pHExample: Mg(OH)2(s) Mg2+(aq) + 2OH-(aq)In acidic buffers, S increasesRule: The solubility of slightly soluble salts containing basic anions, increases as [H+] increases.THE ENDSaltKspSolubility (mol/L)

CuS8.5 x 10-459.2 x 10-23

Ag2S1.6 x 10-493.4 x 10-17

Bi2S31.1 x 10-731.0 x 10-15