In sediments and waterlogged soil, dissolved O2 concentrations are so low that t

Question

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. What is the oxidation number of sulfur in these compounds? SO4^2-(aq): H2S(g): 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 half-reaction that describes the formation of oxygen. Include phases in the balanced chemical equation. Type an open parenthesis "(" to add a phase. Phases should not be subscripted. Use the left and right arrow keys to move the cursor out of a superscript or subscript in the module.

Explanation / Answer

a)

SO4-2

first,

S + 4(O) = -2

note that O will be -2 oxidizes (typically)

S + 4(-2) = -2

S -8 = -2

S = -2 + 8

S = +6

ii)

H2S = 0, since it is neutral

2H + S = 0

note that H = +1 almost always so

2(+1) + S = 0

S = -2

b)

net ionic equation in:

SO4-2 + H+ --> H2S(g)

First, define the “ACIDIC” solution/conditions as H+ presence and

Basic solution implies OH- once it is balanced.

Also; note that ALL species must be balanced, as well as charges

Typical steps:

1) split half redox cells

2) balance atoms other than O,H

3) balance O by adding H2O

4) balance H by adding H+

5) balance charge by adding e-

6) balance e- by multiplying by the Greatest common divisor

7) Add both equations

8) simplify repeating elements, H+, H2O, and e- typically

9) add OH- if we need basic media, otherwise this is done.

10) Simplify again

balance O:

SO4-2 + H+ = H2S + O2

SO4-2 + H+ = H2S + 2O2

SO4-2 + 2H+ = H2S + 2O2

Add H2O for H3O+ presence:

2H2O + SO4-2 + 2H+ = H2S + 2O2 + H2O

SO4-2 + 2H3O+ = H2S + 2O2 + H2O

add phases

SO4-2(aq) + 2H3O+(aq) = H2S(g) + 2O2(g) + H2O(l)

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