Application of Electrochemical Series

Elements, when arranged in order of increasing standard reduction potential, at standard conditions (1 atmospheric pressure, 1M concentration, and 298 K) is called electrochemical series.

The Electrochemical Series and their Reduction Potentials:

Application of Electrochemical Series:

(I) Checking the feasibility of a cell reaction –

A chemical reaction is feasible if the EMF is positive in that particular direction; otherwise, the reaction won’t be viable.

Ex: Zn l ZnSO4 ll CuSO4 l Cu           E= +1.10 volt (Feasible)

      Cu l CuSO4 ll ZnSO4 l Zn           E= -1.10 volt (Not Feasible)

From the values of E, it can be seen that zinc can reduce copper, but copper cannot reduce zinc. 

(II) Checking the reaction’s spontaneity –

Under the standard state,   △Go = – n F Eo

If EMF is positive, △G is negative when the reaction is spontaneous.

If EMF is negative, △G is positive when the reaction is non-spontaneous.

If EMF is 0, △G is 0, which is the condition for equilibrium.

(II) Reactivity of metals –

Metals are reactive if they lose electrons easily, i.e., their tendency to form cations. Those metals with a high negative value or smaller positive value of standard reduction potential lose electrons quickly and are highly reactive. Thus, the metals placed at the top in the electrochemical series are more active than those placed lower in the electrochemical series. So, the electrochemical series imply that Li is more reactive than Cu.

Group 1 and group 2 metals have high negative reduction potential, and therefore they easily dissolve in acids.

Reactivity of metals can also be studied as the electropositive nature of elements.

Just like metals, it decreases as the value of reduction potential increases. It can be studied with the following division:

• Strongly electropositive metals have a standard reduction potential around -2.0 volt or more negative.

• Moderately electropositive metals have a standard reduction potential between 0.0 and about -2.0 volt. 
• Weakly electropositive metals are placed below hydrogen and have positive reduction potential values.

(III) The pattern of displacement reactions –

A metal higher in the electrochemical series will displace the metal from its solution, which is lower in the series, i.e., the higher the negative value of standard reduction potential, the more be its tendency to displace another metal.

Ex: Iron and the metals above iron can liberate hydrogen from water. 

(IV) Reducing nature of metals – 

Reducing the nature of metals depends on the tendency of losing electrons. The higher the negative reduction potential value, the more is the tendency to lose electrons. Their reducing nature decreases as we move from lithium to fluorine in the electrochemical series. Hence, we can say that Lithium is the strongest reducing agent. 

(V) Oxidising nature of non-metals –

Oxidising nature of metals depends on the tendency of accepting electrons. More is the potential reduction value. The higher is the tendency to accept electrons. Thus, oxidising nature increases from top to bottom in the electrochemical series. Hence, we can say that F2 is a stronger oxidant than Cl2.

(VI) Thermal stability of metallic oxides –

Thermal stability refers to the stability of metallic oxide at high temperatures, and it depends on the electropositive nature of metals. The electropositive nature of metals decreases from top to bottom, and therefore, thermal stability also decreases from top to bottom. The metals that come below in the electrochemical series form unstable oxides, decomposition on heating.

(VII) Product of electrolysis –

In case two or more types of ions are present in the solution. They are liberated at the electrode according to their preference during electrolysis. The cations with a high value of standard reduction potential are discharged first at the cathode. Similarly, the anion with a low standard reduction potential value is liberated first at the anode.

Ex: When an aqueous solution of CuSO4  containing Cu2+, SO42-, H+, and OH– ions are electrolysed, Cu2+ ions are discharged at the cathode and OH– ions at the anode.

Conclusion

Electrochemical series is an arrangement of elements based on their value of standard reduction potential. And by looking at this arrangement, the comparative behaviour of various elements can be understood. Some of the properties which can be explained are –  thermal stability of oxides, oxidising nature of non-metals, reducing nature of metals, the pattern of displacement reactions, etc.