Chem. 31 – 3/4 Lecture

Announcements I Exam 1 –Still Grading –Key Posted Next Lab Report Due: Cl lab report –Due next Wednesday –Must turn in in Excel format Homework Set 2 –See website (sorry, but I did not make print outs) –Still need to add additional problem 2.2 –Additional Problem 2.1 due next Wed. (+ quiz)

Announcements II Today’s Lecture –Solubility Product Problems (precipitation and selective precipitations) –Complex Ions –Acid Base Chemistry

Solubility Product Problems Precipitation Problems What occurs if we mix 50 mL of M BaCl 2 with 50 mL of 3.0 x M (NH 4 ) 2 SO 4 ? Does any solid form from the mixing of ions? What are the concentrations of ions remaining?

Precipitations Used for Separations Example: If we wanted to know the concentrations of Ca 2+ and Mg 2+ in a water sample. EDTA titration gives [Ca 2+ ] + [Mg 2+ ]. However, if we could selectively remove Ca 2+ or Mg 2+ (e.g. through titration) and re-titrate, we could determine the concentrations of each ion. Determine if it is possible to remove 99% of Mg 2+ through precipitation as Mg(OH) 2 without precipitating out any Ca(OH) 2 if a tap water solution initially has 1.0 x M Mg 2+ and 1.0 x M Ca 2+.

Complex Ions Example Reaction: Ag + + 2NH 3 (aq) ↔ Ag(NH 3 ) 2 + Metal Ligand Complex Ion Why does reaction occur? Metal is a Lewis acid (electron pair acceptor) NH 3 is a Lewis base (electron pair donator) Metal-ligand bonds are intermediate strength

Complex Ions – Why Study? Useful in separations –Complexed metals become more organic soluble Effects on metal solubility (e.g. addition of NH 3 on AgCl solubility) Complexometric titrations (e.g. water hardness titration) Some Complexes are Colored (use as indicators or for spectroscopic measurements) Na + Crown ether (12-crown-4) Diethyl ether water Crown ether added Sodium conc. given by gray shading

Complex Ions Step-wise vs. full reactions: Example: addition of NH 3 to Ag + Reaction occurs in steps: 1) Ag + + NH 3 (aq) ↔ AgNH 3 + K 1 (= β 1 ) 2) AgNH NH 3 (aq) ↔ Ag(NH 3 ) 2 + K 2 Net) Ag + + 2NH 3 (aq) ↔ Ag(NH 3 ) 2 + β 2 = K 1 ·K 2

Complex Ions Due to large exponents on ligand concentration, a small change in ligand concentration has a big effect on how metal exists Example: Al C 2 O 4 2- ↔ Al(C 2 O 4 ) 3 3- β 3 = 4.0 x [C 2 O 4 2- ][Al(C 2 O 4 ) 3 3- ]/[Al 3+ ] M M M0.004

Complex Ions – “ U ” Shaped Solubility Curves Many sparingly soluble salts release cations and anions that form complexes with each other Example: calcium oxalate (CaC 2 O 4 ) CaC 2 O 4 (s) ↔ Ca 2+ + C 2 O 4 2- (K sp = 1.3 x M) increased [C 2 O 4 2- ] decreases Ca 2+ solubility for above reaction only, but... Ca 2+ + C 2 O 4 2- ↔ CaC 2 O 4 (aq) K 1 = 46 CaC 2 O 4 (aq) + C 2 O 4 2- ↔ Ca(C 2 O 4 ) 2 2- K 2 = 490 β 2 = K 1 ·K 2 = 2.3 x 10 4 = [Ca(C 2 O 4 ) 2 2- ]/([Ca 2+ ][C 2 O 4 2- ] 2 )

Complex Ions – “ U ” Shaped Solubility Curves Solubility in water Common ion effect Complex ion effect Note: looks “U” shaped if not on log scale (otherwise “V” shaped)

Some Questions 1.In the reaction: Ca 2+ + Y 4- ↔ CaY 2- (where Y 4- = EDTA), which species is the Lewis acid? 2.List two applications in which the formation of a complex ion would be useful for analytical chemists. 3.List two applications in the lab in which you used or are using complex ions. 4.AgCN is a sparingly soluble salt. However, a student observed that adding a little of a NaCN solution to a saturated solution of AgCN did not result in more precipitation of solid. Addition of more NaCN solution resulted in total dissolution of the AgCN. Explain what is happening.

One More Question 1.Cu 2+ reacts with thiosulfate (S 2 O 3 2- ) to form a complex which is most stable when two moles of thiosulfate to one mole of Cu 2+ are present. The  2 value is found to be 2.00 x If a solution containing both Cu 2+ and S 2 O 3 2- is prepared and found to contain 1.7 x M free (uncomplexed) S 2 O 3 2- at equilibrium, what is the ratio of complexed to free Cu? Assume that little CuS 2 O 3 forms.

Acids, Bases and Salts Definitions of Acids and Bases - Lewis Acids/Bases (defined before, most general category) - Br ø nsted-Lowry Acids/Bases: acid = proton donor base = proton acceptor (must have free electron pair so also is a Lewis base) - definitions are relative

Br ø nsted-Lowry Acids - examples HCO 2 H(aq) + H 2 O(l) ↔ HCO H 3 O + acid base conjugate conjugate base acid CH 3 NH 2 (aq) + H 2 O(l) ↔ CH 3 NH OH - base acid conjugate conjugate acid base H 2 SO 4 + CH 3 CO 2 H(l) ↔ HSO CH 3 CO 2 H 2 + acid base conjugate conjugate base acid