CHEM 1312H 1st Edition Lecture 32 17 4 SOLUBILITY EQUILIBRIA The Solubility Product Constant Ksp a saturated solution is one in which the solution is in contact with un dissolved solute o the extent to which the dissolution reaction occurs is expressed by the magnitude of the equilibrium constant which expresses how soluble a solid is in water o Ksp writing rules are the same for Kc rules products over reactants no solids etc o ifKsp is small only a small amount of the solid dissolves in water at a certain temperature Solubility and Ksp solubility the quantity that dissolves to form a saturated solution typically g L o solubility of a substance changes considerably in response to a number of factors pH of solution concentrations of other ions in a solution especially common ions Ksp the equilibrium constant for the equilibrium between an ionic solid and its saturated solution no units o Kspis one set value for a given solute at any one temperature 17 5 FACTORS THAT AFFECT SOLUBILITY 1 Presence of a Common Ion If you have CaF2 s Ca2 aq 2F aq the presence of either Ca2 or F will shift the equilib concentrations to the left and reduce the overall solubility of CaF2 2 pH The solubility of any ionic compound that contains a basic anion the anion of a weak acid increases as the solution becomes more acidic as more H is present For instance Mg OH 2 s becomes a lot more soluble as the acidity of the solution increases 3 Formation of Complex Ions These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute For example AgCl s solubility is increased in the presence of NH 3 because NH3 is a Lewis base which attracts the Ag ions that form after AgCl dissolves The presence of NH 3 drives the dissolution of AgCl to the right In order for a Lewis base to increase the solubility of a metal salt the base must be able to interact more strongly with the metal ion than water can Something like Ag NH3 2 is a complex ion which are very soluble in water The equilibrium constant for the formation of a complex ion is called K f and its magnitude tells us the stability of the complex ion in aqueous solution Other common Lewis bases that increase metal salt solubility are CN and OH Amphoterism o amphoteric oxides and hydroxides are soluble in strong acids and bases because they re capable of behaving as either an acid or base themselves o examples Al3 Cr3 Zn2 Sn2 o metal hydroxides such as Ca OH 2 and Fe OH 3 can dissolve in acidic solution but can t react with excess base and therefore aren t amphoteric 17 6 PRECIPITATION AND SEPARATION OF IONS How can we predict whether a precipitate will form under various conditions o Q is the reaction quotient that can determine the direction in which a rxn will proceed to reach equilibrium if Q Ksp the system is at equilibrium the solution is saturated if Q Ksp the reaction will proceed to the right towards the soluble ions no preciipate will form if Q Ksp the reaction will shift left and a precipitate will form Selective Precipitation of Ions when ions are separated from each other based on the solubilites of their salts o for ex if a solution has both Ag and Cu2 adding HCl to the solution will cause AgCl to precipitate will Cu2 will remain in the solution because CuCl2 is still soluble 17 7 QUALITATIVE ANALYSIS FOR METALLIC ELEMENTS Qualitative analysis determines the presence of absence of a particular metal ion relative to some threshold Quantitative analysis determines how much of a given substance is present both proceed in three stages o the ions are separated into broad groups based on solubility properties o the ions in each group are separated by selectively dissolving members in the group o the ions are identified by the conductance of specific tests