Reactivity of Silver and Tarnish
Location: Great Britain
Date: July 2007
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 ?
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
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."
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Update: June 2012