Expt 063 -- One Pot Silver Reactions

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.)

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2. Place 30 drops 0.1 M AgNO₃ into one well of a 24-well plate.

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3. Add 1 drops 1 M NaHCO₃. Stir. Note and record evidence of change.

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4. Add 5 drops 0.2 M Na₃PO₄. Stir. Note and record evidence of change.

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5. Add 2 drops 1.0 M NaOH. Stir. Note and record evidence of change.

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6. Add 5 drops 1 M NaCl. Stir. Note and record evidence of change.

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7. Add 25 drops 6.0 M NH₃. Stir. Note and record evidence of change.

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8. Add 5 drops 0.1 M KBr. Stir. Note and record evidence of change.

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9. Add 4 drops 1.0 M Na₂S₂O₃. Stir. Note and record evidence of change.

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10. Add 20 drops 0.1 M Na₂S. Stir. Note and record evidence of change.

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11. List each of the different silver ions or compounds observed with the respective color.

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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)₂^3- zero or just very small? Explain the answer.
3. A solutions contains I^-, S^2-, 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

Watch the movies.

List each reagent added and the formula of each observed silver ion or compound with its respective color.

Answer the questions.

Curriculum-

This 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.

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.

Time-

Teacher Preparation: 20 minutes

Class Time: 20 minutes

Materials-

• 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.
• 24-well plate
• toothpick
• plastic transfer pipets
• cotton swab

Disposal-

Filter 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-

• 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)

• 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 PO₄^3- --> 2 Ag₃PO₄ + 3 CO₃^2-

  3. Silver hydroxide is less soluble than silver phosphate:

     Ag₃PO₄ + 3 OH^- --> 3 AgOH + PO₄^3-

  4. Silver hydroxide is less soluble than silver hydroxide:

     AgOH + Cl^- --> AgCl + OH^-

  5. Silver chloride dissolves in ammonia:

     AgCl + 2 NH₃ --> Ag(NH₃)₂⁺ + Cl^-

  6. 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 S₂O₃^2- --> Ag(S₂O₃)₂^3- + Br^-

  8. 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)₂^3- + I^- --> AgI + 2 S₂O₃^2-

  9. 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 + S^2- --> Ag₂S + 2 I^-

• 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.

Answers-

Q1. 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)₂^3- 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^-, S^2-, 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 Use-

A spreadsheet can be used with the equilibrium constants to determine equilibrium concentrations.

Literature Data-

Equilibrium constants are taken from Clifford, A. F. Inorganic Chemistry of Qualitative Analysis, Prentice-Hall: Englewood Cliffs, NJ, 1961, p. 448-468.

Reaction	ΔG0 (kJ/mol Ag+)
Ag+ + 1/2 CO32- --> 1/2 Ag2CO3	-31
Ag+ + 2 NH3 --> Ag(NH3)2+	-41
Ag+ + Cl- --> AgCl	-54
Ag+ + 2 S2O32- --> Ag(S2O3)3-	-75
Ag+ + I- --> AgI	-92
Ag+ + 1/2S2- --> 1/2 Ag2S	-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).

Reference-

This experiment is modeled after experiment E21 of the Doing Chemistry series.

Key Words 1-

precipitation, complex ion, predominant species, equilibrium constant, solubility product, simultaneous reactions, photographic processes

Elements-

Ag Cl I S N