Presentation on theme: "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."— Presentation transcript:
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
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
The Nernst equation now becomes substituting the known concentrations (neglecting activity coefficients) and
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?
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
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.
As for the pe-pH diagram, we assume that p O 2 = 1 atm. This results in This yields a line with slope of
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
O2/H2O C 2 HO
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
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?
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???)
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 changes