Uses of Standard Potentials

Standard cell potentials find many uses as sources of thermodynamic information of interest. We have already encountered the relation ΔGº_r =  – νFEº. From this, and the thermodynamic relation (δG/δT)_P = – S , we obtain:

we can now use the fundamental relation ΔGº_r = ΔHº_r  –  T ΔSº_r to write

Standard potentials may also be used to construct an electrochemical series:

Consider a general cell, composed of two redox couples Ox₁/Red₁ and Ox₂/Red₂ , with associated standard potentials Eº₁ and Eº₂. We know that for the following cell, the cell potential may be written as shown:

Red₁ | Ox₁ || Ox₂ | Red₂ , Eº = Eº₂ – Eº₁

Further, we know that the cell reaction, Red₁  +  Ox₂  =>  Ox₁ +  Red₂ , is spontaneous as written if Eº > 0, i.e. if Eº₂ > Eº₁ .

Since in the cell reaction  Red₁ reduces Ox₂ to Red₂ , we can conclude that Red₁ is more reducing than Red₂ (otherwise the reaction would go in the other direction) , and hence, quite generally, the lower the standard potential of a couple, the more powerfully reducing the reduced species in the couple is.

This knowledge may be used to construct the electrochemical series, a list of the metallic elements in order of their reducing ability.

Quite simply, this series is the metals from metal/metal ion couples listed in order of their standard potentials in aqueous solution; any metal may reduce the ions of a metal with a larger value of Eº.

The list can be extended to include non-metallic elements. (Note that this information is purely thermodynamic, and as always reactions which are thermodynamically possible may not be observed for kinetic reasons.)

Partial Electrochemical series in order from least reducing (largest Eº) to most reducing (smallest Eº)

(Note that the table can also be viewed as reading from most to least reducing.) As can be seen from the table, typical electrode potentials lie within the range ± 3 V.