In sediments and waterlogged soil, dissolved O2 concentrations are so low that the microorganisms living there must rely on other sources of oxygen for respiration. Some bacteria can extract the oxygen in sulfate ions, reducing the sulfur in them to hydrogen sulfide gas and giving the sediments or soil a distinctive rotten-egg odor.

Required:
a. What is the oxidation number of sulfur in these compounds?
b. Write the net ionic equation for the reaction under acidic conditions (H3O+) that releases O2 from sulfate and forms hydrogen sulfide gas. Use water as the reactant In the hall-reaction that describes the formation of oxygen.

a) Sulfate reducers are widespread in muds and other sediments, water-logged soils, etc., environments that contain SO₄ ²⁻ and become anoxic as a result of microbial decomposition.

Sulfate (SO₄ ²⁻), the most oxidized form of sulfur (+6), is reduced by these

sulfate-reducing bacteria. The end product of sulfate reduction is hydrogen sulfide, H₂S, (oxidation number -2) an important natural product that participates in many biogeochemical processes. The H₂S they generate is responsible for the pungent smell (like that of rotten eggs) often encountered near coastal ecosystems. When sulfate-reducing bacteria grow, the H₂S formed from SO₄ ²⁻ reduction combines with the ferrous iron to form black, insoluble ferrous sulfide, which is not toxic. This is important for the conservation of the environment.

b) The net ionic equation under acidic conditions is:

4 H₂ + SO₄²⁻ + H⁺ → HS⁻ + 4 H₂O

Global reaction: SO₄²⁻ + 2H⁺ → H₂S + O₂