1. Chapter 10 More Equilibria in Aqueous Solutions: Slightly Soluble Salts and Complex Ions

2. The Solubility Product Constant, Ksp
Many ionic compounds are only slightly soluble in water: ex. Ag salts, sulfides
Equations are written to represent the equilibrium between the compound and the ions present in a saturated aqueous solution
AgCl(s) Ag+(aq) + Cl–(aq)
EOS
Ksp = [Ag+][Cl–]
Chapter 10: Slightly Soluble Salts and Complex Ions

3. Chapter 10: Slightly Soluble Salts and Complex Ions
Ksp’s (25 oC)
EOS
Chapter 10: Slightly Soluble Salts and Complex Ions

4. Ksp and Molar Solubility
The solubility product constant is related to the solubility of an ionic solute
Ksp = [Ag+][Cl–]; solubility given by [Ag+]
From stoichiometry, the ion ratio is 1:1, so [Ag+] = [Cl–], both of which are unknown (x)
Ag+
Cl–
Ag+
Cl– +
EOS
Ksp = x2 and [Ag+] = (Ksp)1/2
Chapter 10: Slightly Soluble Salts and Complex Ions

5. Chapter 10: Slightly Soluble Salts and Complex Ions
The Common Ion Effect
Le Châtelier’s principle is followed for the shift in concentration of products and reactants upon addition of either products or reactants to a solution
The solubility of a slightly soluble ionic compound is lowered when a second solute that furnishes a common ion is added to the solution
Ag2SO4(s) 2 Ag+(aq) + SO4–2(aq)
Solubility of Ag2SO4  if MgSO4 is added to solution
EOS
Chapter 10: Slightly Soluble Salts and Complex Ions

6. Common Ion Effect Illustrated
VideoClip
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Chapter 10: Slightly Soluble Salts and Complex Ions

7. Does Precipitation Occur?
Qip is the ion product reaction quotient and is based on initial conditions of the reaction
EOS
Precipitation should occur if Qip > Ksp
Precipitation cannot occur if Qip < Ksp
A solution is just saturated if Qip = Ksp
Chapter 10: Slightly Soluble Salts and Complex Ions

8. Is Precipitation Complete?
Precipitation to be essentially complete if about 99.9% of the target ion is precipitated
Conditions that generally favor completeness of precipitation
A very small value of Ksp  few ions in solution
A high initial concentration of the target ion
EOS
A concentration of common ion that greatly exceeds that of the target ion  use a soluble salt, e.g., KCl
Chapter 10: Slightly Soluble Salts and Complex Ions

9. Selective Precipitation
EOS
Chapter 10: Slightly Soluble Salts and Complex Ions

10. Effect of pH on Solubility
The solubility of an ionic solute may be greatly affected by pH if an acid–base reaction also occurs as the solute dissolves
Precipitate anions of weak acids will dissolve somewhat when the pH is lowered; precipitate anions of strong acids are not affected by pH
EOS
Cl– is the conjugate base of HCl, a strong acid – pH doesn’t affect the solubility of AgCl
Chapter 10: Slightly Soluble Salts and Complex Ions

11. Chapter 10: Slightly Soluble Salts and Complex Ions
Complex Ion Formation
Experimental data suggests a counterintuitive behavior in solubility of AgCl(s)
EOS
Chapter 10: Slightly Soluble Salts and Complex Ions

12. Chapter 10: Slightly Soluble Salts and Complex Ions
Complex Ion Formation
A complex ion is a polyatomic cation or anion consisting of a central metal atom or ion that has other groups called ligands bonded to it
Common ligands
EOS
An example complex ion
Chapter 10: Slightly Soluble Salts and Complex Ions

13. Chapter 10: Slightly Soluble Salts and Complex Ions
Complex Ion Formation
The formation reaction of a complex ion is reversible, and the equilibrium state in the reaction is described through an equilibrium constant called a formation constant, represented by the symbol Kf
EOS
Chapter 10: Slightly Soluble Salts and Complex Ions

14. Some Formation Constants
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Chapter 10: Slightly Soluble Salts and Complex Ions

15. Complex Ion Formation and Solubilities
Even though AgCl is insoluble in water, it does dissolve in aqueous ammonia to form the complex ion [Ag(NH3)2]+
EOS
Chapter 10: Slightly Soluble Salts and Complex Ions

16. Complex Ion Formation and Solubilities
The Kc values are directly proportional to Ksp because Kf is the same for each reaction
The source of Ag+ ions doesn’t matter ... AgCl, AgBr, AgI, etc ... the complex ion is still Ag(NH3)2+
In this case, as Kc , Ksp 
EOS
In NH3, order of decreasing solubility is
AgCl > AgBr > AgI
Chapter 10: Slightly Soluble Salts and Complex Ions

17. Complex Ions in Acid–Base Reactions
Many metal cations, particularly those with a small size and a high charge, hydrolyze water
EOS
Water molecules are quite commonly found as ligands in complex ions because H2O is a Lewis base
Chapter 10: Slightly Soluble Salts and Complex Ions

18. Complex Ions in Acid–Base Reactions
In some cases, the electron-withdrawing power of the small, highly charged metal ion weakens an O—H bond in a ligand water molecule, causing it to give up a proton to a water molecule in the solution
EOS
Chapter 10: Slightly Soluble Salts and Complex Ions

19. Amphoteric Substances
Certain metal hydroxides, insoluble in water, will react with both strong acids and strong bases
EOS
Chapter 10: Slightly Soluble Salts and Complex Ions

20. Amphoteric Substances
The hydroxides of Al, Zn, and Cr are amphoteric
Like hydroxides, oxides that can react with and dissolve in either an acid or a base are often said to be amphoteric
EOS
The oxides of Al, Zn, and Cr behave toward acids and bases much as their hydroxides do
Chapter 10: Slightly Soluble Salts and Complex Ions

21. Qualitative Inorganic Analysis
Acid–base chemistry, precipitation reactions, oxidation–reduction, and complex-ion formation all come into sharp focus in an area of analytical chemistry called classical qualitative inorganic analysis
“Qualitative” signifies that the interest is in determining what is present, not how much is present
EOS
Applies knowledge of color changes, precipitates, and equilibrium as a determination scheme
Chapter 10: Slightly Soluble Salts and Complex Ions

22. Qualitative Inorganic Analysis
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Chapter 10: Slightly Soluble Salts and Complex Ions

23. Chapter 10: Slightly Soluble Salts and Complex Ions
Group 1 Cations
If aqueous HCl is added to an unknown solution of cations, and a precipitate forms, then the unknown contains one or more of these cations: Pb2+, Hg22+, or Ag+
These are the only ions to form insoluble chlorides
The supernatant liquid and any precipitate are saved for further analysis
Test for Hg2+2
EOS
Test for Pb+2
Chapter 10: Slightly Soluble Salts and Complex Ions

24. Chapter 10: Slightly Soluble Salts and Complex Ions
Group 2 Cations
H2S is added to the supernatant liquid – insoluble sulfide formation is the positive test
The concentration of HS– is so low in a strongly acidic solution (HCl) that only the most insoluble sulfides precipitate
EOS
Chapter 10: Slightly Soluble Salts and Complex Ions

25. Chapter 10: Slightly Soluble Salts and Complex Ions
Group 3 Cations
Of the eight cations in Group 3, five form sulfides that are soluble in acidic solution but insoluble in an alkaline ammonia/ammonium chloride buffer solution
EOS
The other three group 3 cations form hydroxide precipitates in the alkaline solution
Chapter 10: Slightly Soluble Salts and Complex Ions

26. Chapter 10: Slightly Soluble Salts and Complex Ions
Group 4 and 5 Cations
The group 4 cations are precipitated as carbonates from a buffered alkaline solution
The cations of group 5 remain soluble in the presence of all common reagents
EOS
Within each of the qualitative analysis groups, additional reactions to dissolve group precipitates and to separate and selectively precipitate individual cations for identification and confirmation are used
Chapter 10: Slightly Soluble Salts and Complex Ions

27. Chapter 10: Slightly Soluble Salts and Complex Ions
Summary of Concepts
The solubility product constant, Ksp, represents equilibrium between a slightly soluble ionic compound and its ions in a saturated aqueous solution
The common ion effect is responsible for the reduction in solubility of a slightly soluble ionic compound
EOS
Precipitation is assumed to be complete if no more than 0.1% of the target ion remains in solution
Chapter 10: Slightly Soluble Salts and Complex Ions

28. Chapter 10: Slightly Soluble Salts and Complex Ions
Summary of Concepts
The solubilities of some slightly soluble compounds depends strongly on pH
Certain solutes become more soluble in the presence of species that can serve as ligands in complex ions
The ability of water as ligand molecules to donate protons accounts for the acidic character of some complex ions
EOS
Precipitation, acid–base, redox, complex-ion formation, and amphoteric behavior are all used for the qualitative analysis of common cations
Chapter 10: Slightly Soluble Salts and Complex Ions