Redox GeochemistryWHY?J. Willard GibbsEquilibrium ConstantActivityIons in solutionActivity CoefficientsSpeciationMass Action & Mass BalanceGeochemical modelsOxidation – Reduction ReactionsHalf ReactionsHalf-reaction vocabulary part IIELECTRON ACTIVITYTHE pe OF A HALF REACTION - IDEFINITION OF EhFree Energy and ElectropotentialElectropotentialsElectromotive SeriesSlide 20ELECTRON TOWERSlide 22Microbes, e- flowNAD+/NADH and NADP+/NADPHSlide 25Proton Motive Force (PMF)HOW IS THE PMF USED TO SYNTHESIZE ATP?ATP generation IIATPMinimum Free Energy for growthREDOX CLASSIFICATION OF NATURAL WATERSThe Redox ladderRedox GeochemistryWHY?•Redox gradients drive life processes!–The transfer of electrons between oxidants and reactants is harnessed as the battery, the source of metabolic energy for organisms•Metal mobility redox state of metals and ligands that may complex them is the critical factor in the solubility of many metals–Contaminant transport–Ore deposit formationJ. Willard Gibbs•Gibbs realized that for a reaction, a certain amount of energy goes to an increase in entropy of a system.•G = H –TS or G0R = H0R – TS0R•Gibbs Free Energy (G) is a state variable, measured in KJ/mol or Cal/mol•Tabulated values of G0R available…)reactants()(000iiiiiiRGnproductsGnGEquilibrium Constant•for aA + bB cC + dD:•Restate the equation as:GR = G0R + RT ln Q• GR= available metabolic energy (when negative = exergonic process as opposed to endergonic process for + energy) for a particular reaction whose components exist in a particular concentrationQRTaaaaRTbBaAdDcClnln Activity•Activity, a, is the term which relates Gibbs Free Energy to chemical potential:i-G0i = RT ln ai•Why is there now a correction term you might ask…–Has to do with how things mix together–Relates an ideal solution to a non-ideal solutionIons in solution•Ions in solutions are obviously nonideal states! •Use activities (ai) to apply thermodynamics and law of mass actionai = imi•The activity coefficient, i, is found via some empirical foundationsActivity Coefficients•Extended Debye-Huckel approximation (valid for I up to 0.5 M):•Where A and B are constants (tabulated), and a is a measure of the effective diameter of the ion (tabulated)IaBIIIAz2.0log21212Speciation•Any element exists in a solution, solid, or gas as 1 to n ions, molecules, or solids•Example: Ca2+ can exist in solution as: Ca++ CaCl+ CaNO3+ Ca(H3SiO4)2 CaF+ CaOH+ Ca(O-phth) CaH2SiO4 CaPO4- CaB(OH)4+ CaH3SiO4+ CaSO4 CaCH3COO+ CaHCO3+ CaHPO40 CaCO30•Plus more species gases and minerals!!Mass Action & Mass Balance•mCa2+=mCa2++MCaCl+ + mCaCl20 + CaCL3- + CaHCO3+ + CaCO30 + CaF+ + CaSO40 + CaHSO4+ + CaOH+ +…•Final equation to solve the problem sees the mass action for each complex substituted into the mass balance equationlcnciHLCHCL][][][][nxLmCamCa22Geochemical models•Hundreds of equations solved iteratively for speciation, solve for GR•All programs work on same concept for speciation thermodynamics and calculations of mineral equilibrium – lots of variation in output, specific info…Oxidation – Reduction Reactions•Oxidation - a process involving loss of electrons.•Reduction - a process involving gain of electrons.•Reductant - a species that loses electrons.•Oxidant - a species that gains electrons.•Free electrons do not exist in solution. Any electron lost from one species in solution must be immediately gained by another.Ox1 + Red2 Red1 + Ox2LEO says GERHalf Reactions•Often split redox reactions in two:–oxidation half rxn e- leaves left, goes right•Fe2+ Fe3+ + e-–Reduction half rxn e- leaves left, goes right•O2 + 4 e- 2 H2O•SUM of the half reactions yields the total redox reaction 4 Fe2+ 4 Fe3+ + 4 e- O2 + 4 e- 2 H2O 4 Fe2+ + O2 4 Fe3+ + 2 H2OHalf-reaction vocabulary part II•Anodic Reaction – an oxidation reaction•Cathodic Reaction – a reduction reaction•Relates the direction of the half reaction:•A A+ + e- == anodic•B + e- B- == cathodicELECTRON ACTIVITY•Although no free electrons exist in solution, it is useful to define a quantity called the electron activity:•The pe indicates the tendency of a solution to donate or accept a proton.•If pe is low, there is a strong tendency for the solution to donate protons - the solution is reducing.•If pe is high, there is a strong tendency for the solution to accept protons - the solution is oxidizing.eape logTHE pe OF A HALF REACTION - IConsider the half reactionMnO2(s) + 4H+ + 2e- Mn2+ + 2H2O(l)The equilibrium constant isSolving for the electron activity242eHMnaaaK2124HMneKaaaDEFINITION OF EhEh - the potential of a solution relative to the SHE.Both pe and Eh measure essentially the same thing. They may be converted via the relationship:Where = 96.42 kJ volt-1 eq-1 (Faraday’s constant).At 25°C, this becomesorEhRTpe303.2Ehpe 9.16peEh 059.0Free Energy and Electropotential•Talked about electropotential (aka emf, Eh) driving force for e- transfer•How does this relate to driving force for any reaction defined by Gr ?? Gr = - nE–Where n is the # of e-’s in the rxn, is Faraday’s constant (23.06 cal V-1), and E is electropotential (V)•pe for an electron transfer between a redox couple analagous to pK between conjugate acid-base pairElectropotentials•E0 is standard electropotential, also standard reduction potential (write rxn as a reduction ½ rxn) – EH is relative to SHE (Std Hydrogen Electrode)At non-standard conditions:dDcCbBaAHHaaaanFRTEE ln0dDcCbBaAHHaaaanVEE log0592.00At 25° C:Electromotive Series•When we put two redox species together, they will react towards equilibrium, i.e., e- will move which ones move electrons from others better is the electromotive series•Measurement of this is through the electropotential for half-reactions of any redox couple (like Fe2+ and Fe3+)–Because Gr =-nE, combining two half reactions in a certain way will yield either a + or – electropotential (additive, remember to switch sign when reversing a rxn)+E - Gr, therefore spontaneous•In order of
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