1. Consider the following half-cell redox reaction involving NO3^- at a concentr

Question

1. Consider the following half-cell redox reaction involving NO3^- at a concentration of 10^-5M (mol/L) and ammonium at a concentration of 10^-3 M in groundwater with a pH of 8. What is the Eh of the system at 25 degrees Celsius?

1/8NO3^- + 5/4H^+ +e^- 1/8 NH^4+ +3/8H2O

Thermodynamic data     Delta Gf (KJ/mol)

Al^3+                                                    -485.0

Ni^2+                                                    -45.6

NH4^+                                                  -79.31

NO3^-                                                   -108.74

SO4^2-                                                 -177.75

HS^-                                                      2.93

H2O                                                       -237.129

Explanation / Answer

Gf = Gi - RT ln(K)

K=Keq = [NH4+]^(1/8)*[H2O]^(3/8)/([NO3-]^1/8 * [H+]^(5/4))

= (10^-3)^(1/8)*1/((10^-5)^(1/8)*[10^-8]^(5/4))

=1.77*10^10

no Gi = (-79.31/8) - (3*237.129/8) + (108.74/8) + 0

= -85.244

or Gf = -85.244 - 8.31*273 *ln(1.77*10^10)

=-53.617 kJ/mol

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