The Solubility Product Constant Dissolution and Precipitation n

The Solubility Product Constant

Dissolution and Precipitation n 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

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

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

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: n Aa. Bb (s) a. A+(aq) + b. B-(aq) n The solubility product constant is n Ksp = [A+]a[B-]b n

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

Solubility Product n At 25°C, the concentration of Pb+2 ions in a saturated sol’n of Pb. F 2 is 1. 9 x 10 -3 M. What is the value of K for Pb. F ? sp 2

Pb. F 2 (s) Pb+2 (aq) + 2 F- (aq) n For every molecule of Pb. F 2, there will be one Pb+2 ion and two F- ions. If [Pb. F 2] = x, then [Pb+2] = x and [F-] = 2 x n Ksp = [Pb+2] [F-]2 n Ksp = x (2 x)2 n Ksp = 4 x 3 n

Solubility Product n 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 -5 M at equilibrium. What is the value of Ksp for Cd(OH)2?

Solubility n What will be the equilibrium concentrations of lithium and phosphate ions in a saturated solution of lithium phosphate? (Ksp = 3. 2 x 10 -9)

Solubility n What will be the equilibrium concentrations of strontium and phosphate ions in a saturated solution of strontium phosphate? (Ksp = 1. 0 x 10 -31)

Precipitates n n 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

Precipitation Reactions n 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

What is the common ion effect? n Common ion: an ion that comes from two or more substances making up a chemical reaction – example: Ba. SO 4 and Na 2 SO 4; common is SO 4 -2 n 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

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

a) Saturated silver sulfate solution, Ag 2 SO 4 (aq), is colorless. A schematic of the solution is shown above, omitting the water for simplicity. (b) Following the addition of Na 2 SO 4 (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.