1. PRECIPITATION REACTIONS
Chapter 19
PRECIPITATION REACTIONS
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2. Solubility of Ionic Solids
Depends on the balance of two forces:
Attraction between H2O molecules and ions of solid.
Force of attraction between oppositely charged ions within solid.
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3. WOLPA/AP CHEMISTRY/CDO
Solubility Rules
Use in predicting results of precipitation reactions. MEMORIZE THE SOLUBILITY RULES!!!!!
Determine ions present and possible products.
Use solubility rules to determine if any are insoluble.
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4. WOLPA/AP CHEMISTRY/CDO
Example
Ba(NO3)2(aq) + Na2CO3(aq)
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5. WOLPA/AP CHEMISTRY/CDO
Solubility Rules
1. Salts containing Group I elements are soluble (Li+, Na+, K+, Cs+, Rb+). Exceptions to this rule are rare. Salts containing the ammonium ion (NH4+) are also soluble.
2. Salts containing nitrate ion (NO3-) are generally soluble.
3. Salts containing Cl-, Br-, I- are generally soluble. Important exceptions to this rule are halide salts of Ag+, Pb2+, and (Hg2)2+. Thus, AgCl, PbBr2, and Hg2Cl2 are all insoluble.
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6. Solubility Rules Continued
4. Most silver salts are insoluble. AgNO3 and Ag(C2H3O2) are common soluble salts of silver; virtually anything else is insoluble.
5. Most sulfate salts are soluble. Important exceptions to this rule include BaSO4, PbSO4, Ag2SO4 and SrSO4.
6. Most hydroxide salts are only slightly soluble. Hydroxide salts of Group I elements are soluble. Hydroxide salts of Group II elements (Ca, Sr, and Ba) are slightly soluble. Hydroxide salts of transition metals and Al3+ are insoluble. Thus, Fe(OH)3, Al(OH)3, Co(OH)2 are not soluble.
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7. Solubility Rules Continued
7. Most sulfides of transition metals are highly insoluble. Thus, CdS, FeS, ZnS, Ag2S are all insoluble. Arsenic, antimony, bismuth, and lead sulfides are also insoluble.
8. Carbonates are frequently insoluble. Group II carbonates (Ca, Sr, and Ba) are insoluble. Some other insoluble carbonates include FeCO3 and PbCO3.
9. Chromates are frequently insoluble. Examples: PbCrO4, BaCrO4
10. Phosphates are frequently insoluble. Examples: Ca3(PO4)2, Ag2PO4
11. Fluorides are frequently insoluble. Examples: BaF2, MgF2 PbF2.
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8. WOLPA/AP CHEMISTRY/CDO
Example Continued
Ba(NO3)2(aq) + Na2CO3(aq)
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9. WOLPA/AP CHEMISTRY/CDO
Stoichiometry
Mole Relations
Coefficients in the net ionic equation can be used in the usual way to relate the moles of reactants and products.
Moles of ions can be deduced from solute concentrations.
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10. WOLPA/AP CHEMISTRY/CDO
Example
What is the molar concentration of Ba2+ and F- in a solution containing M BaF2
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11. Precipitation Titrations
Used to determine the concentration of species in solution or in a solid mixture.
Indicator shows, usually by color change, when the species being analyzed for has been consumed
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12. WOLPA/AP CHEMISTRY/CDO
General Principles
Involves formation of a precipitate
Must determine the volume of a
standardized titrant needed to just
precipitate all of the ion.
Need an indicator or electrode to
determine when the precipitation is
complete
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13. Solubility Equilibria
Solubility Product Constant, Ksp
Precipitation reactions like all reactions, reach a position of equilibrium.
Expression for Ksp
MaXb < > aM+b + bX-a
Ksp = [M+b]a [X-a]b
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14. Solubility Product Principle
In any water solution in equilibrium with a slightly soluble ionic compound, the product of the concentrations of its ions, each raised to a power equal to its coefficient in the solubility equation is a constant. This constant, Ksp, has a fixed value at a given temperature, independent of the concentrations of the individual ions.
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15. WOLPA/AP CHEMISTRY/CDO
Two Ion Compound
AgCl
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16. WOLPA/AP CHEMISTRY/CDO
Three Ion Compound
PbCl2
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17. WOLPA/AP CHEMISTRY/CDO
Four Ion Compound
Al(OH)3
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18. WOLPA/AP CHEMISTRY/CDO
Calculation of Ksp
Calculated from measured solubility
AgCl Ksp = (s) (s) = s2
PbCl2 Ksp = (s) (2s)2 = 4s3
Al(OH)3 Ksp = (s) (3s)3 = 27s4
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19. WOLPA/AP CHEMISTRY/CDO
Example
At 20 oC, a saturated aqueous solution of silver acetate, AgC2H3O2, contains 1.0 g dissolved in mL of solution. Calculate the Ksp for AgC2H3O2.
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20. Determination of Solubility
In pure water
Ksp = s2 s = (Ksp)1/2
Ksp = 4s3 s = (Ksp/4)1/3
Ksp = 27s4 s = (Ksp/27)1/4
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21. WOLPA/AP CHEMISTRY/CDO
Example
Estimate the solubility of lead (II) bromide in (a) moles per liter and (b) grams per liter of pure water. Ksp = 6.3 x 10-6
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22. WOLPA/AP CHEMISTRY/CDO
Uses of Ksp
Calculation of concentration of one ion, knowing that of the other
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23. WOLPA/AP CHEMISTRY/CDO
Example
You have a solution that has a lead (II) concentration of M. What is the maximum concentration of chloride ions that would be present? Ksp = 1.7 x 10-5
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24. WOLPA/AP CHEMISTRY/CDO
Uses of Ksp
Determination of whether a precipitate will form
Compare original concentration product, P, to Ksp
if P < Ksp, no precipitate will form
if P > Ksp, precipitate forms until P becomes equal to Ksp
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25. WOLPA/AP CHEMISTRY/CDO
Example
You have m of a solution that has a lead (II) concentration of M. Does PbCl2 precipitate when 1.20 g of solid NaCl is added?
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26. Determination of Solubility
In solution containing a common ion
Solubility is much less than in pure water
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27. WOLPA/AP CHEMISTRY/CDO
Example
Calculate the solubility of silver carbonate, Ag2CO3, in moles per liter, in pure water. Compare this with the molar solublity of Ag2CO3 in 225 mL of water to which 0.15 g of Na2CO3 has been added.
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28. Simultaneous Equilibria
Two or more reactions occur at the same time is a solution, all of them being described as equilibrium processes.
The equilibrium for the overall reaction is the product of the equilibrium constants for the summed reactions.
That is Knet = K1 x K2
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29. WOLPA/AP CHEMISTRY/CDO
Solubility and pH
Any salt containing an anion that is the conjugate base of a weak acid dissolves in water to a greater extent than that given by Ksp because the ions undergo a hydrolysis reaction.
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30. WOLPA/AP CHEMISTRY/CDO
Example
PbS
PbS(s) < > Pb2+(aq) + S2-(aq) Ksp = 8.4 x 10-28
S2-(aq) + H2O(l) < > HS-(aq) + OH-(aq) Kb = 1 x 105
Overall
PbS(s) + H2O(l) <---> Pb2+(aq) + HS-(aq) + OH-(aq) Knet = 8.4 x 10-23
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31. WOLPA/AP CHEMISTRY/CDO
In general, the solubility of a salt containing the conjugate base of a weak acid is increased by addition of a stronger acid to the solution. In contrast, the salts are not soluble in strong acid if the anion is the conjugate base of a strong acid.
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32. WOLPA/AP CHEMISTRY/CDO
CaCO3 < > Ca2+(aq)+ CO32-(aq) K = Ksp = 3.8 x 10-9
CO32-(aq) + H2O(l) < > HCO3-(aq) + OH-(aq) K = Kb = 2.1 x 10-4
OH-(aq) + H3O+(aq) < > 2H2O(l) K = 1/Kw = 1 x 1014
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33. WOLPA/AP CHEMISTRY/CDO
NET
CaCO3(s) + H3O+(aq) <------> Ca2+(aq) + HCO3-(aq) + H2O(l)
Knet = (Ksp) (Kb)/(Kw) = 79.8
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34. Solubility and Complex Ions
Examples of complex ions: AgCl2-, Ag(S2O3)23-, Ag(CN)2-
The solubility of certain “insoluble “ compounds can be increased when a complex ion is formed. Complex ions usually refer to cations in which surrounding water molecules have been replaced by some other electron pair donor. The equilibrium constant will equal the solubility constant times the formation constant for the complex ions. (Note - Chapter 23 in your textbook covers complex ions - their formation and nomenclature)
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35. Solubility and Complex ions
AgCl(s) + 2NH3  Ag(NH3)2+ + Cl-
K = (Ksp) (Kf)
= (1.8 x 10-10) (1.6 x 107) = =
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36. WOLPA/AP CHEMISTRY/CDO
Example
Solid gold (I) chloride AuCl, is dissolved when excess cyanide ions, CN-, are added to give a wter souble complex ion.
AuCl(s) + 2CN- (aq) Au(CN)-(aq) + Cl-
Show that this equation is the sum of twp other equations and calculate the Knet.
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37. Solubility, Ion Separations, and Qualitative Analysis
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