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Iron is a redox-active metal element present in marine sediments and occurs in two oxidation states (Fe2+ and Fe3+). Fe3+ typically precipitates with OH in seawater to form amorphous Fe hydroxide particles (Fe(OH)3 (am)). Examine the table and answer the following questions: + Reaction (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) Electrochemical Energies of Aquatic Redox Half Reactions. pe" (= log k) pew Ei(W) 3H2O2+Ht+e- =H2O +30.0 +23.0 +1.36 102(g)+H++e- = }H20 +20.75 +13.75 +0.81 NO3 + H+e- = to N2(g) + H20 +21.05 +12.65 +0.75 MnO2 (s) + 2H+ +e- = Mn2+ +H20 +20.8 +9.86 +0.58 2NO, +H++e- = NO2 + 2H2O +14.15 +7.15 +0.42 NO3 + H+ +e- NHẸ + 2H2O +14.9 +6.15 +0.36 NO; + H+ + = NH; + 3H20 +15.2 +5.82 +0.34 2O2(9)+H+ +e- = H202 +11.5 +4.5 +0.27 Fe(OH)2(am)+3H+ +e- = Fe2+ + 3H2O + 16.0 +1.06 +0.06 CH3OH+H+ +e- = CH4(9) + 3H20 +9.88 +2.88 +0.17 "CH20"+H* +e = CH (9) + H2O +6.94 -0.06 +0.00 50% + H+e- = S(Col)+ 3H20 +5.9 -3.4 -0.20 50%-+*+- = 3H2S(9) + 3H2O +5.25 -3.50 -0.21 50% + H+ +e = BHS + 4H20 +4.25 -3.75 -0.22 "CH20"+H++e- = CH3OH +3.99 -3.01 -0.18 HCO3 + H+ + = CH.(9) + H20 +3.8 -4.0 -0.24 CO2+H*+e- = CH.(g) + H2O +2.9 -4.13 -0.24 S8(col)+H+ +e- = {H (9) +3.2 -3.89 -0.22 Tess (col)+ 3H++e- = HS -0.8 -4.3 -0.25 N2 + 4H+ + +4.65 -4.7 -0.28 H++e = 3H2(9) +0.00 - 7.0 -0.41 HCO3 + H++e = "CH,0"+ 2H20 +1.8 - 7.0 -0.41 + + + (20) NH (21) (22)

a) [3 pts] In some environments, Fe(OH)3 (am) can serve as an electron accepter to oxidize organic matter. Please write down the two half reactions and the overall reaction. b) [2 pts] Could the above reaction occur when oxygen level is high? Why or why not? c) [3 pts] Under anoxic environments, if Fe2is available, some microorganisms can drive the oxidation of Fe2+ to amorphous iron hydroxides, Fe(OH)3 (am), to gain energy for growth using oxygen as the electron acceptor (oxidant; electron moves from Fe2+ to O2). What is the reaction of Fe2+ oxidation by oxygen? list the complete reaction and also the two half reactions. d) [4 pts] In anoxic sediments, another group of microbes can oxidize Fe2+ using NO3- as the electron acceptor (electron moves from Fe2+ to NO3-) and NO3- can be reduced to various forms with different oxidation states, NO2 , N2, and NH4+. List the reactions of Fe2+ oxidation by NO3 to NO2, N2, and NH4*, respectively (three reactions). e) [3 pts] Which of the above reaction in question d) yields the highest energy at standard state?