The Solubility Product Constant

1. The Solubility Product Constant

2. Dissolution and Precipitation • Remember: ionic substances separate into their ions in solution and become uniformly distributed in the sol’n • Dissolution- the process in which an ionic solid dissolves in a polar liquid • Can write an equation for dissociation • Only dissociated substances are written as ions in equations • Must balance numerically and electrically

3. Precipitation • Precipitation- the process in which ions leave a sol’n and regenerate an ionic solid • Precipitate- insoluble solid formed • Dissolution and precipitation are opposite process • Solubility equilibrium- rate of dissolution= rate of precipitation

4. What is a solubility product constant, and what is it used for? • An equilibrium constant for slightly soluble ionic substances • symbolized Ksp • Used to determine solubility of sparingly soluble compounds • Cannot be applied successfully to salts that are more soluble

5. How is a solubility constant written? • The equation for a slightly soluble ionic substance in a saturated sol’n can be written in the following general form: • AaBb (s)  aA+(aq) + bB-(aq) • The solubility product constant is • Ksp = [A+]a[B-]b

6. Association Equations and Solubility Product Constants • Write the dissociation equation and solubility product constant for each of the following substances. • Strontium arsenite • Calcium oxalate • Barium sulfide • Magnesium hydroxide

7. Solubility Product • At 25°C, the concentration of Pb+2 ions in a saturated sol’n of PbF2 is 1.9 x 10-3M. What is the value of Ksp for PbF2?

8. PbF2 (s)  Pb+2 (aq) + 2F- (aq) • For every molecule of PbF2, there will be one Pb+2 ion and two F- ions. If [PbF2] = x, then [Pb+2] = x and [F-] = 2x • Ksp = [Pb+2] [F-]2 • Ksp = x (2x)2 • Ksp = 4x3

9. Solubility Product • A sample of Cd(OH)2 (s) is added to distilled water and allowed to come to equilibrium at 25°C. The concentration of Cd+2 is 1.7 x 10-5M at equilibrium. What is the value of Ksp for Cd(OH)2?

10. Solubility • What will be the equilibrium concentrations of lithium and phosphate ions in a saturated solution of lithium phosphate? (Ksp = 3.2 x 10-9)

11. Solubility • What will be the equilibrium concentrations of strontium and phosphate ions in a saturated solution of strontium phosphate? (Ksp = 1.0 x 10-31)

12. Precipitates • Supersaturated solutions are unstable • Non equilibrium state achieved by manipulating conditions • Precipitates will form in a supersaturated solution • To determine supersaturated solution calculate Q, the ion product • Ksp < Q = Supersaturated • Ksp > Q = Unsaturated • Ksp = Q = Saturated

13. Precipitation Reactions • Reaction in which 2 solutions are mixed and a precipitate is formed • Described by a chemical equation • Remember ionic substances dissociate in solution • The precipitate that forma is a combination of ions present • The precipitate formed can be identified by using solubility rules but can only be truly confirmed experimentally

14. What is the common ion effect? • Common ion: an ion that comes from two or more substances making up a chemical reaction • example: BaSO4 and Na2SO4; common ion is SO4-2 • Common ion effect: a process in which an ionic compound becomes less soluble upon the addition of one of its ions by adding another compound

15. Why does the common ion effect work? • The common ion effect is an example of Le Chatelier’s principle • When a product is added to a system in equilibrium, it will cause the equilibrium to shift to the left, making more insoluble reactant

16. a) Saturated silver sulfate solution, Ag2SO4 (aq), is colorless. A schematic of the solution is shown above, omitting the water for simplicity.(b) Following the addition of Na2SO4 (aq), most of the Ag+ ions originally present (about 7 of 8 shown) have precipitated. The schematic shows the only remaining silver ion as a silver + ball.