This reaction happens in nature so I can't deny it of course.
This is what I thought about the Half cell reactions
Oxidizing half cell reaction
The oxidizing half cell potential value was taken by this document link below pg 3.
Ag2S + 2e- → 2Ag + S2- E = -0.691 V SHE
Reducing half cell reaction
2H2O + 2 e- -> H2 + 2OH- E = -0.828 V SHE
Ecell = Ecathode(re) - Eanode(ox)
Ecell = -0.828 V + 0.691 V = -0.137 V
We get a negative value for Ecell thus non spontaneous of course.
According to the equation of thermodynamics
But aren't we talking about Silver Tarnish so its a poluted media with acidic H+ from H2S
Reducing reaction depends on pH acidity off the polluted media
Reducing reaction pH dependence equation
Reffer here link
E = Ecell - (RT/2F)ln([OH-]2) (H2 is in 1 atm pressure standard state)
See at acidic medium the E value gets higher thus
Ecell = (-0.828 V <-- ++++) + 0.691 V = (-0.137 V <--- ++++);
When the cell voltage is positive thermodynamic-ally feasible thus reaction happens
(ΔG < 0 spontaneous reaction)
2Ag + S2- + 2H2O -> H2 + 2OH- + Ag2S
But where dose S2- come from its H2S
2Ag + 2H+ + S2- + 2H2O -> H2 + 2H+ + 2OH- + Ag2S
Simplifying water goes away!
There we have it final answer
2Ag + H2S⟶Ag2S + H2