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Published byRudy Marmion Modified about 1 year ago

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Nernst Equation Consider the half reaction: NO H + + 8e - NH H 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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Let’s assume that the concentrations of NO 3 - and NH 4 + have been measured to be M and 3 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 r G° = 3(-237.1) + (-79.4) - (-110.8) = kJ mol -1

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

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Reaction directions for 2 different redox couples brought together?? More negative potential reductant // More positive potential oxidant Example – O 2 /H 2 O vs. Fe 3+ /Fe 2+ O 2 oxidizes Fe 2+ is spontaneous! Biology’s view upside down?

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Stability Limits of Water H 2 O 2 H + + ½ O 2(g) + 2e - Using the Nernst Equation: Must assign 1 value to plot in x-y space (P O2 ) Then define a line in pH – Eh space

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

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As for the pe-pH diagram, we assume that p O 2 = 1 atm. This results in This yields a line with slope of

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LOWER STABILITY LIMIT OF WATER (Eh-pH) Starting with H + + e - 1/2H 2 (g) we write the Nernst equation We set p H 2 = 1 atm. Also, G r ° = 0, so E 0 = 0. Thus, we have

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O2/H2O C 2 HO

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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, P O2 -pH, T-[x], [x]-[y], [x]/[y]-[z], etc

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Construction of these diagrams For selected reactions: Fe H 2 O FeOOH + e H + How would we describe this reaction on a 2-D diagram? What would we need to define or assume?

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How about: Fe H 2 O FeOOH (ferrihydrite) + 3 H + K sp =[H + ] 3 /[Fe 3+ ] log K=3 pH – log[Fe 3+ ] 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???)

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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)

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

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