H2S as strong reductant?

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Zacch
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H2S as strong reductant?

Post by 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.

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- 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!
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