One Pot Silver Reactions

Description

A series of reactions is performed in a sequence such that a visible change appears in a system. First, solid silver carbonate precipitate is produced. It is tan in color. This precipitate is dissolved in ammonia to give a colorless solution. Sodium chloride produces a white precipitate which dissolved in sodium thiosulfate to give a colorless solution. Sodium iodide gives a yellow solid that turns black on the addition of sodium sulfide.

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The sequence of reactions is arranged so that, for each succeeding step, silver is reacted with some constituent of the newly added reactant. The reactions are driven in that direction.

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1. Add 300 mL of 0.05 M AgNO₃ to a tall form 1-L beaker.
2. Add 30 mL of 1 M Na₂CO₃ and stir. Note evidence of reaction.
3. Add 150 mL of 3.0 M NH₃ and stir. Note evidence of reaction.
4. Add 60 mL of saturated NaCl and stir. Note evidence of reaction.
5. Add 210 mL of 0.32 M Na₂S₂O₃ and stir. Note evidence of reaction.
6. Add 150 mL of 2 M NaI. and stir. Note evidence of reaction.
7. Add 120 mL of 0.5 M Na₂S and stir. Note evidence of reaction.
8. Note the different ions and compounds observed during the experiment.

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For each succeeding addition, a more stable silver compound or complex is formed. Note the thermodynamic data and the relative equilibrium constants.

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⁺ + CO₃^2- Ag₂CO₃

Addition of ammonia causes the silver ion to be complexed as Ag(NH₃)₂±. This complex is formed at the expense of silver carbonate. The silver carbonate supports a higher concentration of Ag± than the silver ammine complex.

Ag₂CO₃ + 4 NH₃ 2 Ag(NH₃)₂⁺ + CO₃^2-

The concentration of Ag± in equilibrium with the silver ammine complex, at these concentrations, is greater than silver chloride will support. For this chloride ion concentration, then, silver chloride forms at the expense of the silver ammine complex.

Ag(NH₃)₂⁺ + Cl^- AgCl + 2NH₃

Addition of a source of thiosulfate ions allows the formation of the silver(I) thiosulfate complex. This complex forms at the expense of silver chloride. The concentration of Ag⁺ in the solution is still lower:

AgCl + 2 S₂O₃^2- Ag(S₂O₃)₂^3- + Cl^-

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-

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 + 2I^-

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Name ___________________________ Class ________

Teacher__________________________

DoChem 109 One Pot Silver Reactions

Watch the movie and record the colors of each mixture and whether a precipitate is present.

Predict the effect of adding the sodium sulfide solution before adding the sodium iodide solution.

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Name ___________________________ Class ________

Teacher__________________________

DoChem 109 One Pot Silver Reactions

Watch the movies and record the colors of each mixture and whether a precipitate is present.

Predict the effect of adding the sodium sulfide solution before adding the sodium iodide solution.

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Purpose

To illustrate that some reactions near completion more than others.

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• 1-L tall form beaker
• 1 large stirring rod
• 500-mL graduated cylinder
• 0.05 M AgNO₃ (300 mL, 8.5 g/L)
• 2 50-mL graduated cylinder
• 1 M Na₂CO₃ (30 mL, 106 g/L)
• 3 M NH₃ (30 mL, 200 mL conc. NH₃ /L)
• 100-mL graduated cylinder
• saturated NaCl (60 mL, 5.3 M, 36 g/100 mL H₂O)
• 3 250-mL graduated cylinder
• 0.32 M Na₂S₂O₃ (210 mL, 51 g/L)
• 2 M NaI (150 ml, 300 g/L)
• 0.5 M Na₂S (120 mL, 39 g/L)

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• Mixing the solutions out of order ruins the demonstration.
• Silver nitrate is expensive. The reactions may be accomplished with half of the recommended volumes in circumstances where all students will still be able to see clearly.
• You may wish to make a transparency of the equilibrium data handout and display on the overhead during the demonstration or during discussion after the demonstration.

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Teacher preparation: 30 minutes.

Presentation: 15-25 minutes

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Sulfide and silver solutions are toxic. The sulfide fumes and ammonia fumes are toxic. Silver nitrate causes black stains on skin.

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Store sulfide and ammonia solutions under hood until just before use. Make the sulfide solution fresh, and use only the amount required. Wash any silver spills immediately. Provide adequate ventilation.

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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.

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Presentation Question:

• Predict the effect of adding the sodium sulfide solution before adding the sodium iodide solution.
  • The sulfide compound is more stable than the iodide compound. Therefore, adding the iodide after the sulfide will have no effect upon the sulfide mixture. The mixture will remain black when the iodide is added.

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There are many practical circumstances in which sequences of reactions are used. In photography, the unexposed, unreduced, insoluble silver halide is removed from the emulsion after development using sodium thiosulfate. Several cleaning agents contain chelating agents intended to form soluble complexes of such insoluble compounds as CaSO₄ from bath tubs and sinks. Many proteins in the body serve as complexing agents to assist in the transport of needed metal ions.

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• See Discussion section.
• If any of the reagents are added out of sequence, the compounds and complexes missed will never be formed as the predominant species.

Background -- Reaction Free Energies

An excellent way to look at this series of observations is to examine the ΔG° for the reactions. These are tabulated below: 

Background -- Equilibrium Constants

A different way to look at these reactions is to examine their equilibrium constants. (Of course, the equilibrium constants are related to the free energies.) Below there is a Table in which the equilibrium concentration of Ag⁺ has been calculated assuming only one reacting species to be present. The concentration of that species has been corrected for the dilution conditions of the experiment. For example, when the 0.32 M thiosulfate solution was added, 210 mL was added to a volume of 540 mL, so the approximate total volume was 750 mL. It was also assumed that nearly all of the silver was tied up in the complex or precipitate. Therefore, the equilibrium concentration of the reacting species was lowered by the stoichiometric amount needed to bring about complete reaction.

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The sulfide compound is more stable than the iodide compound. Therefore, adding the iodide after the sulfide will have no effect upon the sulfide mixture. The mixture will remain black when the iodide is added.

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• precipitation
• complex ion
• predominant species
• equilibruim
• constant
• solubility product
• simultaneous reactions

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