Considering the following reactions and boundary conditions, construct a pe-pH speciation diagram for the iron system consisting of Fe(s)- Fe2+ – Fe(OH)2(s) – Fe3+ – Fe(OH)3 in freshwater. Use 1.0 x 10-5 M as the maximum soluble [Fe2+] or [Fe3+].

Given:      

Reactions:

Fe3+  +  e-  ====  Fe2+ Fe2+   +  2 e-  ===Fe(s)

Fe(OH)3(s)  + 3H++  e-   ==== Fe2+ + 3H2O

Fe(OH)3(s)  +  H++ e-=== Fe(OH)2(s) + H2O

Fe(OH)2(s)  +  2 H+  +  2 e-  ====  Fe(s)  +  2 H20  

Fe(OH)3(s)  +  3 H+  ====  Fe3+ 3 H2O  

Fe(OH)2(s)  + 2 H+  ====  Fe2+  + 2H2O

Thermodynamic Data for Fe(s)- Fe2+ – Fe(OH)2(s)- Fe3- Fe(OH)3 system: 

Ksp (Fe(OH)2) = 4.8 X 10-17

Ksp (Fe(OH)3) = 1.0 X 10-38

Standard Free Energy of Formation (∆G0f) in kJ/mol

e-

 (aq)H+

(aq)OH-

 (aq)H2O(l)H30+(aq)Fe(s)

Fe2+(aq)

Fe3+ (aq)Fe(OH)2 (s)Fe(OH)3(s)

00-157.3-237.2-237.20-78.9-4.60-486.6-699

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