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When silver objects are exposed to pollutants normally found in air, they gradually become dull and discolored. This darkening of silver is called tarnishing.

Materials | Procedure | Examining the Reactions

It occurs when silver undergoes a chemical reaction, classified as an oxidation reaction, with sulfur-containing substances. These substances may be airborne, but they can also be introduced through use, such as drinking wine from a silver goblet. The silver combines with sulfur and forms silver sulfide, Ag2S (see the equation below).

2 Ag  +    S   → Ag2S
silver + sulfur → silver sulfide

The reflective silver surface can be recovered in two ways: 1. by removing or dissolving the dark silver sulfide oxidation products, or 2. by chemically reducing the silver sulfide back to metallic silver. The first method removes some of the metallic silver along with the silver sulfide. The second option, to chemically reduce the silver sulfide, is recommended because it does not remove any silver, and it actually converts the black silver sulfide to metallic silver. Instructions for the second option are described in the procedure below.
 

Materials

  • Gloves and safety glasses
  • Tarnished piece of silver*
  • Pan or dish, preferably aluminum or Pyrex, large enough to completely immerse the silver object
  • Aluminum foil
  • Water
  • Sodium bicarbonate, NaHCO3 (baking soda)
  • Hot plate
  • Tongs (plastic or wood)
*See the supplementary information section in the appendix if you need to create tarnish.

Note: the waste from this experiment may be washed down the sink.

Procedure

Please read through the entire procedure before you begin.
Demonstration of hands-free silver polishing.
  • Put on gloves and safety glasses.
  • Line the bottom of the pan with aluminum foil.
  • Set the silver object on top of the aluminum, ensuring that the silver touches the aluminum.
  • Add water and baking soda (about one cup of baking soda for each gallon of water) to the pan to a level that covers the silver object. Heat the water to boiling using a hot plate. Almost immediately the tarnish will begin to disappear. If the silver is only lightly tarnished, all of the tarnish will disappear within several minutes. Keep the water boiling until all of the tarnish has been removed from the silver.
  • Turn off the hot plate.
  • Allow the water to cool, remove the object from the pan using tongs, and rinse it with water.

Examining the Reactions

In this experiment, the silver sulfide reacts with aluminum. Sulfur atoms are transferred from silver to aluminum, freeing the silver metal and forming aluminum sulfide. Chemists represent this reaction with the following equation:

3 Ag2S        + 2 Al0        → 6 Ag0 +        Al2S3
silver sulfide + aluminum → silver + aluminum sulfide

This type of reaction is called an electrochemical reaction because a tiny electric current flows between the silver and aluminum when the two are in contact and immersed in a baking soda solution. The addition of the baking soda improves the ease with which the electrons move between the silver and aluminum.

The reduction reaction restores metallic silver:

3 e- + 3 Ag+ → 3 Ag0 (s)

The aluminum oxidation reaction drives the reduction of silver:

Al0s → Al+3 (aq) +3 e-

Combining these two half reactions gives us the complete redox reaction:

Al0 (s) + 3 Ag+ → Al+3 + 3 Ag0 (s)

Referring to the Metal Activity Series helps us to understand why silver is reduced and aluminum is oxidized.

Aluminum is the reducing agent and thereby reduces silver sulfide to elemental silver while forming aluminum sulfide. We use boiling water because the reaction is faster when the solution temperature is higher. The solution carries sulfur released from the silver to the aluminum, where a layer of aluminum sulfide is formed. The aluminum sulfide then hydrolyzes to form aluminum hydroxide, Al(OH)3, and gaseous hydrogen sulfide, H2S. Did you detect a foul odor during this experiment? If the silver was heavily tarnished you may have! Hydrogen sulfide evolved in this reaction is the same smelly gas that rotting eggs give off. Can you write a balanced equation for the dissolution of aluminum sulfide in water?

  Al2S3 +   H2O →   Al(OH)3 +   H2S

What is the role of sodium bicarbonate? Aluminum in pure water would not remove silver tarnish because there is a film of aluminum hydroxide that stops the oxidation reaction. Sodium bicarbonate removes the surface film of aluminum hydroxide (by dissolving it) to expose fresh metallic aluminum, ready for oxidation. The dissolved sodium bicarbonate also increases the ionic strength of the solution, which increases the rate of the reaction.

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