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Name: Yogesh
Status: student
Grade: 9-12
Location: Great Britain
Country: N/A
Date: July 2007

Question:
The equation for silver tarnishing is: Ag + H2S AgS + H2. Then how does silver displace hydrogen from hydrogen sulphide though it is less reactive than hydrogen ?



Replies:
Yogesh,

The pertinent reaction is an oxidation-reduction; silver is oxidized and hydrogen is reduced. Thus, Ag = Ag(+) + e(-) [an oxidation] and 2H(+) + 2e(-) = H2 [a reduction]. A quick look at a standard reductions table shows the relative potentials for the half-cell reactions to be: 0.00V for the hydrogen transformation, and for the *reduction* of silver: +0.80V - which means that for the oxidation of silver the potential change is -0.80V. This tells me that the reaction for the tarnishing of silver cannot be through a direct reaction with hydrogen sulfide because the galvanic potential change for that reaction would be (-)0.80V; a non-spontaneous process.

Another look at the standard reductions table shows that the reduction of oxygen with hydrogen has a potential change of +1.23V. In combination with the oxidation of silver (-0.80V), this would give a potential change of +0.43V (a spontaneous process). This suggests to me that silver is oxidized by the presence of oxygen gas and acids (H+) in the air and that the final product, Ag2S, results from other transformations for the oxidized silver.

I believe that what you see written as: Ag + H2S = Ag2S + H2 is a short-hand or end-result of many transformations.

Greg (Roberto Gregorius)


I am not sure that H2S is the only substance that causes silver to "tarnish". In any case the reaction you propose is probably not the correct one. Silver almost always has an oxidation number of (+1) so the more likely stoichiometry is:

2 Ag + H2S ----> Ag2S + H2.

The heat generated by this reaction is -11.5 kJ and the change in free energy, which is a more precise measure of the reactivity is -7.4 kJ. In either case the reaction "goes" to completion.. I think that your definition of "reactivity" needs to be refined. It is more subtle than a list of "What displaces what."

Vince Calder



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