Expt 063 -- One Pot Silver Reactions
Description
A series of reactions is conducted in one
vessel or pot. Evidence for the reactions is
based upon visual observations of
precipitates forming and dissolving, and
color changes.
Safety
Sulfide and silver solutions are toxic. The
sulfide fumes and ammonia fumes are toxic.
Silver nitrate causes black stains on skin.
Store sulfide and ammonia solutions under
hood until just before use. Wear goggles and
apron. Wash any silver spills immediately.
Provide adequate ventilation. Wash hands
after the experiment.
Procedure
1. Stirring must be vigorous in order to accelerate the reactions. Equilibrium is frequently approached
slowly if solutions are not stirred. Solids are particularly difficult to mix. Use a toothpick or a beral pipet
to stir the solution. (Draw the mixture into the pipet from the well, and then expel it back into the well as
fast as possible without spilling. The same pipet may be used throughout the experiment without
cleaning.)
2. Place 30 drops 0.1 M AgNO3 into one well of a 24-well plate.
3. Add 1 drops 1 M NaHCO3. Stir. Note and record evidence of change.
4. Add 5 drops 0.2 M Na3PO4. Stir. Note and record evidence of change.
5. Add 2 drops 1.0 M NaOH. Stir. Note and record evidence of change.
6. Add 5 drops 1 M NaCl. Stir. Note and record evidence of change.
7. Add 25 drops 6.0 M NH3. Stir. Note and record evidence of change.
8. Add 5 drops 0.1 M KBr. Stir. Note and record evidence of change.
9. Add 4 drops 1.0 M Na2S2O3. Stir. Note and record evidence of change.
10. Add 20 drops 0.1 M Na2S. Stir. Note and record evidence of change.
11. List each of the different silver ions or compounds observed with the respective color.
12. Carefully transfer the mixture in the reaction well to the disposal jar provided by the instructor. Rinse
with a small amount of water into this jar.
13. Wash the plate at the sink.
14. Wash hands.
Questions
1. Pure AgCO3 is white. Ag2O is blackish-brown. The product of reacting Ag+ with Na2CO3 is tan.
Account for this color. Suggest a way to test the hypothesis.
2. After KI is added, is the concentration of Ag(S2O3)23- zero or just very small? Explain the answer.
3. A solutions contains I-, S2-, NH3, and Cl-. If Ag+ is added to the solution, predict the appearance of the
mixture. Write the formula of the silver ion or compound which contains most of the silver which was
added.
Handout Makeup
Name ___________________________ Class _______
Teacher __________________________
SmallScale 063 One Pot Silver Reactions
You can watch movies of each step on David Brooks website. Look under “small scale” chemistry.
List each reagent added and the formula of each observed silver ion or compound with its respective color.
Answer the questions.
CurriculumThis experiment fits in when discussing equilibria. It is an excellent AP chemistry activity. Because the
manipulation of silver halides is a part of common photographic processes, a phenomenological version of the
activity works in applied chemistry classes.
SafetySulfide and silver solutions are toxic. The sulfide fumes and ammonia fumes are toxic. Silver nitrate causes
black stains on skin. Store sulfide and ammonia solutions under hood until just before use. Wear goggles and
apron. Wash any silver spills immediately. Provide adequate ventilation. Wash hands after the experiment.
TimeTeacher Preparation: 20 minutes
Class Time: 20 minutes
Materials
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2 mL of 0.1 M AgNO3 --(Dissolve 1.70 g of AgNO3 in enough water to make 100 mL of solution.)
0.5 mL of 1 M NaHCO3 -- (Dissolve 8.4 g sodium bicarbonate (NaHCO3) in enough water to make 100
mL solution.)
0.5 mL of 0.2 M Na3PO4 -- (Dissolve 3.3 g sodium phosphate (Na3PO4) in enough water to make 100
mL solution.)
0.5 mL of 1.0 M NaOH -- (Add 33 mL 3 M sodium hydroxide (NaOH) to enough water to make 100
mL solution.)
0.5 mL of 1 M NaCl -- (Dissolve 5.844 g sodium chloride (NaCl) in enough water to make 100 mL
solution.)
2 mL of 6 M NH3 -- (Add 40.0 mL 15 M ammonia (NH3) to enough water to make 100 mL solution.)
0.5 mL of 0.1 M KBr -- (Dissolve 1.190 g potassium bromide (KBr) in enough water to make 100 mL
solution.)
0.5 mL of 1.0 M Na2S2O3 -- (Dissolve 24.8 g sodium thiosulfate pentahydrate (Na2S2O3•5H2O) in
enough water to make 100 mL solution.)
0.5 mL of 0.1 M KI -- (Dissolve 1.660 g potassium iodide (KI) in enough water to make 100 mL
solution.)
1 mL of 0.1 M Na2S -- purchase as a solution of this concentration.
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24-well plate
toothpick
plastic transfer pipets
cotton swab
DisposalFilter the silver sulfide keeping the solid and discarding filtrate with 20 volumes of water per volume of filtrate.
Recover silver from the solid by treating with concentrated nitric acid under a hood to dissolve the silver sulfide
and oxidize the sulfide ion. Dilute the resulting solution 10-fold, and treat with metal zinc. Filter the resulting
solid silver.
Lab Hints
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If a dissolving reaction does not "go" completely even after stirring for a minute or two, ask the students
to add one more drop of the reacting chemical.
Please note that the concentrations of reagents added often are not suffucuent to drive the reactions "to
completion."
Background(Modified from Doing Chemistry)
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For each succeeding addition, a more stable silver compound or complex is formed:
1. The silver in the water solution exists as an aquo complex, Ag+(aq), which is represented simply
as Ag+. Addition of a carbonate source leads to the formation of silver carbonate.
2 Ag+ + HCO3-  Ag2CO3 + H+
2. Silver phosphate is less soluble than silver carbonate:
3 Ag2CO3 + 2 PO43-  2 Ag3PO4 + 3 CO323. Silver hydroxide is less soluble than silver phosphate:
Ag3PO4 + 3 OH-  3 AgOH + PO434. Silver hydroxide is less soluble than silver hydroxide:
AgOH + Cl-  AgCl + OH5. Silver chloride dissolves in ammonia:
AgCl + 2 NH3  Ag(NH3)2+ + Cl6. Silver bromide forms from silver ammine:
Ag(NH3)2+ + Br-  AgBr + 2 NH3
7. Addition of a source of thiosulfate ions allows the formation of the silver(I) thiosulfate complex.
This complex forms at the expense of silver bromide. The concentration of Ag+ in the solution is
still lower:
AgBr + 2 S2O32-  Ag(S2O3)23- + Br8. Addition of an iodide source allows the formation of silver iodide which, at these concentrations,
supports a concentration of Ag+ still lower than that of the thiosulfate complex:
Ag(S2O3)23- + I-  AgI + 2 S2O329. Finally, addition of a sulfide source allows the formation of silver sulfide, which supports the
lowest concentration of silver ion of any of the complexes or solids formed in this reaction
sequence.
2 AgI + S2-  Ag2S + 2 I
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If any of the reagents are added out of sequence, the compounds and complexes missed will never be
formed as the predominant species.
From the perspective of free energy, each of the silver compounds formed has a lower free energy than
its predecessor. Equilibrium calculations suggest progressively lower concentrations of free Ag+
available for reaction. See the literature data section.
AnswersQ1. Pure Ag2CO3 is white. Ag2O is brown or black. The product of reacting Ag+ with Na2CO3 is tan. Account
for this color. Suggest a way to test the hypothesis.
A1. Carbonate solutions are basic. a small amount of Ag2O forms together with mostly Ag2CO3. This causes the
color to be off white. By buffering the solution at a lower pH and adding a carbonate ion source, the precipitate
can be made cleaner.
Q2. After KI is added, is the concentration of Ag(S2O3)23- zero or just very small? Explain the answer.
A2. It is very small. So long as all of the reacting species are present, the equilibrium expression must be
satisfied.
Q3. 3. A solutions contains equal molar amounts of I-, S2-, NH3, and Cl-. If Ag+ is added to the solution, predict
the appearance of the mixture. Write the formula of the silver ion or compound which contains most of the
silver which was added.
A3. A black precipitate forms. The black precipitate of Ag2S will be the predominate compound containing
silver.
Computer UseA spreadsheet can be used with the equilibrium constants to determine equilibrium concentrations.
Literature DataEquilibrium constants are taken from Clifford, A. F. Inorganic Chemistry of Qualitative Analysis, PrenticeHall: Englewood Cliffs, NJ, 1961, p. 448-468.
Reaction
Ag+ + ½ CO32-  ½ Ag2CO3
Ag+ + 2 NH3  Ag(NH3)2+
Ag+ + Cl-  AgCl
Ag+ + 2 S2O32-  Ag(S2O3)3Ag+ + I-  AgI
Ag+ + ½ S2-  ½ Ag2S
ΔG0 (kJ/mol Ag+)
-31
-41
-54
-75
-92
-146
Note that ammonia dissolves solid AgCl. This is accomplished by a concentration effect; the ammonia is kept at
a high concentration relative to the chloride ion. The silver chloride solid (ΔG0 = -54 kJ/mol) is more stable than
the silver ammine complex (ΔGâ = -41 kJ/mol).
Reactant
CO32NH3
ClS2O32IS2-
Approx. [ ]
0.091
0.94
0.64
0.09
0.33
0.06
K
6.2 x 10-12
1.23 x 107
1.2 x 10-10
1.1 x 1013
1.5 x 10-16
1.6 x 10-49
[Ag+]eq
6.4 x 10-7
3.1 x 10-9
2.0 x 10-10
7.4 x 10-13
4.7 x 10-16
1.9 x 10-24
ReferenceThis experiment is modeled after experiment E21 of the Doing Chemistry series.
Key Words 1precipitation, complex ion, predominant species, equilibrium constant, solubility product, simultaneous
reactions, photographic processes
ElementsAg Cl I S N