[Zacch]

Looking at some standard redox potentials, I am having trouble seeing why H₂S is frequently described as a strong reductant, and would appreciate some help.

My reading of the tables is thus:

1. S + 2H⁺ + 2e  <-> H₂S is slightly thermodynamically favourable. Bad news.

2. S^--  <-> S + 2e is also thermodynamically favourable. The sulphide ion is about as strong a reductant as Cu(s), which is to say not all that strong.

So I must be missing something. H₂S is reputedly a stronger reductant than, say, FeS, but why? Does HS^- and water dissociation come into it? Does pH matter? Is the product of the reaction not S(s) but some more oxidised form of sulphur?

Thanks for any enlightenment!

PS: I tried to use a search-engine for "H₂S", but couldn't find a SE!