Nernst EquationSlide 2Slide 3Slide 4Stability Limits of WaterUPPER STABILITY LIMIT OF WATER (Eh-pH)Slide 7LOWER STABILITY LIMIT OF WATER (Eh-pH)Slide 9Slide 10Making stability diagramsConstruction of these diagramsSlide 13Redox titrationsRedox titration IINernst EquationConsider the half reaction:NO3- + 10H+ + 8e- NH4+ + 3H2O(l)We can calculate the Eh if the activities of H+, NO3-, and NH4+ are known. The general Nernst equation isThe Nernst equation for this reaction at 25°C isQnRTEEh log303.2010034log80592.0HNONHaaaEEhLet’s assume that the concentrations of NO3- and NH4+ have been measured to be 10-5 M and 310-7 M, respectively, and pH = 5. What are the Eh and pe of this water?First, we must make use of the relationshipFor the reaction of interest rG° = 3(-237.1) + (-79.4) - (-110.8) = -679.9 kJ mol-1nGEor0 volts88.0)42.96)(8(9.6790EThe Nernst equation now becomessubstituting the known concentrations (neglecting activity coefficients)and 1034log80592.088.0HNONHaaaEh volts521.01010103log80592.088.010557E h81.8)521.0(9.169.16 EhpeReaction directions for 2 different redox couples brought together?? More negative potential reductant // More positive potential oxidant Example – O2/H2O vs. Fe3+/Fe2+ O2 oxidizes Fe2+ is spontaneous!Biology’s view upside down?Stability Limits of Water•H2O 2 H+ + ½ O2(g) + 2e-Using the Nernst Equation:•Must assign 1 value to plot in x-y space (PO2)•Then define a line in pH – Eh space202121log0592.0HOapnEEhUPPER STABILITY LIMIT OF WATER (Eh-pH)To determine the upper limit on an Eh-pH diagram, we start with the same reaction1/2O2(g) + 2e- + 2H+ H2Obut now we employ the Nernst eq.202121log0592.0HOapnEEh202121log20592.0HOapEEhAs for the pe-pH diagram, we assume that pO2 = 1 atm. This results inThis yields a line with slope of -0.0592.2212log0296.023.1HOapEhpHpEhO0592.0log0148.023.12 volts23.1)42.96)(2()1.237(00nGErpHEh 0592.023.1 LOWER STABILITY LIMIT OF WATER (Eh-pH)Starting withH+ + e- 1/2H2(g)we write the Nernst equationWe set pH2 = 1 atm. Also, Gr° = 0, so E0 = 0. Thus, we havepHEh 0592.0HHapEEh212log10592.00O2/H2OC2HOMaking stability diagrams•For any reaction we wish to consider, we can write a mass action equation for that reaction•We make 2-axis diagrams to represent how several reactions change with respect to 2 variables (the axes)•Common examples: Eh-pH, PO2-pH, T-[x], [x]-[y], [x]/[y]-[z], etcConstruction of these diagrams•For selected reactions:Fe2+ + 2 H2O FeOOH + e- + 3 H+How would we describe this reaction on a 2-D diagram? What would we need to define or assume?230log10592.0FeHaaEEh•How about:•Fe3+ + 2 H2O FeOOH(ferrihydrite) + 3 H+Ksp=[H+]3/[Fe3+]log K=3 pH – log[Fe3+]How would one put this on an Eh-pH diagram, could it go into any other type of diagram (what other factors affect this equilibrium description???)Redox titrations•Imagine an oxic water being reduced to become an anoxic water•We can change the Eh of a solution by adding reductant or oxidant just like we can change pH by adding an acid or base•Just as pK determined which conjugate acid-base pair would buffer pH, pe determines what redox pair will buffer Eh (and thus be reduced/oxidized themselves)Redox titration II•Let’s modify a bjerrum plot to reflect pe changesGreg Mon Oct 25 2004-4 -2 0 2 4 6 8 10 125060708090100peSome species w/ SO4--
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