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

we can now use the fundamental relation ΔGºr = ΔHºr  –  T ΔSºr 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)

Electrochemical Series Redox Couple (Ox/Red) Eº/V

Fluorine

F2/F

+ 2.87

Gold

Au3+/Au

+ 1.40

Chlorine

Cl2/Cl

+ 1.36

Platinum

Pt2+/Pt

+ 1.20

Bromine

Br2/Br

+ 1.09

Silver

Ag+/Ag

+ 0.80

Mercury

Hg22+/Hg

+ 0.79

Iodine

I2/I

+ 0.54

Copper

Cu2+/Cu

+ 0.34

Hydrogen

H+/H2

0

Lead

Pb2+/Pb

– 0.13

Tin

Sn2+/Sn

– 0.14

Nickel

Ni2+/Ni

– 0.23

Iron

Fe2+/Fe

– 0.44

Zinc

Zn2+/Zn

– 0.76

Chromium

Cr2+/Cr

– 0.91

Aluminium

Al3+/Al

– 1.66

Magnesium

Mg2+/Mg

– 2.36

Sodium

Na+/Na

– 2.71

Calcium

Ca2+/Ca

– 2.87

Barium

Ba2+/Ba

– 2.91

Potassium

K+/K

– 2.93

Lithium

Li+/Li

– 3.05

(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.