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Nernst Equation

Nernst Equation. Consider the half reaction: NO 3 - + 10H + + 8e -  NH 4 + + 3H 2 O(l) We can calculate the Eh if the activities of H + , NO 3 - , and NH 4 + are known. The general Nernst equation is The Nernst equation for this reaction at 25°C is.

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Nernst Equation

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  1. Nernst Equation Consider 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 is The Nernst equation for this reaction at 25°C is

  2. Let’s assume that the concentrations of NO3- and NH4+ have been measured to be 10-5 M and 310-7 M, respectively, and pH = 5. What are the Eh and pe of this water? First, we must make use of the relationship For the reaction of interest rG° = 3(-237.1) + (-79.4) - (-110.8) = -679.9 kJ mol-1

  3. The Nernst equation now becomes substituting the known concentrations (neglecting activity coefficients) and

  4. Biology’s view  upside down? Reaction 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!

  5. 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 space

  6. UPPER STABILITY LIMIT OF WATER (Eh-pH) To determine the upper limit on an Eh-pH diagram, we start with the same reaction 1/2O2(g) + 2e- + 2H+ H2O but now we employ the Nernst eq.

  7. As for the pe-pH diagram, we assume that pO2 = 1 atm. This results in This yields a line with slope of -0.0592.

  8. LOWER STABILITY LIMIT OF WATER (Eh-pH) Starting with H+ + e- 1/2H2(g) we write the Nernst equation We set pH2 = 1 atm. Also, Gr° = 0, so E0 = 0. Thus, we have

  9. O2/H2O C2HO

  10. Making 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], etc

  11. Construction 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?

  12. 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???)

  13. 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)

  14. Redox titration II • Let’s modify a bjerrum plot to reflect pe changes

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