GG325 -- GEOCHEMISTRY
1. Plot the pE and pH changes associated with each step in the model calculations in of
Fig 11.12 of lecture 11 notes (page 13) on a pE=PH diagram. Be sure to label your
axes and to also include the two lines for the upper and lower water stability limits.
2. Upon what half reaction is the rigorous definition of pE based?
3. The acid-base reaction for the dissociation of acetic acid is
CH3COOH + H2O H3O+ + CH3COO-
a) Break the reaction above into two half-reactions involving hydrogen ions, H+
b) Next, break the redox reaction for iron and hydrogen (below) into two redox half-
reactions.
Fe2+ + H+ Fe3+ + 2H2
Compare the acid- base reaction to the redox reaction by drawing an analogy between
the roles of H+ and e- in the two reactions.
4. What determines the oxidizing and reducing limits for the thermodynamic stability of
water (and thus, the exogenic hydrosphere)? Hint: Write chemical equations for the two
reactions that are involved in the decomposition of water outside of its stability limit (the
oxidation of water and reduction of water).
5. a) Calculate [Fe3+], pE and pH at the point in the figure where Fe2+, Fe(OH)2 and
Fe(OH)3 are in equilibrium, for maximum Fe concentration in solution of 10-5 M
b) Calculate the pE at the point
on the Fe2+ - Fe(OH)3 boundary
line where pH =5 in a solution with
a soluble iron concentration of 10-5
M?
c) Calculate the pE for a soluble
iron concentration of 10-4 M?
Ksp for the reaction Fe(OH)3 (s) +
3H+ Fe3+ + 3H2O is 9.1x103 pE =
13.2 + log [Fe3+]/[Fe2+] for the
reaction Fe2+ Fe3+ + 1e-
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