E2 Precipitation and Water Purity
Chem.125/126 Information
 GSI SLC Office hours start Wednesday, January 16
“Will we do
the entire
experiment?”
“E2 is a one
session lab!”
Experiment 2
Pre-lab Report (page 54)
 Due at the start of Lab (8am or 11am or 2pm)
 Question 3 refers to Part 2A (not Part 2)
 Office hours occur 10 am - 3pm on Mondays,
Tuesdays, Wednesdays, and Fridays.
 Chem.125/126 Hourly I March 11 occurs prior to
and on same day as the chem.130 exam.
- Labs on Hourly I exam will be completed
BEFORE spring break (Feb. 23 - March 3)
- Discussion of labs on Hourly I exam will take
place Feb. 21, Feb. 22 or March 4.
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.
 Complete team report and turn in at the end
of lab or by the grace period deadline
indicated in the lab manual, p. 221.
Discussion: first hour of next session
 Prepare discussion abstract and presentation
during lab or before next session.
1
Discussion Preparation
 Refer to the two grading rubricks, pages 65-66.
- Note that one discussion grading rubrick refers to
an exam question.
Precipitation
 A solid comes out of solution
 Refer to the discussion information, page 222.
Solutions before mixing
E2 SALTS
 Ionic compounds with cations (+) and anions (-) in
fixed positions in a crystalline solid.
 Salts only melt at temperatures exceeding 1000°C.
“I’m a cation.
Note my eyes!”
After mixing
Background: Salt Formulas
 In the solid, the salt ions are
fixed in a rigid lattice.
 The simplest ratio of the
ions in the solid is represented
by the formula of the salt.
2
Salt Formulas
Background: Water and Salt Solubility
In the solid salt, the ions are
fixed in a rigid crystal lattice
 In water solution the salt ions
are free to move about.
Formula ?
Cr+3 O-2
= Cr2O3
Salt Solubility in Water
Salt Solubility in Water
 Water (H2O) is polar.
 Polar water molecules reduce the effective charges of the ions
in the solid and thus salt ions dissociate and the salt dissolves.
DEMO
NaCl(s) = Na+(aq) + Cl-(aq)
BaCl2(s) = Ba2+(aq) + 2 Cl-(aq)
3
Salt Solubility in Water
Salt Solubility
 Salt ions in aqueous solution are surrounded by and
• Aqueous salt solutions are electrolytes due
to the fact that the mobile salt ions can
carry a charge.
bonded to water molecules.
• Example: [Na(H2O)6]+ = Na+ (aq)
DEMO
Precipitation reactions
Hg2+(aq)
+
2Cl-(aq)
+
K+(aq)
+
Part 1. What is the precipitate?
I-(aq)
KCl
HgI2
HgCl2 (aq) +
__?__ (s)
__?
1. The positive ion of a dissolved salt combines with
the 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”.
KI (aq) →
clear and colorless salt solutions
ppt.
__?__ ↓
DEMO
4
REFERENCE BLANK TEST
 Test mixture designed to identify reactants
(forming the precipitate) and spectators (non-reactants).
Reference Blank Tests
Reaction:
HgCl2 (aq) + KI (aq) →
Clear and colorless
?
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 Test Design
HgCl2 (aq) + KI (aq) →
?
Reference blank test:
Hg(NO3)2 + KI →
Clear and colorless
Same
properties
Conclusion?
Cl- is a spectator ion.
DEMO
Reference Blank Test Design
HgCl2 (aq) + KI (aq) →
colorless solutions
?
Valid Reference Blank Test
 produces the identical products or no
reaction.
Invalid test:
CuCl2 (aq) + KI (aq) →
DEMO
• The test produced a different reaction with different products
and is therefore invalid.
5
BaCl2 (aq) + AgF (aq) → white (s)?
Test Observations:
1. FeCl3 (aq) + AgF (aq) → green (s)
2. NaCl (aq) + AgF (aq) → white (s)
3. Ba(NO3)2 (aq) + AgF (aq) → no reaction
Course theme
“There are structure and
property and periodic table
relationships”
Q. What do you know about the white (s)?
1. Nothing.
2. Ba2+ is a spectator.
3. Cl- is a reactant.
Part 2A. Precipitation Studies
Discussion questions 1-3, p.67
 Is the solubility of a cation predictable from the
position of its element in the Periodic Table?
 Is precipitate color predictable from the position of
the cation’s element in the Periodic Table?
Useful web sites are:
www.merlot.org
www.webelements.com
Discussion Preparation
Manipulate the class data.
You will NOT get points for just
reproducing the class data.
 Is there a relationship between cation charge and
solubility? Compare the solubility data of multiply
charged ions and….
6
Nitrate salts 
Part 2B. Can I identify it?
Sodium Salts
Cations
Cl-
CrO4
Hg2+


2-
I
2-
C2O4
S2-
SO4



2-
 Identify an unknown metal ion based on
precipitation observations from part 2A:
Caution: Save your acetate sheet results from Part
2A for use in Part 2B
Part 2A. Table, page 44; Team cation assignments, page 216
Part 3. Concentration and Precipitation.
Discussion (Question 4, page 67)
Examine the relationship between concentration and
amount of precipitate. Organize the class data…
Concentration and Salt Precipitation
Saturated solution = contains the maximum amount of
salt that can be dissolved in a given solution volume.
Laboratory
 Combine assigned ion combinations at different
concentrations
 Observe and record the relative amounts of
precipitate formed at different concentrations.
7
Concentration and Salt Precipitation
Concentration and Precipitation
• Precipitation (ppt.) is dependent on concentration
and occurs only if the solubility limit of the salt is
exceeded.
DEMO
DEMO
Supersaturation
and
precipitation.
Concentration and Precipitation
• Precipitation reactions are equilibrium systems and
thus there are always reactants and products present
when precipitation occurs
0.10 M KI(aq) + 0.10 M HgCl2 (aq) →
ppt.
10-3 M KI(aq) + 10- 4 M HgCl2 (aq) →
NO ppt.
Concentration and Precipitation
 Precipitation reactions are equilibrium systems
 When precipitation occurs, some reactant ions
remain in solution:
Hg2+ and 2I- →
X HgI2(s)
DEMO:
1.Remove the ppt. by
filtration
2. Check for presence of
reactant ions in the filtrate.
“In reality, it is understood there is
both a forward and reverse arrow!”
8
Solvent Polarity
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.
 Water (H2O) is very polar.
O
H
/
\
H
 Hexane (C6H14) is nonpolar
CH3CH2CH2CH2CH2CH3
Discussion Question 5, page 67
What is the relationship, if any, between salt solubility,
precipitation, and solvent polarity?
 Acetone (CH3COCH3) is moderately polar.
O

C
/
\
CH3 CH3
Solvent Polarity
Salt solubility and solvent polarity
 Polar and non-polar solvents are immiscible
 Salts will not dissociate in non-polar solvents.
DEMO
Acetone is less polar than water.
Acetone is added to a saturated
solution of CuSO4(aq).
Q1. Will the solubility of the CuSO4
Increase?
Decrease?
Remain the same?
Q2. What will you observe?
DEMO
9
Solvent Polarity and Solubility
B = Acetone is added to A = CuSO4(aq)
After mixing
CuSO4(s) forms
Any Questions?
Contact [email protected]
 The less polar the polar solvent environment, the
lower the solubility of the salt.
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