Lesson # 5 The Solubility Product Chemical Equilibrium Lesson # 5 The Solubility Product

Ksp The solubility product, Ksp, is another equilibrium constant term for dissolution. When a substance is placed in water, if it is soluble in the solvent the solid will dissolve into its component ions, and the system obtains an equilibrium. This is a heterogeneous equilibrium, and so the concentration of the initial solid before dissolution will not be in the equilibrium expression; only the concentration of the dissolved ions.

Ksp & Solubility Ksp values help us determine solubility. If the number is very small, this means that very little products are formed (quite insoluble). The greater the value of Ksp, the more soluble the solid.

Example 1 Write the equilibrium expression for the following reaction: H2O BaSO4 (s) = Ba2+ (aq) + SO42- (aq)

Example 2 A chemist has a closed vessel containing a chunk of solid zinc hydroxide, surrounded by a saturated solution at 25°C. He determines the concentration of aqueous zinc ions and aqueous hydroxide ions to be 2.7x10-6 mol/L and 5.4x10-6 mol/L, respectively. Calculate the solubility product constant, and write a statement about the solubility of the solid.

Example 3 A technician places a quantity of solid copper (I) bromide ions into a beaker of distilled water. The measured molar solubility of copper (I) bromide is 2.0x10-4 mol/L at 25°C. Calculate the Ksp of copper (I) bromide.

Example 4 The solubility product constant of solid copper (II) iodate is 6.9x10-8 at 25°C. Calculate the molar solubility of copper (II) iodate.

Predicting Precipitation When compounds containing ions are mixed, sometimes a product with low solubility is forms, which then precipitates from the solution. This is commonly seen in double displacement reactions, and this was predicted using your solubility rules in grade 11. We can determine the reaction quotient, Q, for a reaction and compare it to the Ksp value.

Predicting Precipitation If Q is greater than Ksp, the equilibrium lies to the right (saturated) so will shift to the left and precipitation occurs. If Q is less than Ksp, the equilibrium lies to the left (unsaturated) so will shift to the right and no precipitation occurs. If Q is equal to Ksp, the solution is at equilibrium and no precipitation occurs.

Example 5 A student is planning to combine a 1.0x10-3 mol/L aqueous solution of silver nitrate with a 5.0x10-3 mol/L aqueous solution of potassium bromide at 25°C. Will a precipitate form when the solutions are mixed? If so, identify the precipitate.