Temperature Dependence of Silver Oxide Formation

Equipment

Bunsen burner or propane torch, lighter. If artificial tarnishing is necessary: 5 volt power supply and leads, container to hold silver object.

Reagents

Piece of tarnished silver. We use a pitcher, but any item large enough to be visible to the audience should work. Silver oxide powder will also work, though it doesn't have the same impact as a familiar household item. If artificial tarnishing is necessary: 1 M sodium hydroxide solution of sufficient quantity to dip silver object into it

Presentation

1. Go through the free energy calculations and show the temperature dependence of the silver oxide formation.
2. Display the tarnished item to the audience, note the tarnished blackened appearence. If your object is not tarnished sufficiently, follow the tarnishing procedures below before the demonstration.
3. Light the torch or Bunsen burner and heat a portion of the tarnished object. The black silver oxide will be replaced by metallic silver wherever the temperature has been raised sufficiently. Apply the heat carefully so as not to melt your object.

Electrochemical Tarnishing Procedure

1. Make up enough 1 M sodium hydroxide solution to immerse a significant portion of your item in.
2. Mechanically polish your item until it is smooth and shiny.
3. Immerse your item in the sodium hydroxide solution.
4. Connect the pitcher to the positive lead of the power supply. Connect the negative lead to a large counter electrode (graphite works well).
5. Turn on the power supply and adjust the voltage until the item blackens, this should require less than 5 volts.
6. Allow the item to to tarnish for approximately 10 minutes.
7. Turn off the power supply, disconnect the power supply leads, remove the item from the solution, and rinse thoroughly with deionized water.

Hazards

Either the propane torch or the Bunsen burner can produce an intense and very hot flame. Severe burns can result either directly from the flame or by touching objects heated in them. Solid sodium hydroxide and concentrated solutions can cause severe burns to eyes, skin, and mucous membranes.

Discussion

Silver metal will oxidize spontaneously upon exposure to free oxygen. This process is commonly referred to as "tarnishing". The chemical reaction describing this proces is shown below.

4 Ag  (s)  +  O2 (g) ® 2 Ag2O  (s)

Silver metal is a grayish white color, silver oxide is a black color. This contrast in colors makes tarnished silver appear much different in appearence than untarnished silver. This explains why so much physical and chemical effort is spent in removing the tarnish from silver objects.

Thermodynamic Constants of Compounds of Interest [1]

Compound	DHof (kJ/mol)	DGof (kJ/mol)	So (J/mol.K)
Ag (s)	0	0	42.6
O2 (g)	0	0	205.2
Ag2O (s)	-31.1	-11.2	121.3

The standard state enthalpy ( DH^o_rxn) and entropy ( DS^o_rxn) changes for the reaction are -62.2 kJ and -0.133 kJ/K respectively as calculated from the thermodynamic data in the above table. These values tell us that the reaction is exothermic and that the entropy of the reaction is negative. The decrease in entropy is to be expected when there are fewer moles of gaseous products than there were moles of gaseous reactants. The entropy and enthalpy terms are in conflict. The enthalpy term favors the reaction being spontaneous, but the entropy term favors the reaction being non-spontaneous. When the terms conflict in such a manner, the temperature at which the reaction occurs will determine the spontaneity. The following equation will allow the standard Gibb's free energy ( DG^o_rxn) of the reaction to be calculated. DG^o_rxn = DH^o_rxn - T DS^o_rxn             Eq. 1 Substituting the previously calculated values for the standard state enthalpy and entropy changes and the standard state temperature of 298 K into the previous equation yields: DG^o_rxn = -62.2 kJ - (298 K)(-0.133 kJ/K) DG^o_rxn = -22.6 kJ Since DG^o_rxn< 0, the reaction is spontaneous at room temperature. This agrees with our experience that silver does spontaneously tarnish as it sits in air. By rearranging Eq. 1 we may determine at what temperature the reaction would be at equilibrium ( DG^o_rxn = 0). NOTE: It is not entirely accurate to use standard state thermodynamic quantities away from T = 298 K, since they do have a temperature dependence to them, but this usually introduces an acceptably small error in the resulting calculations.
T = DH^o_rxn/ DS^o_rxn
T = (-62.2 kJ)/(-0.133 kJ/K)
T = 468 K For T < 468 K the reaction is spontaneous, for T = 468 K the reaction is at equilibrium and for T > 468 K the reaction would be non-spontaneous (or the reverse reaction, see reaction below, would be spontaneous). In order to remove the tarnish from our silver object all we need do is raise the temperature to above 468 K. The animated GIF below shows the temperature being raised by applying a propane torch's flame to the side of a badly tarnished silver pitcher.

2 Ag2O  (s)  ®  4 Ag (s)  +  O2 (g)

References

1. Electronic version, CRC Handbook of Chemistry and Physics, 81st ed., link