Standard electrode potentials

The electrochemical series described in a basic level article is, in fact, a tabulation of the half-cell standard electrode potentials, Eo, of various chemical species. Just as the metal reactivity series compares the reactivity of metals, the electrochemical series lists the reactivity of metals and ions in an electrochemical cell. A summary of the electrochemical series is as follows:-

The more positive the potential of a half-cell is, the more spontaneous its reaction is from left to right. Hence, when we connect two half-cells,

the overall cell potential is:


For example, the combined redox reaction of Zn2+/Zn and Cu2+/Cu is

E_{right}\; or\; E_{red,cathode}:H_2(g)+Cu^{2+}(aq)\rightleftharpoons 2H^+(aq)+Cu(s)

E_{left}\; or\; E_{red,anode}:Zn(s)+2H^+(aq)\rightleftharpoons Zn^{2+}(aq)+H_2(g)

Adding the two cell reactions, we have the Ecell reaction equation, whose Ecell value is Eright Eleft:

Zn(s)+Cu^{2+}(aq)\rightleftharpoons Zn^{2+}(aq)+Cu(s)

E_{cell}=0.34\; V-(-0.76\; V)=+1.10\; V

The above diagram can be simplified by omitting the standard hydrogen electrodes to give:



What If we place the Cu2+/Cu half-cell on the left and the Zn2+/Zn half-cell on the right?


We will have

E_{cell}=-0.76\; V-0.34\; V=-1.10\; V

This implies that the overall electrochemical cell reaction is not spontaneous, which is not true.



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