oxidation-reduction - io-warnemuende.de chemie... · ¾many elements in the periodic table can...
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Oxidation-Reduction
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Many elements in the periodic table can exist in more than one oxidation state.
Oxidation states are indicated by Roman numerals in parentheses (e.g. (+I), (-IV) etc.).
The oxidation state can be expressed as the excess or deficiency of electrons relative to the elemental state.
Element Oxidation State SpeciesNitrogen N (+V)
N (+III)N (O)N (-III)
NO3-
NO2-
N2NH3, NH4
+
Sulfur S (+VI)S (+II)S (O)S(-II)
SO42-
S2O32-
S¼H2S, HS-, S2-
Iron Fe (+III)Fe (+II)
Fe3+
Fe2+
Manganese Mn (+VI)Mn (+IV)Mn (+III)Mn (+II)
MnO42-
MnO2 (s)MnOOH (s)Mn2+
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Why study redox (reduction/oxidation) reactions:
Redox reactions are a major determinant of chemical species present in natural environments.
Redox reactions are characterized by the transfer of electrons between chemical species, so that they change their oxidation state
Full reaction2 Fe2+ + MnO2 + 4 H+ ⇔ 2 Fe3+ + Mn2+ + 2H2OHalfreactionsFe2+ ⇔ Fe3+ + e-
Mn2+ + 2H2O ⇔ MnO2 + 4H+ + 2e-
Easier
Zn° - 2e Zn2+ an oxidation half reaction, electrons lost
Cu2+ + 2e Cuº an reduction half reaction, electrons gained
Cu2++ Znº Cuº + Zn2+ Full reaction
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Galvanische Zelle (Daniell-Element)
Elektronenaustausch an der Phasengrenze Metallstab / wässrige Lösung
Elektrisches Potential = “Druck” mit dem die e- in den Draht gezwungen werden.
Quellspannung (E) = Potentialdifferenz zwischen den beiden Elektrode [V]
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Galvanische Zelleaus einer Norm-Wasserstoff-Elektrode und einer Cu2+ I Cu-Elektrode
= Referenzelektrode zur Messung des Potentials (Elektronendruck) im Metallstab (= Potential zwischen Metallstab und Lösung)
Normalpotential (Standardpotential, E0) = Relativwerte bezogen auf die Normal-Wasserstoffelektrode, deren Standardpotential willkürlich gleich Null gesetzt wurde
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Elektrochemische Spannungsreihe
Normal- Standardbedingungen:• 25°C• 1,013 bar• c (H3O+) = 1mol L-1 (bzw. a = 1)
Normal-Wasserstoffelektrode
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Elektrochemische Spannungsreihe
• Hinweis auf Reduktionspotential
• je ausgeprägter bei einem Redoxpaardie Tendenz der reduzierten Form ist,Elektronen abzugeben, um so schwächerist die Tendenz der korrespondierendenoxidierten Form, Elektronen aufzunehmen.
• Redoxpaare werden aufgrund dieser Tendenzin eine Redoxreihe (ECSR) angeordnet.
• Redoxprozesse laufen nur freiwillig zwischeneiner reduzierten Form mit einer in der ECSRdarunter stehenden oxidierten Form ab.
Starke Oxidationsmittel (e- - Akzeptoren)
Starke Reduktionsmittel (e- - Donatoren)
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Quellspannung (E, Elektromotorische Kraft EMK)der Galvanischen Zelle
• Halbreaktion an der Anode in umgekehrter Reihenfolge (Wert = -E0)
• Addition der E0-Werte ergibt die Quellspannung der elektrochemischen Zelle
• Nur dann, wenn die Quellspannung der Gesamtreaktion positiv ist,kommt es zur Reaktion!
Nur bei Standardbedingungen!!!
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NERNST-Gleichung
Bei Abweichungen von den Standardbedingungen
Berechnung des EMK eines Redoxpaares (Halbzelle)oder eines Redoxsystems (Zelle)
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NERNST-Gleichung - Abweichung der Konzentration
Zur Berechnung des EMK eines Redoxpaares (Halbzelle)
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Zur Berechnung des EMK eines Redoxpaares (Halbzelle)
[Ox] = Produkt der Konzentrationen aller Reaktionsteilnehmer auf der Seite der oxidiertenForm (Oxidationsmittel) des Redoxpaares.
[Red] = Produkt der Konzentrationen aller Reaktionsteilnehmer auf der Seite der reduziertenForm (Reduktionsmittel) des Redoxpaares.
NERNST-Gleichung - Abweichung der Konzentration
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Zur Berechnung des EMK eines Redoxpaares (Halbzelle)
NERNST-Gleichung - Abweichung der Konzentration
bei Metallreaktionen (25°C)=log c (Mn+)
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Zur Berechnung des EMK eines Redoxsystems (Zelle)
NERNST-Gleichung - Abweichung der Konzentration
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NERNST-Gleichung - Abweichung vom pH-Wert
Zur Berechnung des EMK eines Redoxpaares (Halbzelle)
• Potentiale einiger Redoxsysteme sind stark von pH-Wert abhängig
• Das Redoxverhalten dieser Systeme kann nicht mehr ausschließlich aus denStandardpotentialen vorhergesagt werden.
oxidierte Formred. Form
oxidierte Form
reduzierte Form
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Freie Energie (G)
• Chemische Reaktionen werden von Energieänderungen begleitet.
• Für die Mikrobiologie ist die freie Energie (G) wichtig, hierunter verstehenwir die Energie, die für nützliche Dinge zur Verfügung steht! (ATP-Synthese)
• Veränderungen der freien Energie während einer Reaktion wir durch ∆G0‘ [Joule](= Reaktionsenthalpie) ausgedrückt („0“ und „ ‘ “ = Standardbedingungen)
Freie Bildungsenthalpie G0f = Energiegehalt oder Energie, die benötigt wird, um eine
Verbindung aus den jeweiligen Elementen zu bilden
Produkt Edukt
∆G ≈ ∆U – T ∆S ST = isotherm nicht verfügbare Energie∆U = Änderung der inneren Energie
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Redox reactions
• Über die Gibbs´sche freie Energie läßt sich bestimmen in welche Richtung eine Reaktion aA + bB ⇔ cC + dD abläuft.
• Unter Standardbedingungen gilt:
∑∑ −=Δ )()(Pr0 EdukteGnodukteGnG iiiiR
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Redoxreaktionen
• Allgemein gilt zur Berechnung der Gibbs´schen freien Energie:
[ ] [ ][ ] [ ]ba
dc
RRBADCRTGG ln0 ⋅+Δ=Δ
[ ] [ ][ ] [ ]ba
dc
RBADCRTG ln0 ⋅−=Δ kRTGR ln0 ⋅−=Δ
• bei Gleichgewicht gilt:oder
R=Gaskonstante [kJ/(Grad mol), T=Temperatur [K]
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Redoxreaktionen
• Elektrochemische Energie ist nur eine andere Form der Gibbs´schen freien Energie. Es gilt folgende Verknüpfung:
– n=Anzahl der ElektronenübergängeF=Faraday-konstante (96.42 kJ/(Vg(eq))E=Spannungspotential (V)
00 nFEGR =Δ
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Produkt Edukt
Stabile Verbindung: ∆G0f < 0
Instabile Verbindung: ∆G0f > 0
Freie Energie (G)
• Chemische Reaktionen werden von Energieänderungen begleitet.
• Für die Mikrobiologie ist die freie Energie (G) wichtig, hierunter verstehenwir die Energie, die für nützliche Dinge zur Verfügung steht! (ATP-Synthese)
• Veränderungen der freien Energie während einer Reaktion wir durch ∆G0‘ [Joule](= Reaktionsenthalpie) ausgedrückt („0“ und „ ‘ “ = Standardbedingungen)
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Die Änderung der freien Enthalpie ∆G0‘ ist das eigentliche Kriterium für die Reaktionsfähigkeit (Triebkraft). Sie beschreibt die reale (thermodynamische) Möglichkeit für den Ablauf einer Reaktion.
- ∆G0‘ = Reaktion kann in der angegebenen Richtung ablaufen.
+∆G0‘ = Reaktion kann in der angegebenen Richtung nicht ablaufen.
Produkt Edukt
Freie Energie (G)
• Chemische Reaktionen werden von Energieänderungen begleitet.
• Für die Mikrobiologie ist die freie Energie (G) wichtig, hierunter verstehenwir die Energie, die für nützliche Dinge zur Verfügung steht! (ATP-Synthese)
• Veränderungen der freien Energie während einer Reaktion wir durch ∆G0‘ [Joule](= Reaktionsenthalpie) ausgedrückt („0“ und „ ‘ “ = Standardbedingungen)
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Redoxreaktionen
• Die elektromotorische Kraft ∆E0 einer galvanischen Zelle ist ein Maß für die freie Reaktionsenthalpie (∆G0, Joule)
n = Anzahl der ElektronenübergängeF = Faraday-konstante (96.42 kJ/(Vg(eq))∆E0 = Potentialdifferent [V]
00 EnFG Δ=Δ
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1/4O2 + H+ + e- ↔ 1/2 H2O ,
20.8 = – 1/4 log pO2 + pH + pe
when pO2 = 1atm, pe = 20.8 – pH
oxic
suboxic
anoxicpe
pH
Lower boundary for stability of water:
H+ + e- ↔ 1/2 H2
log Keq = pH + pe
since log Keq = 0, pe = −pH
pe vs pH Stability Fields
13.8
8.1
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(CH2O)106(NH3)16(H3PO4) +138O2 →
106CO2 + 16HNO3 + H3PO4 +122H2O
(CH2O)106(NH3)16(H3PO4) +138O2 →
106CO2 + 16HNO3 + H3PO4 +122H2O
Oxidized & Reduced species
Electron donor: ‘fuel’Electron acceptor: O2, NO3
-, Fe2O3, MnO2, SO4-2, CO2
CH2O + O2 → CO2 + H2O (aerobic respiration)
Composed of two half reactions:Reduction: O2 + 4H+ + 4e- 2H2O Oxidation: CH2O + H2O CO2 + 4H+ + 4e-
Respiration
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most favorable
least favorable
EnergeticallyMicrobial Redox-Chain
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Marines Beispiel (1):
Schwarzes Meer
Wassersäule
Wasserkörper geschichtet(Stratifizierung)
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Vertical mixing processes in the Black Sea basin. The approximate depth range of these processes in meter is displayed on the left bar (after Özsoy et al., 1993).
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most favorable
least favorable
Energetically
Marines Beispiel (1): Schwarzes Meer Wassersäule
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Ammonification(NH2 groups of protein > NH3)
Nitrification
Denitrification
Sulfate ReductionMethane ProductionAmmonification(NH2 groups of protein > NH3)
Main features of the vertical biogeochemical structure of the upper Black Sea water column.
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Marines Beispiel (2):
Sauerstoff-Minimum Zonen (OMZ)
OMZ
CH2O + O2 → CO2 + H2O (intense aerobic respiration)
OMZupwelling
high bioproductivity
remineralization > nutrient rich deep water
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Mn+2 + O2 MnO2 (s)
Mn- distribution
OMZ
OMZ
Korrekt2MnO2 + 4H+ ↔ 2Mn+2 + O2 + 2H2O
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Marines Beispiel (3): Hydrothermale Quellen
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Marines Beispiel (3): Hydrothermale Quellen
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Marines Beispiel (3): Hydrothermal Quellen
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Fe+2 + O2 Fe2O3Mn+2 + O2 MnO2
Fe+2 + S-2 FeS
Marines Beispiel (3): Hydrothermale Quellen
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Marines Beispiel (3): Hydrothermale Quellen
SPP1144, MAR, IFM-GEOMAR
ROV, MARUM (Bremen)
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Marines Beispiel (4): Schwarzes Meer, Kalte Quellen
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Marines Beispiel (4): Schwarzes Meer, Kalte Quellen
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Cold Vent Abbildung
CH4 + SO4 → HS- + HCO3- +H2O
Anaerobic Oxidation of Methane (AOM)
Marines Beispiel (4): Schwarzes Meer, Kalte Quellen
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Marines Beispiel (4): Schwarzes Meer, Kalte Quellen
GHOSTDABS, Paleo Dnepr Area, IFBM (HH)
JAGO, IFM-GEOMAR (Kiel)
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Early diagenesisDefinition (modified after Berner 1980):
Early diagenesis refers to the sum total of physical, chemical and biological processes that bring about changes in unconsolidated surface sediments subsequent to deposition in water.
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Sedimentkern Gotlandtief (Zentrale Ostsee)
Gotlandtief
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Diagenetic sequence - Primary redox reactions
-3190 kJ/mol
-3090 kJ/mol
-2750 kJ/mol
-1410 kJ/mol
-380 kJ/mol
-350 kJ/mol
Oxidation by dissolved Oxygen
(CH2O)106(NH3)16(H3PO4) + 138 O2
106 CO2 + 16 HNO3 + 122 H2O + H3PO4
Oxidation by Manganese-Oxides
(CH2O)106(NH3)16(H3PO4) + 236 MnO2
236 Mn2+ + 106 CO2 + 8 N2 + 366 H2O + H3PO4
Oxidation by Nitrate
(CH2O)106(NH3)16(H3PO4) + 84.4 HNO3
106 CO2 + 42.2 N2 + 16 NH3 + 148.4 H2O + H3PO4
Oxidation by Iron-Oxides
(CH2O)106(NH3)16(H3PO4) + 212 Fe2O3 + 848 H+
424 Fe2+ + 106 CO2 + 16 NH3 + 530 H2O + H3PO4
Oxidation by Sulphate
(CH2O)106(NH3)16(H3PO4) + 53 SO42-
106 CO2 + 16 NH3 + 53 S2- + 106 H2O + H3PO4
Methane Fermentation
(CH2O)106(NH3)16(H3PO4) 53 CO2 + 53 CH4 + 16 NH3 + H3PO4
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The diagenetic sequence
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The diagenetic sequenceSecondary redox processes, e.g.
Fe2+ + H2S + 2OH-→FeS + 2H2O
2Fe(OH)3 + H2S →2Fe2+ + 2H2O + OH- + S0
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The diagenetic sequence
• Understanding pore water profiles
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Early diagenesis on a global scale
30 40 50 75 100 125 150 20020
PP [gC m-2 yr-1]
Antoine et. al. (1996)
Global distribution map of the surface ocean
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Calculation of the POC-flux from Primary Production
(Export Production)
Early diagenesis on a global scale
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POC-fluxes from the surface water to the
sediment in gC·m-2·yr -1
Early diagenesis on a global scale
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Global Benthic Mineralization-RatesOxidant Mineralization in
1x1014 gC a-1 Percentage Source
Total
18-31
26
23
100 Middelburg et al. (1997)
Smith & Hollibaugh (1993)
Jørgensen (1983)
O2 23
17
11
~57
(total minus anoxic)
Jørgensen (1983)
Henrichs &Reeburgh (1983)
Jahnke (1996) – deep-sea only
NO3 0.93
0.96
2.4-3.0
~11
Jørgensen (1983)
Christensen et al. (1987)
Middelburg et al. (1996)
SO4 1.3-13
4.4
~26
Henrichs &Reeburgh (1983)
Canfield (1993)
CH4 0.44-0.9 3.8 Canfield (1993)
Mn-Oxides 0.23-0.46 1.4 Canfield (1993)
Fe-Oxides 0.07-0.23 0.6 Canfield (1993)
Early diagenesis on a global scale
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Early diagenesis on a global scale
30 40 50 75 100 125 150 20020
PP [gC m-2 yr-1]
Antoine et. al. (1996)
Global distribution map of the surface ocean
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Early diagenesis on a global scale
Seiter et. al. (2005)
Estimated mineralization rates
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Early diagenesis on a global scale
Seiter et. al. (2005)
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Early diagenesis on a global scale
Seiter et. al. (2004)
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Early diagenesisDefinition (modified after Berner 1980):
Early diagenesis refers to the sum total of physical, chemical and biological processes that bring about changes in unconsolidated surface sediments subsequent to deposition in water.
•Primary productivity is the driving force for most benthic activity•The amount of POC reaching the sea floor is determined by the export production (mineralization in the water column)•Early diagenetic processes in the sediments determine how much of the POC is mineralized and much organic carbon is buried on long time scales