The Effect of a Common Ion on Solubiity溶解对常见离子的影响.docVIP

Section 16.6. Slightly Soluble Ionic Salts Ionic salts are collections of cations (M+) and anions (X-). When an ionic salt dissolves in water, it does so by the ions separating as they become surrounded by H2O molecules. A very soluble ionic salt (e.g., NaCl) dissolves in water completely, giving Na+(aq) and Cl(aq), and there is no solid left. However, some salts are only slightly soluble, and an equilibrium exists between dissolved and undissolved (i.e. solid) compound. Consider the addition of PbSO4(s) (lead sulfate) to water. PbSO4 (s) ? Pb2+ (aq) + SO42 (aq) At equilibrium, the rate at which more PbSO4(s) dissolves (i.e. the forward reaction) is equal to the rate at which Pb2+(aq) and SO42(aq) ions come together to give PbSO4(s) (the reverse reaction). We say the solution is saturated i.e. the concs are as big as they can be. If the reaction has not yet reached equilibrium, Qc is Qc = The conc of a solid (= its density) is a constant ( combine it with Qc. Qc [PbSO4] = Qsp = [Pb2+][SO42] Qsp = “ion-product expression” or “solubility product expression” At equilibrium, Qsp = Ksp ( Ksp = [Pb2+][SO42] ** Ksp = “solubility product constant” or just solubility product ** Ksp, like all other equilibrium constants, only changes with temperature. In general: MpXq (s) ? p Mn+ (aq) + q Xz (aq) Ksp = [Mn+]P[Xz]q We usually only consider systems at equilibrium ( use Ksp (not Qsp). Occasionally, we will consider Qsp (see later) Examples: Cu(OH)2 (s) ? Cu2+ (aq) + 2 OH (aq) Ksp = [Cu2+][OH]2 CaCO3 (s) ? Ca2+ (aq) + CO32 (aq) Ksp = [Ca2+][CO32] Ca3(PO4)2 (s) ? 3 Ca2+ (aq) + 2 PO43 (aq) Ksp = [Ca2+]3[PO43]2 ** the greater is Ksp, the more soluble is the substance ** e.g., PbSO4 Ksp = 1.6 x 108 insoluble CoCO3 Ksp = 1.0 x 1010 more insoluble (or less soluble) Fe(OH)2 Ksp = 4.1 x 1015 most insoluble (or least soluble) Calculations Involving Solubility Products Two types: Use Ksp to find conc of dissolved ions Use concs to fin