I Chapter 17 Additional Aspects of Aqueous Equilibria A 17 1 The Common Ion Effect Common Ion effect Whenever a weak electrolyte and a strong electrolyte containing a common ion are together in solution the weak electrolyte ionizes less than it would if it were alone in solution B Buffered solutions Solutions that contain a weak conjugate acid base pair resist drastic changes in pH when small amounts of strong acid or strong base are added to them 17 2 Buffered Solutions Composition and Action of Buffered Solutions Calculating the pH of a Buffer Henderson Hasselbalch equation In general pH p K a log base acid Buffer Capacity and pH Range 1 2 3 Buffer capacity the amount of acid or base the buffer can neutralize before the pH begins to change to an appreciable degree pH range the pH range of any buffer is the pH of range over which the buffer acts effectively Buffers usually have a usable range within 1 pH unit of pKa 4 Addition of Strong Acids or Bases to Buffers Reactions between strong acids and weak bases proceed essentially to completion as do those between strong bases and weak acids To calculate how the pH of the buffer responds to the addition of a strong acid or a strong base o 1 Consider the acid base neutralization reaction and determine it effect on HX and X this step is a stoichiometry calculation o 2 Use the calculated values of HX and X along with Ka to calculate H This step is an equilibrium calculation and is most easily done using the Henderson Hasselbalch equation 17 3 Acid Base titrations C Equivalence point the point at which stoichiometrically equivalent quantities of acid and base have been brought together pH titration curve a graph of the pH as a function of the volume of titrant added 1 Strong Acid Strong Base Titrations When a strong base is added to a strong acid the shape of the titration curve can be divided into four general regions o 1 Initial pH The pH of the solution before the addition of any base is determined by the initial concentration of the strong acid o 2 Between initial pH and equivalence point A the strong base is added the pH increases slowly at first and the rapidly in the vicinity of the equivalence point The pH before the equivalence point is determined by the concentration of acid not yet neutralized o 3 Equivalence point at the equivalence point an equal number of moles of strong acid and strong base have reacted leaving only a solution of their salt The pH of the solution is 7 because the cation of the strong base and the anion of the strong acid are neither acids nor bases and therefore have no appreciable effect on pH o 4 After the equivalence point the pH of the solution after the equivalence point is determined by the concentration of excess strong base in the solution 2 Weak Acid Strong Base Titrations The curve for titration of a weak acid by a strong base can be divided into four regions o 1 Initial pH We use Ka to calculate this pH o 2 Between initial pH and equivalence point prior to reaching the equivalence point the acid is being neutralized and its conjugate base is being formed Calculating the pH in this region involves two steps first calculate the neutralization reaction between the weak acid and the strong base to determine the concentration of the weak acid and its conjugate base Next calculate the pH of this buffer pair o 3 Equivalence point in general the pH at the equivalence point is always above 7 in a weak acid strong base titration because the anion of the salt form is a weak base o 4 After equivalence point excess base In this region the pH is determined by the concentration of OH from the excess strong base The titration curve for a weak acid strong base titration differs from the curve for a strong acid strong base titration in three noteworthy ways o 1 The solution of the weak acid has a higher initial pH than a solution of a strong acid of the same concentration o 2 The pH changes in the rapid rise portion of the curve near the equivalence point is smaller the weak acid than for the strong acid o 3 The pH at the equivalence point is above 7 00 for the weak acid titration o The weaker the acid the more pronounced these differences become As the acid becomes weaker Ka becomes smaller the initial pH increases and pH change near the equivalence point becomes less marked The pH at the equivalence point steadily increase as Ka decrease because the strength of the conjugate base of the weak acid increases 3 Titrations of Polyprotic Acids When the neutralization steps of a polyprotic acid or polybasic base are sufficiently separated the titration has multiple equivalence points D Titrating with an Acid Base Indicator 4 17 4 Solubility Equilibria 1 The Solubility Products Constant K sp The solubility product constantK sp is equal to the product of the concentration of the ions involved in the equilibrium each raised to the power of its coefficient in the equilibrium equation 2 Solubility and K sp The magnitude of K sp is the measure of how much of the solid dissolves to form a saturated solution 17 5 Factors that Affect Solubility E Presence of common ions Solution pH Presence of complexing agents Common Ion Effect 1 2 3 4 In general the solubility of a slightly soluble salt is decrease by the presence of a second solute that furnishes a common ion Solubility and pH as the solution becomes more acidic increases as pH is lowered The solubility of a compound containing a basic anion that is the anion of a weak acid increase The solubility of a slightly soluble salt containing basic anions increase as the concentration of H Formation of Complex Ions A complex ion an assembly of a metal ion and the Lewis bases bonded to it Formation constant K f the equilibrium constant for a complex ion Amphoterism 17 6 Precipitation and Separation of Ions Amphoteric oxides and hydroxides metal oxides and hydroxides that are relatively insoluble in water dissolve in strongly acidic and strongly basic solutions F If Q K sp the precipitation occurs reducing ion concentrations until Q K sp If Q K sp equilibrium exists saturated solution If Q K sp solid dissolves increasing ion concentrations until Q K sp 1 Selective Precipitation of Ions Selective precipitation separation of ions in an aqueous solution by using a reagent that forms a precipitate with one or more but not all of the ions G 17 7 Qualitative Analysis for Metallic Elements