Just as we use the O2 we breathe to oxidize the organic chemicals (i.e., food) w
ID: 997334 • Letter: J
Question
Just as we use the O2 we breathe to oxidize the organic chemicals (i.e., food) we eat to CO2(g) and H2O(L), many aerobic microorganisms oxidize organic chemicals, such as benzene, using O2 dissolved in water as electron acceptor. After dissolved O2 is used up, some microbes can use nitrate (NO3 —) as an electron acceptor to oxidize benzene. And when NO3 — is exhausted, other (anaerobic) organisms can degrade benzene using Mn4+ (e.g., MnO2), Fe3+ (e.g., Fe2O3), sulfate (SO4 = ), and finally CO2 as an electron acceptor. f Go = –84.9 kJ/mol for Fe2+ (aq), –744.8 kJ/mol for SO4 = (aq), and –27.8 kJ/mol for H2S(aq). Show, quantitatively, that this electron acceptor sequence for organic compound degradation in the environment can be predicted by free energy. Write a balanced equation for microbial oxidation of 1 mole of C6H6(L) using O2(g), Fe2O3(S), or SO4 = (aq) as electron acceptor. Calculate the r Go for each reaction, and explain the EA sequence based on your r Go values.
Explanation / Answer
Sequentially the reactions are:
1. C6H6 + 7.5 O2 + 3 H2O 6 HCO3- + 6 H+
2. C6H6 + 6 NO3- 6 HCO3- + 3 N2
3. C6H6 + 18 H2O + 30 Fe3+ 6 HCO3- + 30 Fe2+ + 36 H+
4. C6H6 + 3 H2O + 3.75 SO42 6 HCO3- + 1.875 H2S + 1.875 HS- + 0.375 H+
5. C6H6 + 6.75 H2O 2.25 HCO3- + 3.75 CH4 + 2.25 H+
You have not given the value of G0 for all the compounds in the reactions. The values are necessary to calculate G for the reactions.
Use the equation , G of reaction = sum of G of products - sum of G of reactants
and calculate the G values for the reactions.
G value should be more negative for the first reaction and least for the last reaction. More negative G value indicates that the first reaction is more thermodynamically favorable. This explains the sequence of the reactions.
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