The electrochemical series
In a nutshell
An electrochemical series shows you the standard electrode potentials of elements in order of voltage. This allows you to determine element reactivity and reaction feasibility. A reaction is thermodynamically feasible if it has a positive cell potential, however these predictions can sometimes be incorrect.
Elements and standard electrode potentials
Reactive metals will have a more negative standard electrode potential than less reactive metals as they form positive ions more quickly. The reactive metal has reduced the less reactive metal and the less reactive metal has oxidised the reactive metal.
Example
Li has a more negative standard electrode potential than Fe so will form its positive ions more quickly and Li will reduce Fe.
Reactive non-metals will have a more positive standard electrode potential than less reactive non-metals as they form negative ions more quickly. The reactive non-metal has oxidised the less reactive non-metal whereas the less reactive non-metal has reduced the reactive non-metal.
Example
F2 has a more positive standard electrode potential than Cl2 so will form its negative ions more quickly and F2 will oxidise Cl2.
An electrochemical series will show you the standard electrode potentials of different species in order of voltage.
Example
half equation | standard electrode potential (V) |
F2+2e−⇋2F− | |
Ag++e−⇋Ag | |
2H++2e−⇋H2 | |
Fe2++2e−⇋Fe | |
Li++e−⇋Li | |
As the standard electrode potential becomes more negative down the table, the species on the right are more readily oxidised so the species on the left become more stable.
As the standard electrode potential becomes more positive up the table, the species on the left are more readily reduced so the species on the right become more stable.
Predicting the feasibility of reactions
By using the standard electrode potentials of the metal species, you can determine if the reaction is likely to occur in terms of thermodynamics. A thermodynamically feasible reaction should have a positive cell potential.
Procedure
1. | Work out the half equations for the reaction |
2. | Decide which species are being oxidised or reduced |
3. | Input the standard electrode potentials into the equation |
4. | Determine the feasibility of the reaction by the cell potential value |
Example
Determine if this reaction is thermodynamically feasible:
Mg2++Pb→Mg+Pb2+
Half equation | standard electrode potential (V) |
Mg2++2e−⇋Mg | |
Pb2++2e−⇋Pb | |
First, work out the half equations for the reaction. Mg2+ needs to gain 2e− to form Mg and Pb needs to lose 2e− to form Pb2+. This gives you the half equations:
Mg2++2e−→Mg
Pb→Pb2++2e−
Mg has been reduced as it has gained electrons and decreased in oxidation number whereas Pb has been oxidised as it has lost electrons and increased in oxidation number.
Now you can input the standard electrode potentials into the following equation:
Ecell=Ereductionθ−Eoxidationθ
Ecell=−2.36−(−0.76)
Ecell=−2.36+0.76
Ecell=−1.60 V
This reaction is not thermodynamically feasible as it has a negative cell potential of −1.60 V.
Note: The reverse of this reaction would be thermodynamically feasible as it would have a cell potential of +1.60 V.
Incorrect predictions
When predicting the feasibility of reactions using standard electrode potentials, these can sometimes be incorrect. Standard electrode potentials use standard conditions, therefore if a reaction is carried out under non-standard conditions such as a change in temperature or concentration, this will alter the electrode potentials.
Example
Co+Ni2+→Co2++Ni
HALF EQUATION | STANDARD ELECTRODE POTENTIAL (V) |
Co2++2e−⇋Co | |
Ni2++2e−⇋Ni | |
In this reaction, Co has been oxidised and Ni has been reduced. This would produce an Ecell value of:
Ecell=Ereductionθ−Eoxidationθ
Ecell=−0.28−(−0.23)
Ecell=−0.28+0.23
Ecell=−0.05 V
The cell potential is −0.05 V so is thermodynamically unfeasible. However, if the concentration of Co is increased, the equilibrium will shift to the right and the oxidation potential would become more negative. The cell potential becomes more positive overall so would be thermodynamically feasible.
Alternatively, if the concentration of Ni2+ was increased from 1.00 mol dm−3, equilibrium will shift to the right and the reduction potential would become less negative, resulting in a more positive cell potential and a thermodynamically feasible reaction.
A reaction may be thermodynamically feasible but not kinetically feasible. A high activation energy or a slow rate of reaction may prevent a reaction occurring. Despite the cell potential showing that the reaction is thermodynamically feasible, if the reaction is not kinetically feasible, it will not occur.