1. Chapter 18 Solubility

2. Equilibria of Slightly Soluble Ionic Compounds Explore the aqueous equilibria of slightly soluble ionic compounds. Chapter 5. Precipitation Reactions: AgNO 3(aq) + NaCl (aq)  ?? Want to consider quantitative predictions

3. The Formation & Dissolution of Precipitates Solubility: maximum amount of solute that will dissolve in a given amount of solvent (depends on solvent, temperature and pressure) No compound is infinitely soluble and no compound is perfectly insoluble. Solute Solubility (g solute/100 g solvent) Qualitative Solubility Description Less than 0.1Insoluble 0.1 – 1Slightly soluble 1 – 10Soluble Greater than 10Very soluble

4. The Formation & Dissolution of Precipitates Saturated solution: contains maximum concentration of solute –Equilibrium between undissolved and dissolved solute. Solutes (even those called “soluble”) have a limited solubility in a particular solvent. Slightly soluble (often called “insoluble”) ionic compounds have a relatively low solubility –Reach equilibrium with little solute dissolved –Heterogeneous equilibrium

5. Why is this important? Dissolving and Precipitation occurs around us: –Tooth enamel dissolves in acidic soln (tooth decay) –Ppt of certain salts in kidneys causes kidney stones –Waters of Earth contains dissolved salts as water passes over and through the ground –Ppt of CaCO 3 from groundwater is responsible for cave formation. Let’s look at the factors that affect solubility!

6. Solubility-Product Constant (K sp ) Solubility-product constant (K sp ): equilibrium constant for equilibrium between slightly soluble ionic solid and a solution of its ions –Indicates how soluble the solid is in water Solubility: quantity that dissolves to form a saturated solution (g/L) Molar solubility: number of moles of solute that dissolves in forming a liter of saturated solution of solute (mol/L) Solubility depends on concentrations of other ions and pH but K sp is a constant.

7. Solubility-Product Constant (K sp ) Practice: Write an ionic equation for the dissolution, and the equation for the solubility product for: (a)Calcium carbonate (b)Magnesium hydroxide (c)Ag 3 PO 4 Magnitude of K sp is measure of how far to the right dissolution proceeds at equilibrium (saturation). –Used to compare solubilities if same total number of ions

8. K sp of Selected Ionic Compounds (25 °C) Name, FormulaK sp Aluminum hydroxide, Al(OH) 3 3 x 10 -34 Cobalt(II) carbonate, CoCO 3 1.0 x 10 -10 Iron(II) hydroxide, Fe(OH) 2 4.1 x 10 -15 Lead(II) fluoride, PbF 2 3.6 x 10 -8 Lead(II) sulfate, PbSO 4 1.6 x 10 -8 Mercury(I) iodide, Hg 2 I 2 4.7 x 10 -29 Silver sulfide, Ag 2 S8 x 10 -48 Zinc iodate, Zn(IO 3 ) 2 3.9 x 10 -6 See Appendix D in your book for a much more extensive list.

9. Example Predict which of the following compounds will have the greatest molar solubility in water A)AgClK sp = 1.8 x 10 -10 B)AgBrK sp = 5.0 x 10 -13 C)AgIK sp = 8.3 x 10 -17 D)all have the same molar solubility

10. Solubilities and Solubility Products K sp for a slightly soluble solid can be determined from its solubility –as long as there is no other reaction Example 1: Lead(II) sulfate (PbSO 4 ) is a key component in lead-acid car batteries. Its solubility in water at 25 °C is 4.25 x 10 -3 g/100 mL solution. What is the K sp of PbSO 4 ? Example 2: Determine the molar solubility of MgF 2 from its solubility product (K sp = 6.4 x 10 -9 ).

11. Example Calculate the molar solubility of calcium fluoride, CaF 2 K sp = 3.7 x 10 -11

12. Factors that Affect Solubility: Common Ion Effect The presence of a common ion decreases the solubility of a slightly soluble ionic compound. The shift in equilibrium that occurs because of the addition of an ion already involved in the equilibrium reaction. AgCl (s) ⇌ Ag + (aq) + Cl  (aq)

13. The effect of a common ion on solubility PbCrO 4 ( s ) Pb 2+ ( aq ) + CrO 4 2- ( aq ) CrO 4 2- added

14. Example The solubility of Ca(OH) 2 in water is 0.012 M. What is its solubility in 0.10 M Ca(NO 3 ) 2 ? K sp of Ca(OH) 2 is 6.5 x 10 -6

15. Le Ch âtelier’s Principle Determine the effects of solubility when each of the following is added to a mixture of the slightly soluble solid NiCO 3 and water at equilibrium: (a)Ni(NO 3 ) 2 (c) K 2 CO 3 (b)KClO 4 (d) HNO 3

16. Effect of pH on solubility [H 3 O + ] can have a profound effect on the solubility of an ionic compound. Solubility of slightly soluble salts containing basic anions increases as [H + ] increases –More basic anion…more solubility is influenced by pH Predict the effect on solubility of adding a strong acid CaCO 3(s) ⇌ Ca 2+ (aq) + CO 3 2- (aq) AgCl (s) ⇌ Ag + (aq) + Cl - (aq) Ca(OH) 2(s) ⇌ Ca 2+ (aq) + 2OH - (aq)

17. If limestone (CaCO 3 ) deposit is near surface…sinkhole

18. If limestone (CaCO 3 ) deposit is well below the surface….caves A view inside Carlsbad Caverns, New Mexico

19. Copyright © Houghton Mifflin Company.All rights reserved. 1–19 Cango Caves

20. Copyright © Houghton Mifflin Company.All rights reserved. 1–20 Sudwala Caves

21. Example Calculate the molar solubility of MgF 2 in 0.10 M MgCl 2 at 25  C. K sp of MgF 2 = 7.4 x 10 -11

22. Predicting the Formation of a Precipitate Compare Q sp to K sp to predict if a precipitate will form and, if not, what concentrations of ions will cause it to do so. Q sp = K sp soln is saturated & no changes occur Q sp > K sp ppt forms until soln is saturated Q sp < K sp soln is unsaturated & no ppt forms

23. Practice Example 1: Determine whether CaHPO 4 will precipitate from a solution with [Ca 2+ ] = 0.0001 M and [HPO 4 2- ] = 0.001 M. Example 2: Does silver chloride precipitate when equal volumes of a 2 x 10 -4 M solution of AgNO 3 and a 2 x 10 -4 M solution of NaCl are mixed. AgNO 3(aq) + NaCl (aq)  AgCl (s) + NaNO 3(aq)

24. Practice Will a precipitate form when 0.10 L of 8.0 x 10 -3 M Pb(NO 3 ) 2 is added to 0.40 L of 5.0 x 10 -3 M Na 2 SO 4 ? K sp for PbSO 4 = 6.3 x 10 -7

25. Concentration Necessary to Form a Ppt We can also determine the concentration of an ion necessary for precipitation to begin. Assume that precipitation begins when Q sp = K sp Example: If a solution contains 0.0020 mol CrO 4 2- per liter, what concentration of Ag + ion must be added as AgNO 3 before Ag 2 CrO 4 begins to precipitate. (Neglect any increase in volume upon adding the solid silver nitrate.)