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Lecture 20 The Redox Sequence Oxidation State Half-Reactions Balanced Oxidation-Reduction reactions Predicted Sequence of Redox Reactions Tracers for these reactions Distributions in Nature The organic carbon that reaches the sediments drives sedimentary diagenesis. Many elements in the periodic table can exist in more than one oxidation state. Oxidation states are indicated by Roman numerals (e.g. (+I), (-II), etc). The oxidation state represents the "electron content" of an element which can be expressed as the excess or deficiency of electrons relative to the elemental state. Oxidation States Element Nitrogen Sulfur Iron Manganese Oxidation State N (+V) N (+III) N (O) N (-III) S (+VI) S (+II) S (O) S(-II) Fe (+III) Fe (+II) Mn (+VI) Mn (+IV) Mn (+III) Mn (+II) Species NO3NO2N2 NH3, NH4+ SO42S2O32S° H2S, HS-, S2Fe3+ Fe2+ MnO42MnO2 (s) MnOOH (s) Mn2+ Oxidation / Reduction Reactions One Reactant: is oxidized – it loses electrons = the e- doner (a reductant) is reduced – it gains electrons = the e- acceptor (an oxidant) Example: CH2O + O2 e- donor e- acceptor ↔ CO2 + e-acceptor H2O e-donor Why is organic matter an electron donor? photosynthesis Z-scheme for photosynthetic electron transport Falkowski and Raven (1997/2007) ADP→ATP Energy Scale Ferredoxin ADP→ATP to Krebs cycle and carbohydrate formation photooxidation of water e- from water energy from sun converted to C-C, energy rich, chemical bonds Redox half-reactions Redox reactions are written as half-reactions which are in Ox + ne- = Red; the form of reductions ∆G°r K where the more oxidized form of an element is on the left and the reduced form is on the right. n is the number of electrons transferred. We can write an equilibrium constant for this reaction as we can any other reaction. Formally the concentrations should be expressed as activities. Thus: K = (Red) / (Ox)(e-)n We can also rearrange the equation to determine the activity of the electron for any redox couple: (e-) = [ (Red) / K (Ox) ] 1/n Electron activities are usually expressed on either the pE or Eh scales as shown below. pE = - log (e-) = 1/n [logK - log (Red)/(Ox) ] or Eh = 2.3 RT pE / F Redox Half Reactions written as reductants in terms of 1 e- Balanced Redox Reactions A balanced reaction has an electron passed from an electron donor to an electron acceptor. Thus: Ox1 + Red2 = Red1 + Ox2 In this case Red2 is the electron donor, passing electrons to Ox1 which is the electron acceptor. Thus Red2 is oxidized to Ox2 and Ox1 is reduced to Red1. The equilibrium constant for an oxidation-reduction reaction can be determined by combining the constants from Table 1 as follows for O2 with glucose The two half reactions (written as reductions in terms of one electron) with their appropriate values of log K, are: (Rxn 1) 1/4 O2(g) + H+ + e- = H2O pE° = log K = 20.75 (Rxn 18) 1/4 CO2(g) + H+ + e- = 1/24 C6H12O6 + 1/4 H2O pE° = -0.20 We reverse reaction 18 (now it's log K = +0.20) and add it to reaction 1 to get: 1/4 O2(g) + 1/24 C6H12O6 = 1/4 CO2(g) + 1/4 H2O log K = 20.75 + 0.20 = 20.95 Don’t like fractions: x 24 to get 6 O2(g) + C6H12O6 = 6 CO2(g) + 6 H2O log K = 20.95 x 24 = 502.80 Ideal Redox Sequence There is an ideal sequence of redox reactions driven by e- rich organic matter that is based on the energy available for the microbes that mediate the reactions. In this sequence organic matter is combusted in order by O2 → NO3 → MnO2 → Fe2O3 → SO42- (decreasing energy yield). Most of these reactions have slow kinetics if not mediated by bacteria. Bacteria mediate most of these reactions and get the energy for their life processes. Because the energy of the sun is trapped in the C-C bonds, bacteria are indirectly using sunlight when they combust natural organic matter to CO2. Bacteria use the electron acceptors in the order of decreasing energy availability. e- acceptors Electron-Free Energy Diagram Photosynthesis e- donors Energy Scale Oxidation-Reduction reaction log K log Kw Aerobic Respiration 1/4CH2O + 1/4O2 = 1/4H2O + 1/4CO2(g) 20.95 20.95 Denitrification 1/4CH2O + 1/5NO3 + 1/5H+ = 1/4CO2(g) + 1/10N2(g) +7/20H2O 21.25 Manganese Reduction 1/4CH2O + 1/2MnO2(s) + H+ = 1/4CO2(g) + 1/2Mn2+ + 3/4H2O 21.0 Iron Reduction 1/4CH2O + Fe(OH)3(s) + 2H+ = 1/4CO2(g) + Fe2+ + 11/4 H2O 16.20 Sulfate Reduction 1/4CH2O + 1/8SO42- + 1/8H+ = 1/4CO2(g) + 1/8HS- + 1/4H2O 5.33 Methane Fermentation 1/4CH2O = 1/8CO2(g) + 1/8CH4 3.06 Tracers are circled 19.85 17.0 8.2 3.7 3.06 Free energy available ∆Gr° = - 2.3 RT logK = -5.708 logK R = 8.314 J deg-1 mol-1 T = °K = 273 + °C Organic Matter Degradation (using Redfield stoichiometry) “OM” = (CH2O)106(NH3)16(H3PO4) Indicator species are circled Photosynthesis 106CO2 + 16 NO3- + HPO42- + 18H+ 122 H2O → “OM” + 138 O2 Respiration Aerobic Respiration 138 O2 + “OM” + 18 HCO3- → 124 CO2 + 16 NO3- + HPO42- + 140 H2O Denitrification 94.4 NO3- + “OM” → 13.6 CO2 + 92.4 HCO3- + 55.3 N2 + 84.8 H2O + HPO42Manganese Oxide Reduction 236 MnO2 + “OM” + 364 CO2 + 104 H2O → 470 HCO3- + 8N2 + 236 Mn2+ + HPO42Iron Oxide Reduction 212 Fe2O3 + “OM” + 742 CO2 + 318 H2O → 848 HCO3- + 16 NH3 + 424 Fe2+ + HPO42Sulfate Reduction 53 SO42- + “OM” → 39 CO2 + 67 HCO3- + 16 NH4+ + 53 HS- + 39 H2O + HPO42Methane Fermentation “OM” → 53 CO2 + 53 CH4 + 16 NH3 + HPO42- + 2H+ Multi-colored sediments! What’s going on here??? Ideal redox sequence in sediments and anoxic basins Froelich et al (1979) Aerobic Respiration in Red Clay Sediments Murray and Grundmanis, 1980 Denitrification in Oxygen Minimum Zones Oxygen Minimum Zones Eastern Tropical North Pacific NO3- → NO2- → N2O → N2 → NH4+ Denitrification and MnO2 reduction – Hemipelagic Sediments Guatemala Basin, North Pacific Emerson et al Iron reduction – Hemipelagic Sediments East Pacific Rise – North Pacific Emerson et al Saanich Inlet, BC Sediments Sulfate Reduction Methane Production Anaerobic Methane Consumption Kuivila and Murray The Black Sea Why is the Black Sea Interesting to Oceanographers? 1. The classic anoxic basin. Oxic layer over sulfidic layer. 2.Model for modern and ancient anoxic environments. 3. Well developed transition or suboxic zone. Model for world’s organic rich sediments. 4. Suboxic reactions easy to study here because of predictable depth locations. 5. An ideal location to study effect of climate forcing on ocean distributions. A well formed suboxic zone exists in central western gyre. Data from Knorr 2001 Depth Density Results from Knorr 1988 Cruise (w/ Pump Profiling System) Oxygenated surface layer over a sulfide rich deep layer with a suboxic zone at the interface O2 H2S Suboxic Zone NO3 NO2 Murray et al (1995) Mn Fe NH4 PO4