Solubility and Complex Ion Equilibria Chapter 16 Solubility and Complex Ion Equilibria

Solubility - It’s Kind of A Big Deal

When salt is placed in water, the number of ions in solution gradually increase. As the ions’ concentrations increase, they become more likely to collide with one another and reform the solid Eventually, a dynamic equilibrium is reached (a double arrow is used in the reaction) where no more solid will dissolve The equilibrium expression for solubility product ([products]/[reactants]) can be written for the balanced reaction (Ksp)

CaF2(s) <-> Ca2+(aq) + 2F-(aq) Ksp = [Ca2+][F-]2 Ksp = solubility product constant or solubility product Pure liquids and solids are not included in the equilibrium expression (normally reactant) **There is a difference between solubility (equilibrium position) and solubility product (equilibrium constant with ONE value for given solid/temp.) Usually the units for Ksp are omitted

Solubility Expressions Write products and equilibrium expressions for the following dissolution reactions: Ba(OH)2(s) PbCO3(s) Ca3(PO4)2(s)

Calculations Copper (I) bromide has a measured solubility of 2.0 X 10-4 mol/L at 25°C. Calculate its Ksp value. Answer: 4.0 X 10-8 Calculate the Ksp value for bismuth sulfide (Bi2S3), which has a solubility of 1.0 X 10-15 mol/L at 25°C. Answer: 1.1 X 10-73 The solubility of silver sulfide (Ag2S) at 25°C is 3.4 X 10-17M. Calculate Ksp for Ag2S. Answer: 1.6 X 10-49

The Other Way… The solubility of an ionic solid can be calculated knowing its Ksp value Example: The Ksp value for copper(II) iodate, Cu(IO3)2, is 1.4 X 10-7 at 25°C. Calculate its solubility at 25°C. Answer: 3.3 X 10-3 mol/L Calculate the solubility in mol/L and g/L of NiCO3 (Ksp=1.4 X 10-7) For Ba3(PO4)2 (Ksp = 6 X 10-39)

How Does pH Relate When a reaction is shifted right/left with a change in pH (change in [H+] and [OH-]), it affects solubility Depending on whether the pH is decreased (add [H+]/decrease [OH-]) or increased (add [OH-]/decrease [H+]), the solubility will change General rule: if the anion X- is an effective base (if HX is a weak acid), the salt will show increased solubility in acidic solution (OH-, S2-, CO32-, C2O42-, CrO42-) <= more soluble in acidic solution than in pure water

Will A PPT Form?? Ion Product (Q): (calculated just like Ksp), but initial concentrations are used rather than equilibrium concentrations (just like Q from equilibrium expressions in ch. 13) If Q > Ksp, precipitation occurs and will continue until concentrations are reduced enough so Q is not greater than Ksp If Q < Ksp, no precipitation occurs

Example A solution is prepared by adding 750.0 mL of 4.00 X 10-3 M Ce(NO3)3 to 300.0 mL of 2.00 X 10-2 M KIO3. Will Ce(IO3) 3 (Ksp = 1.9 X 10-10) precipitate from this solution?