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E2 Precipitation and Water Purity
“E2 is a onesession lab!”
“Will we dothe entireexperiment?”
Experiment 2
Pre-lab correction Question 3 refers to Part 2A (not Part 2)
Goals for Experiment 2
One session three hour lab Complete Parts 1 and 2A and 2B (all). Complete part 3 OR 4 as assigned. Omit part 5.
Discussion: first hour of next session Prepare discussion abstract and presentation. Complete team report.
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Precipitation
A solid comes out of solution
Salt solutions before mixing After mixing
Ionic compounds with cations (+) and anions (-).
“I’m a cation.Note my eyes!”
Background: SALTS
Background: Water and Salt Solubility
In the solid, the salt ions arefixed in a rigid lattice.
The simplest ratio of theions in the solid is representedby the formula of the salt.
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Cr+3 O-2
Formula ?
= Cr2O3
Salt Formulas
Background: Water and Salt Solubility
In solution the ions are mixedwith water molecules and free tomove about in solution.
Salt Solubility in Water
Water (H2O) is polar.
DEMO
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Background: Water and Salt Solubility
Polar water molecules reduce the effective charges of the ionsin the solid and thus salts dissociate and dissolve.
NaCl(s) = Na+(aq) + Cl-(aq)BaCl2(s) = Ba2+(aq) + 2 Cl-(aq)
Salt Solubility
DEMO
• Aqueous salt solutions are electrolytes.
1. The positive ion of a dissolved salt combines withthe negative ion from a different dissolved salt.
2. The recombined ions may stay in solution or come out of solution in the form of a solid called a “precipitate”.
Hg2+(aq) + 2Cl-(aq) + K+(aq) + I-(aq)
HgI2
KCl
Precipitation reactions
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Part 1. What is the precipitate?
HgCl2 (aq) + KI (aq) → __?__ (s) clear and colorless salt solutions
__? ppt.
__?__ ↓
DEMO
REFERENCE BLANK TEST
Test mixture designed to identify reactants(forming the precipitate) and spectators (non-reactants).
Test Mixture Omit a species (ion) from the reaction mixture. Substitute a known SPECTATOR ion for the
omitted ion such as Na+ or K+ or NO3- Compare the products of the test and original
reaction mixture
Reference Blank Tests� Reaction: HgCl2 (aq) + KI (aq) → ? Clear and colorless
Conclusion?
Reference blank test:Hg(NO3)2 + KI → ___________Clear and colorless
DEMO
______________
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Reference Blank Test Design
�HgCl2 (aq) + KI (aq) → ? colorless solutions
Valid Reference Blank Test produces the identical products or noreaction.
Reference Blank Test Design
�HgCl2 (aq) + KI (aq) →
Invalid test:CuCl2 (aq) + KI (aq) →
?
DEMO
BaCl2 (aq) + AgF (aq) → white (s)?
� 3. Ba(NO3)2 (aq) + AgF (aq) → no reaction
Test Observations:1. FeCl3 (aq) + AgF (aq) → green (s)
1. _____________________2. _____________________3. _____________________
Q. What do you know about the white (s)?
2. NaCl (aq) + AgF (aq) → white (s)
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Part 2A. Precipitation Studies
Discussion questions 1-3, p.67 Is the solubility of a metal ion predictable from theposition of its element in the Periodic Table? Is precipitate color predictable from the position ofthe metal ion’s element in the Periodic Table? Is there a relationship between ion charge andsolubility? Compare the solubility data of multiplycharged ions and….
SO42-S2-C2O42-I- CrO42-Cl-Cations
Part 2A. Table, page 44; Cation assignments, page 216
Hg2+
Nitrate salts Sodium Salts
Part 2B. Can I identify it?
Identify an unknown metal ion based onprecipitation observations from part 2A:
Caution: Save your acetate sheet results from Part2A for use in Part 2B
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Part 3. Concentration and Precipitation.
Laboratory Combine assigned ion combinations at different
concentrations Observe and record the relative amounts of
precipitate formed at different concentrations.
Discussion (Question 4, page 67)Examine the relationship between concentration andamount of precipitate. Organize the class data…
Concentration and Salt Precipitation
Saturated solution = contains the maximum amount ofsalt that can be dissolved in a given solution volume.
Concentration and Salt Precipitation
DEMOSupersaturationandprecipitation.
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Concentration and Precipitation
• Precipitation (ppt.) is dependent on concentrationand occurs only if the solubility limit of the salt isexceeded.
DEMO 0.10 M KI(aq) + 0.10 M HgCl2 (aq) →
10-3 M KI(aq) + 10- 4 M HgCl2 (aq) →
________
________
• Precipitation reactions are equilibrium systems andthus there are always reactants and products presentwhen precipitation occurs
Concentration and Precipitation
DEMO:1.Remove the ppt. byfiltration2. Check for presence ofreactant ions in the filtrate.
Precipitation reactions are equilibrium systems When precipitation occurs, some reactant ions
remain in solution: Hg2+ and 2I- → HgI2(s)X
Concentration and Precipitation
“In reality, it is understood there isboth a forward and reverse arrow!”
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Part 4. Solvent Pollution and Precipitation
Laboratory Combine assigned salts in different solvents (water,
acetone, hexane) Compare salt solubility and amount of precipitate
formed by the salt mixtures in the different solvents.
Discussion Question 5, page 67What is the relationship, if any, between salt solubility,precipitation, and solvent polarity?
Solvent Polarity
Water (H2O) is very polar. O / \ H H
Hexane (C6H14) is nonpolar
Acetone (CH3COCH3) is moderately polar. O
C
/ \ CH3 CH3
CH3CH2CH2CH2CH2CH3
Solvent Polarity
Polar and non-polar solvents are immiscible
Salts will not dissociate in non-polar solvents.
DEMO
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Salt solubility and solvent polarity
Acetone is less polar than water.Acetone is added to a saturatedsolution of CuSO4(aq).
DEMO
Q1. Will the solubility of the CuSO4 Increase? Decrease? Remain the same?Q2. What will you observe?
Solvent Polarity and Solubility
The less polar the polar solvent environment, the__________ the solubility of the salt.
B = Acetone is added to A = CuSO4(aq) After mixing ____________
Any Questions?Contact nkerner@umich.edu
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