H2S as strong reductant?
Posted: Wed Oct 15, 2008 12:56 am
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!
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!