Although a host of chemical processes contribute to acid mine drainage, pyrite o

Question

Although a host of chemical processes contribute to acid mine drainage, pyrite oxidation is by far the greatest contributor. (Wikipedia on “acid-mine drainage” accessed 9 September 2017.) In this chemistry, the sulfur in iron pyrite (FeS 2 , also known as “fools gold”) is oxidized to sulfuric acid (H 2 SO 4 ), a strong, diprotic acid. Molecular oxygen (O 2 ) is the oxidizer. Enzymes produced by microorganisms catalyze the redox process. This problem focuses on writing a balanced redox reaction. [Hint: The oxidation state of Fe in FeS 2 is +II and it does not change in the process; the final state of the iron in this reaction is Fe 2+ .] (a) What is the oxidation state of S in FeS 2 ? (b) What is the oxidation state of S in H 2 SO 4 ? (c) Write a stoichiometrically balanced reaction for the process in which S in FeS 2 is oxidized to H 2 SO 4 in water with dissolved oxygen (O 2 ) as the oxidizer and with the Fe being converted to Fe 2+ .

Explanation / Answer

A) The oxidation state of S in FeS2 is, -1

Since the Fe is in +2 state to balance the compound S is in -1 state.

B) The oxidation state of S in SO4 is, +6

Since the H is +1 so total hydrogen charge is +2 and Oxygen is -2 so total oxygen charge is -8. The charge on S is -6 so the it balances the compound.

C)

4H+  + FeS2  + 4O2   = 2H2SO4+ Fe+2  

Here the actual reaction is,

H+  + FeS2  + O2   = H2SO4+ Fe+2 and the digits are stoichiometry balance.

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