Chem. 1B – 10/13 Lecture. Announcements I Lab –Starting Wednesday: Experiment 5 (Acids/Bases and...
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Transcript of Chem. 1B – 10/13 Lecture. Announcements I Lab –Starting Wednesday: Experiment 5 (Acids/Bases and...
Chem. 1B – 10/13 Lecture
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)
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)
Chem 1B – Aqueous ChemistrySolubility (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 Ca2+ + 0.0010 M F- – do we get CaF2 (s)?
– Using Ksp reaction (backwards of what is occurring), we can compare Q with Ksp
– If Q < Ksp, no precipitation occurs (solution stays clear)
– If Q > Ksp, either precipitation occurs or supersaturated solution [Example problem]
Chem 1B – Aqueous ChemistrySolubility (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 CaCO3. The collected liquid has [Ca2+] = 0.20 M and [Pb2+] = 0.10 M. A chemist wants to save the lead (in form Pb2+) but not the Ca2+. Can she selectively precipitate out >98% of the Pb2+ without precipitating Ca2+ by addition of SO4
2-?Ksp(CaSO4) = 7.10 x 10-5 and Ksp (PbSO4) = 1.82 x 10-8
Chem 1B – Aqueous ChemistrySolubility (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+, Hg2
2+, and Pb2+ which precipitate)
Chem 1B – Aqueous ChemistrySolubility (Chapter 16)
• Qualitative Analysis – cont.– Sulfides are generally insoluble (except with
group I and some group II cations)– Sulfide is a strong base (Kb1 > 1), which
means that addition of acid increases solubility
– Ksp is defined somewhat differently:for CuS, rxn is CuS(s) + H2O(l) ↔ Cu2+(aq) + HS-(aq) + OH-(aq) and Ksp = [Cu2+][HS-][OH-]
– Because of basic nature, addition of acid increases solubility
Chem 1B – Aqueous ChemistrySolubility (Chapter 16)
• Qualitative Analysis – cont.– However, some Ksp values are so small,
that even addition of acid will not all significant dissolution
– Example Ni2+ (Ksp = 3 x 10-20) vs. Cu2+ (Ksp = 1.3 x 10-36)
– Solubility in 0.2 M H+ and 0.2 M H2S: show calculation
– This allows separation into acid insoluble (smaller group) and base insoluble sulfides (also precipitates out insoluble hydroxides)
Chem 1B – Aqueous ChemistrySolubility (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 + NH4+ are
always soluble)
Chem 1B – Aqueous ChemistryComplex 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):Cu2+(aq) + 4NH3(aq) ↔ Cu(NH3)4
2+(aq) orCu2+(H2O)4 + 4NH3(aq) ↔ Cu(NH3)4
2+(aq) + 4H2O(l)– Kf values tend to be large (Kf = 1.7 x 1013), but
stoichiometry limits importance to higher concentrations of ligand
Chem 1B – Aqueous ChemistryComplex Ion Formation (Chapter 16)
• Complex Ions – cont.– Example:Cu2+(aq) + 4NH3(aq) ↔ Cu(NH3)4
2+(aq)– Kf values tend to be large (Kf = 1.7 x 1013), but
stoichiometry limits importance to higher concentrations of ligand
Kf = [Cu(NH3)42+]/[Cu2+][NH3]4
Equil. [NH3] (M) [Cu(NH3)42+]/[Cu2+]
1.0 x 10-2 170,000
1.0 x 10-3 17
1.0 x 10-4 0.0017
Chem 1B – Aqueous ChemistryComplex Ion Formation (Chapter 16)
• Complex Ions – Example problem– The complex Zn(CN)4
2- forms (from Zn2+ + CN-) with a formation constant (Kf) of 2.1 x 1019. If 1.0 x 10-5 moles of ZnCl2 is added to 1.00 L of a 0.0010 M NaCN solution, what is the concentration of each species at equilibrium?
Chem 1B – Aqueous ChemistryComplex 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) + NH3(aq)solubility rxn: AgCl(s) ↔ Ag+(aq) + Cl-(aq) Ksp = 1.77 x 10-
10
complex rxn: Ag+(aq) + 2NH3(aq) ↔ Ag(NH3)2+(aq) Kf = 1.7
x 107
sum: AgCl(s) + 2NH3(aq) ↔ Ag(NH3)2+(aq) + Cl-(aq) K =
3.0 x 10-3
Chem 1B – Aqueous ChemistryComplex Ion Formation (Chapter 16)
• Complex Ions – Effects on Solubility– Examples: 2) Ca2+ + C2O4
2- (oxalate anion) – anion can lead to both precipitation and complex formation
solubility rxn: CaC2O4(s) ↔ Ca2+(aq) + C2O42-(aq) 1.3 x 10-8
complex rxn: Ca2+(aq) + 2C2O42-(aq) ↔ Ca(C2O4)2
2-(aq) Kf = 2.3 x 104
At low [C2O42-] (e.g. 1.0 x 10-7 M at equilibrium), Ca2+ is fairly
soluble (0.13 M) and almost no complex forms ([Ca(C2O4)22-] = 3
x 10-13 M and doesn’t contribute to solubility)At moderate [C2O4
2-] (e.g. 1.0 x 10-3 M), Ca2+ 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 [C2O42-] (e.g. 0.5 M), very little Ca2+ is present (2.6 x 10-8
M), but complex starts to increase net solubility (1.5 x 10-4 M)
Complex Ions – “U” Shaped Solubility Curves
Calcium Oxalate
1.00E-10
1.00E-09
1.00E-08
1.00E-07
1.00E-06
1.00E-05
1.00E-04
1.00E-03
1.00E-02
1.00E-01
1.00E+00
1.00E-06 1.00E-05 1.00E-04 1.00E-03 1.00E-02 1.00E-01 1.00E+00
[C2O42-] (M)
Con
c. (M
) CaC2O4(aq)Ca2+Ca(C2O4)22-Total Ca
Solubility in water
Common ion effect
Complex ion effect
Note: looks “U” shaped if not on log scale (otherwise “V” shaped)
Chem 1B – Aqueous ChemistryComplex 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 Zn2+ in buffers at pH
= 7, 10 and 13. Ksp(Zn(OH)2) = 3 x 10-17 and Kf (Zn(OH)4
2-) = 2 x 1015
Chem 1B – ThermodynamicsChapter 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
Chem 1B – ThermodynamicsChapter 17
• Chapter 6 – Review II– Enthalpy (H)
• Energy related to heat• H = qp (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