1. Chem. 1B – 10/13 Lecture

2. Announcements I Lab –Starting Wednesday: Experiment 5 (Acids/Bases and Buffers) –Report for Lab #3 due Mastering Assignments –16b due today Bonus In-Class Problems: –Can pick up after class (max of 0.25 to 0.5 pts awarded to each student in group)

3. Announcements II Today’s Lecture –Solubility and Precipitation: precipitation selective precipitation use in qualitative analysis (emphasized in lab) –Complex Ion Formation –Thermodynamics (Start by reviewing Ch. 6)

4. Chem 1B – Aqueous Chemistry Solubility (Chapter 16) Precipitation –If we mix an ion pair from a sparingly soluble salt together, how do we know if precipitation will occur? –Example: 0.040 M Ca 2+ + 0.0010 M F - – do we get CaF 2 (s)? –Using K sp reaction (backwards of what is occurring), we can compare Q with K sp –If Q < K sp, no precipitation occurs (solution stays clear) –If Q > K sp, either precipitation occurs or supersaturated solution [Example problem]

5. Chem 1B – Aqueous Chemistry Solubility (Chapter 16) Precipitation for selective ion removal –Example: An old battery plant had a leak of lead and sulfuric acid that was neutralized by addition of CaCO 3. The collected liquid has [Ca 2+ ] = 0.20 M and [Pb 2+ ] = 0.10 M. A chemist wants to save the lead (in form Pb 2+ ) but not the Ca 2+. Can she selectively precipitate out >98% of the Pb 2+ without precipitating Ca 2+ by addition of SO 4 2- ? K sp (CaSO 4 ) = 7.10 x 10 -5 and K sp (PbSO 4 ) = 1.82 x 10 -8

6. Chem 1B – Aqueous Chemistry Solubility (Chapter 16) Qualitative Analysis –Based on selective removing specific (less soluble) ions –Anions already examined in lab –Text covers how to separate cations (somewhat similar to experiment 8) –Schemes generally start with addition of Cl - (most chlorides are soluble – except Ag +, Hg 2 2+, and Pb 2+ which precipitate)

7. Chem 1B – Aqueous Chemistry Solubility (Chapter 16) Qualitative Analysis – cont. –Sulfides are generally insoluble (except with group I and some group II cations) –Sulfide is a strong base (K b1 > 1), which means that addition of acid increases solubility –K sp is defined somewhat differently: for CuS, rxn is CuS(s) + H 2 O(l) ↔ Cu 2+ (aq) + HS - (aq) + OH - (aq) and K sp = [Cu 2+ ][HS - ][OH - ] –Because of basic nature, addition of acid increases solubility

8. Chem 1B – Aqueous Chemistry Solubility (Chapter 16) Qualitative Analysis – cont. –However, some K sp values are so small, that even addition of acid will not all significant dissolution –Example Ni 2+ (K sp = 3 x 10 -20 ) vs. Cu 2+ (K sp = 1.3 x 10 -36 ) –Solubility in 0.2 M H + and 0.2 M H 2 S: show calculation –This allows separation into acid insoluble (smaller group) and base insoluble sulfides (also precipitates out insoluble hydroxides)

9. Chem 1B – Aqueous Chemistry Solubility (Chapter 16) Qualitative Analysis – last step –Phosphate is usually added last (fewer exceptions to the generally insoluble rule) to remove alkaline earth cations –Remaining ions (alkali metals + NH 4 + are always soluble)

10. Chem 1B – Aqueous Chemistry Complex Ion Formation (Chapter 16) Complex Ions –Consists of a metal reacting with ligands –Metal is Lewis Acid (electron pair acceptor) while Ligand is Lewis Base –Water is a common ligand (but often not included in reaction) –Example (2 reactions are essentially the same): Cu 2+ (aq) + 4NH 3 (aq) ↔ Cu(NH 3 ) 4 2+ (aq) or Cu 2+ (H 2 O) 4 + 4NH 3 (aq) ↔ Cu(NH 3 ) 4 2+ (aq) + 4H 2 O(l) –K f values tend to be large (K f = 1.7 x 10 13 ), but stoichiometry limits importance to higher concentrations of ligand

11. Chem 1B – Aqueous Chemistry Complex Ion Formation (Chapter 16) Complex Ions – cont. –Example: Cu 2+ (aq) + 4NH 3 (aq) ↔ Cu(NH 3 ) 4 2+ (aq) –K f values tend to be large (K f = 1.7 x 10 13 ), but stoichiometry limits importance to higher concentrations of ligand K f = [Cu(NH 3 ) 4 2+ ]/[Cu 2+ ][NH 3 ] 4 Equil. [ NH 3 ] (M) [Cu(NH 3 ) 4 2+ ]/[Cu 2+ ] 1.0 x 10 -2 170,000 1.0 x 10 -3 17 1.0 x 10 -4 0.0017

12. Chem 1B – Aqueous Chemistry Complex Ion Formation (Chapter 16) Complex Ions – Example problem –The complex Zn(CN) 4 2- forms (from Zn 2+ + CN - ) with a formation constant (K f ) of 2.1 x 10 19. If 1.0 x 10 -5 moles of ZnCl 2 is added to 1.00 L of a 0.0010 M NaCN solution, what is the concentration of each species at equilibrium?

13. Chem 1B – Aqueous Chemistry Complex Ion Formation (Chapter 16) Complex Ions – Effects on Solubility –The strong binding possible with ligands allows insoluble salts to become soluble in the presence of a ligand –Examples: 1) AgCl(s) + NH 3 (aq) solubility rxn: AgCl(s) ↔ Ag + (aq) + Cl - (aq) K sp = 1.77 x 10 -10 complex rxn: Ag + (aq) + 2NH 3 (aq) ↔ Ag(NH 3 ) 2 + (aq) K f = 1.7 x 10 7 sum: AgCl(s) + 2NH 3 (aq) ↔ Ag(NH 3 ) 2 + (aq) + Cl - (aq) K = 3.0 x 10 -3

14. Chem 1B – Aqueous Chemistry Complex Ion Formation (Chapter 16) Complex Ions – Effects on Solubility –Examples: 2) Ca 2+ + C 2 O 4 2- (oxalate anion) – anion can lead to both precipitation and complex formation solubility rxn: CaC 2 O 4 (s) ↔ Ca 2+ (aq) + C 2 O 4 2- (aq) 1.3 x 10 -8 complex rxn: Ca 2+ (aq) + 2C 2 O 4 2- (aq) ↔ Ca(C 2 O 4 ) 2 2- (aq) K f = 2.3 x 10 4 At low [C 2 O 4 2- ] (e.g. 1.0 x 10 -7 M at equilibrium), Ca 2+ is fairly soluble (0.13 M) and almost no complex forms ([Ca(C 2 O 4 ) 2 2- ] = 3 x 10 -13 M and doesn’t contribute to solubility) At moderate [C 2 O 4 2- ] (e.g. 1.0 x 10 -3 M), Ca 2+ is less soluble (1.3 x 10 -5 M), and complex still hasn’t formed much (3 x 10 -7 ) to affect solubility At high [C 2 O 4 2- ] (e.g. 0.5 M), very little Ca 2+ is present (2.6 x 10 -8 M), but complex starts to increase net solubility (1.5 x 10 -4 M)

15. 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)

16. Chem 1B – Aqueous Chemistry Complex Ion Formation (Chapter 16) Complex Ions – Effects on Solubility –The second example also applies to metal hydroxides (e.g. Zn(OH) 2 = sparingly soluble salt, but solubility increases at high pH due to formation of Zn(OH) 4 2- ) –Calculate the solubility of Zn 2+ in buffers at pH = 7, 10 and 13. K sp (Zn(OH) 2 ) = 3 x 10 -17 and K f (Zn(OH) 4 2- ) = 2 x 10 15

17. Chem 1B – Thermodynamics Chapter 17 Chapter 6 – Review –Types of Energy: kinetic energy (associated with motion) potential energy (stored energy – e.g. ball at the top of a hill) Chemical energy (a type of stored energy) Heat (a molecular scale type of kinetic energy) –Systems and Surroundings used to define energy transfers example: system with reaction that produces heat (conversion from chemical energy) can heat surroundings

18. Chem 1B – Thermodynamics Chapter 17 Chapter 6 – Review II –Enthalpy (H) Energy related to heat  H = q p (heat in a constant pressure system) Endothermic reaction means  H > 0, means heat from surrounding used for reaction Exothermic reaction means  H > 0, means heat from reaction goes to surroundings