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Solubility and Complex-Ion Equilibria in Chemistry

Learn the principles of solubility and complex ion equilibria in chemistry. Understand how to predict precipitation, calculate ion concentrations, and manipulate pH effects. Explore Ksp concept, incomplete dissociation, fractional precipitation, and pH's impact on solubility. Enhance your knowledge with examples and practical applications.

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Solubility and Complex-Ion Equilibria in Chemistry

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  1. Petrucci • Harwood • Herring • Madura GENERAL Ninth Edition CHEMISTRY Principles and Modern Applications Chapter 18: Solubility and Complex-Ion Equilibria Juana Mendenhall, Ph.D. Assistant Professor Morehouse College Lecture 2 Chapter 18 General Chemistry: Chapter 18

  2. Objectives • Determine if a salt will precipitate from solution based on concentration of its ions. • Determine the concentration of ions remaining in solution after precipitation and predict whether precipitation will be complete • Calculate the separation of ions using fractional precipitation • Calculate the effect of pH on the precipitation and dissolving of certain substances General Chemistry: Chapter 18

  3. Sequence of steps for calculating: Calculating Ksp from solubility data Calculating solubility from Ksp data General Chemistry: Chapter 18

  4. Definition of Terms • Molar solubility (@ 25ºC): the number of moles of solute in one liter of a saturated solution (mol/L) • Solubility (@ 25ºC): the number of grams of solute in one liter of a saturated solution (g/L) General Chemistry: Chapter 18

  5. The Ksp Concept • Ksp is usually limited to slightly soluble solutes. • For more soluble solutes we must use ion activities • Activities (effective concentrations) become smaller than the measured concentrations. • The Salt Effect (or diverse ion effect). • Ionic interactions are important even when an ion is not apparently participating in the equilibrium. • Uncommon ions tend to increase solubility. • Review Solubility Rules • Ksp indicates how soluble an ionic compound is in H20; the larger the Ksp the more soluble the compound General Chemistry: Chapter 18

  6. Incomplete Dissociation • Assumption that all ions in solution are completely dissociated is not valid. • Ion Pair formation occurs. • Some solute “molecules” are present in solution. • Increasingly likely as charges on ions increase. Ksp (CaSO4) = 2.310-4 by considering solubility in g/100 mL Table 19: Ksp = 9.110-6 Activities take into account ion pair formation and must be used. General Chemistry: Chapter 18

  7. 18-5 Criteria for Precipitation and Its Completeness AgI(s) Ag+(aq) + I-(aq) Ksp = [Ag+][Cl-] = 8.510-17 Mix AgNO3(aq) and KI(aq) to obtain a solution that is 0.010 M in Ag+ and 0.015 M in I-. Saturated, supersaturated or unsaturated? Q = [Ag+][Cl-] = (0.010)(0.015) = 1.10-4 > Ksp General Chemistry: Chapter 18

  8. The Ion Product Q is generally called the ion product. Q > Ksp Precipitation should occur. Q = Ksp The solution is just saturated. Q < Ksp Precipitation cannot occur. General Chemistry: Chapter 18

  9. Example • Calculate the solubility of copper hydroxide, Cu(OH)2 in g/L. Ksp = 2.2 x 10-20. • Step 1: ICE chart General Chemistry: Chapter 18

  10. 18-6 Fractional Precipitation • A technique in which two or more ions in solution are separated by the proper use of one reagent that can cause precipitation of both ions. • Significant differences in solubility's are necessary. • Key to the technique isslow addition of the reagent. General Chemistry: Chapter 18

  11. Example of fractional ppt • Silver nitrate is slowly added to a solution that is 0.020 M in Cl- ions and 0.020 M in Br- ions. Calculate the concentration of Ag+ ions (in mol/L) required to initiate a) ppt of AgBr (Ksp 7.7 x 10-13) and b) ppt of AgCl (Ksp = 1.6 x 10-10) General Chemistry: Chapter 18

  12. 18-7 Solubility and pH • pH can affect the solubility of a salt. • Especially when the anion of the salt is the conjugate base of a weak acid. • Mg(OH)2 Milk of Magnesia. Adding OH- ions (increasing pH shifts) equilibrium from right to left decreases the solubility of Mg(OH)2 Adding H+ ions (decreasing the pH) shifts equilibrium from left to right increases the solubility of Mg(OH)2 General Chemistry: Chapter 18

  13. Example of solubility and pH • At 25 ºC the molar solubility of Mg(OH)2 in pure H20 is 1.4 x 10-4M. Calculate its molar solubility in a buffer medium whose pH is a) 12.00 and b) 9.00 General Chemistry: Chapter 18

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