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b) “Defend” this by determining the thermodynamic favorability of these different terminal electron-accepting processes, assuming the potential terminal electron-accepting processes would be aerobic respiration (O2 reduction), Mn(IV) (pyrolusite) reduction (to Mn2+), Fe(III) (ferrihydrite) reduction (to Fe2+), sulfate reduction (so sulfide), and methanogenesis. Before calculating the energetic favorability of these processes, produce balanced reactions depicting these terminal electron-accepting processes based on H2 as the electron donor (be sure to normalize all reactions on the basis of one mole of H2).
c) Explain how the thermodynamic favorability of the different terminal electron-accepting processes. A table that includes ∆G0f related to this question is provided at the end of the exam.
d) Why might it be “scientifically defensible” to use H2 for these calculations, and not benzene?
Requirements: long enough to show the work and drawings and answer the questions.
Aerobic Respiration (O2 reduction):
H2 + 0.5O2 → H20 + 2e- ∆G0f = -687kJ/mol
Mn(IV) (pyrolusite) Reduction (to Mn2+):
4H2 + MnO2 → Mn2+ + 2H20 + 4e- ∆G0f = -551kJ/mol
Fe(III) (ferrihydrite) Reduction (to Fe2+):
4H2 + Fe203 → 3Fe2+ + 4H20 + 8e- ∆G0f = -622kJ/mol
Sulfate Reduction (SO sulfide):
3H202+ SO42- → S8s-+ 6OH-∆G0f = -1211 kj/mole Methanogenesis: 8CH4→ CO32- + 12H++ 12e− ∆Go f= −137.9 KJ/mole
The thermodynamic favorability of the different terminal electron accepting processes can be determined by comparing the corresponding reaction’s standard free energy change, or Delta G, for each process. The delta G values for these five reactions are given in the table below:
Reaction ΔGₒ⁰ [kj mol⁻¹] Thermodymanically Favorable
Aerobic respiration (-687) Yes Mn(IV)(pyrolusite reduction (-551)) Yes
Fe(III)(ferrihydrite reduction (-622)) Yes Sulfate reduction (-1211) Yes Methanogenesis (-137.9) No
Total Phosphorylation Potential [-1816]
From this data, we can see that aerobic respiration and Mn(IV), Fe III, and sulfate reductions are all thermodynamically favorable processes with a negative delta G value indicating they are exothermic and release energy during the reaction while methanogenesis is not thermodynamically favorable since it has an endothermic delta G value of 137.9 kj/mol indicating that energy must be put into the system to drive this reaction forward. Additionally, we can calculate the total phosphorylation potential using equation PV=ΔGₒ⁰ which yields a total phosphorylation potential of – 1816 kj mol–1 when we sum up all five electron acceptance processes together with H₂ as our electron donor normalized on one mole of hydrogen gas being used as electron donor material for these 5 terminal acceptors.. This means that 1816 kj of energy could theoretically be harvested from 1 mole of hydrogen gas when used in conjunction with these five terminal acceptors assuming all other metabolic requirements such as cofactors are available to complete their respective redox reactions efficiently.. By understanding how much free energy is released during these various redox reactions then scientists and engineers can determine more efficient ways to generate useful forms of chemical and electrical power from renewable sources such as hydrogen gas.
It is scientifically defensible to use H₂ for calculations because it is a readily available molecule found in nature that contains two hydrogen atoms bound together via covalent bonds capable of donating electrons during redox reactions due to its low oxidization state number compared to higher oxidation numbers like benzene which contain six carbon atoms bonded together instead making them much less suitable donors than diatomic hydrogen molecules due to their increased complexity . Additionally, hydrogen molecules have no charge so they move freely through membranes without requiring active transport mechanisms or ATP hydrolysis making them prime candidates for efficient electrochemical generation systems capable of powering modern technology devices without much effort or expense involved in their production or collection from natural sources containing large concentrations such as water vapor or biomass combustion products..