The Electron Balance and Equilibrium Calculations

In making equilibrium calculations for redox reactions it is often necessary to make use of the electron balance. The equation is analogous to the proton balance and is based on the principle that electrons are conserved. For example, when Cl a is added to a solution, the following half-reactions take place  

Given that these half-reactions are the only ones of importance and that no Cl or HOCl was present before the Cla was added, we obtain the electron balance by establishing an electron lelerence level (ERL) as the species that are either oxidized or reduced and with which the solution was prepared. In this example, the solution was prepared with HgO and 

but only CI2 is oxidized or reduced so it alone is the ERL. Each Cl formed has one electron more than the Cl dt the ERL each Cl in HOCl has one electron less than the ERL. Thus the electron balance is 

The ERL is Fe and Cr207 , One electron is produced for each Fe formed and three electrons are used in the formation of each Cr . Since there are no other sources or sinks of electrons and given that no Fe or Cr was in the solution prior to adding CraOy and Fe, the electron balance yields 

There are five unknowns (Eh, Ct.Fetti) CT.Fedin- CT,cr iii). and Ct.cfjOt) cmd thus five equations are needed. Because 1M H2SO4 is used, [H ] can be considered constant and it is thus not an unknown. 

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