chem ppt

Chapter 19 2 Solubility Product Constants Silver chloride, AgCl,is rather insoluble in water. Careful experiments show that if solid AgCl is placed in pure water and vigorously stirred, a small amount of the AgCl dissolves in the water. 3 Solubility Product Constants The equilibrium constant expression for this dissolution is called a solubility product constant. Ksp = solubility product constant 4 Solubility Product Constants The solubility product constant, Ksp, for a compound is the product of the concentrations of the constituent ions, each raised to the power that corresponds to the number of ions in one formula unit of the compound. Consider the dissolution of silver sulfide in water. 5 Solubility Product Constants The solubility product expression for Ag2S is: 6 Solubility Product Constants The dissolution of solid calcium phosphate in water is represented as: The solubility product constant expression is: 7 Solubility Product Constants In general, the dissolution of a slightly soluble compound and its solubility product expression are represented as: 8 Solubility Product Constants The same rules apply for compounds that have more than two kinds of ions. One example of a compound that has more than two kinds of ions is calcium ammonium phosphate. 9 Determination of Solubility Product Constants Example: One liter of saturated silver chloride solution contains 0.00192 g of dissolved AgCl at 25oC. Calculate the molar solubility of, and Ksp for, AgCl. 10 Determination of Solubility Product Constants Example: One liter of saturated silver chloride solution contains 0.00192 g of dissolved AgCl at 25oC. Calculate the molar solubility of, and Ksp for, AgCl. The molar solubility can be easily calculated from the data: 11 Determination of Solubility Product Constants The equation for the dissociation of silver chloride, the appropriate molar concentrations, and the solubility product expression are: 12 Determination of Solubility Product Constants Substitution of the molar concentrations into the solubility product expression gives: 13 Uses of Solubility Product Constants The solubility product constant can be used to calculate the solubility of a compound at 25oC. 14 Uses of Solubility Product Constants Example: Calculate the molar solubility of barium sulfate, BaSO4, in pure water and the concentration of barium and sulfate ions in saturated barium sulfate at 25oC. For barium sulfate, Ksp= 1.1 x 10-10. 15 Uses of Solubility Product Constants 16 Uses of Solubility Product Constants Make the algebraic substitution of x’s into solubility product expression and solve for x, giving the ion concentrations. 17 Uses of Solubility Product Constants Finally, to calculate the mass of BaSO4 in 1.00 L of saturated solution, use the definition of molarity. 18 Uses of Solubility Product Constants Example: The solubility product constant for magnesium hydroxide, Mg(OH)2, is 1.5 x 10-11. Calculate the molar solubility of magnesium hydroxide and the pH of a saturated magnesium hydroxide solution at 25oC. 19 Uses of Solubility Product Constants Be careful, do not forget the stoichiometric coefficient of 2! 20 Uses of Solubility Product Constants Substitute the algebraic expressions into the solubility product expression. 21 Uses of Solubility Product Constants Solve for the pOH and pH. 22 The Reaction Quotient in Precipitation Reactions The reaction quotient, Q, and the Ksp of a compound are used to calculate the concentration of ions in a solution and whether or not a precipitate will form. Will a precipitate form? {C4B1156A-380E-4F78-BDF5-A606A8083BF9}Qsp < Ksp Forward process is favored No precipitation occurs; if solid is present, more solid can dissolve Qsp = Ksp Solution is just saturated Solid and solution are in equilibrium; neither forward bore reverse process is favored Qsp > Ksp Reverse process is favored Precipitation occurs to form more solid 24 The Reaction Quotient in Precipitation Reactions Example: We mix 100 mL of 0.010 M potassium sulfate, K2SO4, and 100 mL of 0.10 M lead (II) nitrate, Pb(NO3)2 solutions (not at equilibrium). Will a PbSO4 precipitate form? Ksp= 1.8x10-8 25 The Reaction Quotient in Precipitation Reactions Write out the solubility expressions. 26 The Reaction Quotient in Precipitation Reactions Calculate the Qsp for PbSO4. Assume that the solution volumes are additive. Concentrations of the important ions are: 27 The Reaction Quotient in Precipitation Reactions Finally, calculate Qsp for PbSO4 and compare it to the Ksp. 28 The Reaction Quotient in Precipitation Reactions Example: Suppose we wish to remove mercury from an aqueous solution that contains a soluble mercury compound such as Hg(NO3)2. We can do this by precipitating mercury (II) ions as the insoluble compound HgS. What concentration of sulfide ions, from a soluble compound such as Na2S, is required to reduce the Hg2+ concentration to 1.0 x 10-8 M? For HgS, Ksp=3.0 x 10-53. 29 The Reaction Quotient in Precipitation Reactions Example 20-7: What concentration of sulfide ions, from a soluble compound such as Na2S, is required to reduce the Hg2+ concentration to 1.0 x 10-8 M? For HgS, Ksp=3.0 x 10-53. 30 Fractional Precipitation The method of precipitating some ions from a solution while leaving others in solution is called fractional precipitation. If a solution contains Cu+, Ag+, and Au+, each ion can be precipitated as chlorides. 31 Fractional Precipitation Example: If solid sodium chloride is slowly added to a solution that is 0.010 M each in Cu+, Ag+, and Au+ ions, which compound precipitates first? Calculate the concentration of Cl- required to initiate precipitation of each of these metal chlorides. 32 Fractional Precipitation These three calculations give the [Cl-] required to precipitate AuCl ([Cl-] >2.0 x 10-11 M), to precipitate AgCl ([Cl-] >1.8 x 10-8 M), and to precipitate CuCl ([Cl-] >1.9 x 10-5 M). It is also possible to calculate the amount of Au+ precipitated before the Ag+ begins to precipitate, as well as the amounts of Au+ and Ag+ precipitated before the Cu+ begins to precipitate.