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Non Life Sciences => Chemistry => Topic started by: Hugo_P on 30/05/2013 13:51:04

Title: Why do some metal sulfides generate acid when they oxidize, but not all?
Post by: Hugo_P on 30/05/2013 13:51:04

Hello! Does anyone have an idea why metal sulfides having a metal/sulfide ratio <1 produce (sulfuric) acid when they are oxidized by oxygen (for example FeS₂, MoS₂), while metal sulfides having a metal/sulfide ratio >= 1 don´t generate any acid during the same oxidation? In the case of the acid producing sulfides, water always takes part in the reaction (thus supplying hydrogen) but that isn´t the case for the other sulfides.

Title: Re: Why do some metal sulfides generate acid when they oxidize, but not all?
Post by: damocles on 31/05/2013 02:01:14

Well, yes! It is fairly obvious from the stoichiometry, although the chemistry of FeS₂ and MoS₂ is a little different:

FeS + 2 O₂ ==> FeSO₄
PbS + 2 O₂ ==> PbSO₄

but

FeS₂ + ⁵/₂ O₂ + H₂O ==> FeSO₄ + H₂SO₄

The case of MoS₂ is somewhat more complicated.

MoS₂ + 6¹/₂ O₂ + 4 H₂O ==> MoO₃.2H₂O + 2 H₂SO₄

Title: Re: Why do some metal sulfides generate acid when they oxidize, but not all?
Post by: damocles on 31/05/2013 02:41:19

Here is the overall picture: Monovalent and divalent sulfides ( M₂S and MS) are either ionic or network compounds. They will roast to produce the corresponding ionic sulfates (M₂⁺SO₄^2– or M²⁺SO₄^2–.

Trivalent sulfides mostly do not exist; I suspect that when they do they will be exceptions to the rule.

Substances with formula MS₂ or higher are all network compounds. They can be
• equivalent to FeS₂ if they are effectively polysulfides of a metal with a stable +1 or +2 oxidation state, reacting to form an ionic sulfate and sulfuric acid, or
• sulfides of metals with a higher oxidation state -- like MoS₂ -- which will normally react to produce a network oxide and sulfuric acid.

Title: Re: Why do some metal sulfides generate acid when they oxidize, but not all?
Post by: SorryDnoodle on 31/05/2013 09:40:25

Can this method be used to produce concentrated sulfuric acid? Specifically using iron sulphate?

Title: Re: Why do some metal sulfides generate acid when they oxidize, but not all?
Post by: damocles on 31/05/2013 13:18:18

Quote from: SorryDnoodle on 31/05/2013 09:40:25

Can this method be used to produce concentrated sulfuric acid? Specifically using iron sulphate?

FeS₂ is Iron(II) disulfide -- pyrite or marcasite.
It certainly could be used to produce concentrated sulfuric acid.

Even monovalent or divalent sulfide minerals -- CuS, CuFeS₂, PbS, if they are roasted in the open air, will produce CuO, Fe₂O₃, Pb₃O₄ etc., along with SO₂ and a small amount of H₂SO₄, which will be emitted as gases that damage the environment.

If the effluents are collected, purified, and passed over a V₂O₅ catalyst you can obtain SO₃, which is the material you need for making sulfuric acid. Collection of the effluents also reduces the damaging effects on the environment.

http://en.wikipedia.org/wiki/Contact_process