Electrochemical Series

Just now, we have seen that how to find a single electrode potential. In similar manner, the E° value of other electrodes can also be measured.

When the elements are arranged in the order of their standard reduction potential E°Red , an important series known as electro – chemical or electro motive series is obtained. E°Red is the quantitative measure of readiness of the element to lose electrons.

The series is arranged in the order of increasing strength as oxidant and decreasing strength of reducing agent. Those elements having E°Red negative can easily liberate hydrogen from H+ ions in aqueous medium and those with positive value can oxidise H2 to H+ ion.

Thus Li occupying the top position in the series is the strongest reducing agent while fluorine is the strongest oxidising agent as it occupies the bottom of the series.

A close Scrutiny of the numerical values of the standard electrode potentials in the electro chemical series, reveals the following interesting points.

1. Lower values of the standard reduction potential indicate greater tendency of the metal to lose electrons (i..e,) higher metallic or electropositive character.

2. A negative value of reduction potential ( $\large E^o_{Li^+ / Li} = -3.03 V$ )   indicates that the metal has a greater tendency to lose electron (i.e.) to be oxidised into its own ion than the tendency of its ion to be reduced by gaining electron.

3. A positive reduction potential value indicates that the tendency of the metal ion to be reduced by gaining electron, is more than tendency of the metal to be oxidised by losing electron.

4. An element having higher negative potential will replace all others below it with lower negative potential values from their aqueous salt solutions. Thus Zn will replace Cu2+ from an aqueous solution of CuSO4 (i.e.,) Zn will reduce Cu2+ and itself will be oxidised

$\large E^o_{Zn^{2+}/Zn} = -0.76 V$

$\large E^o_{Cu^{2+}/Cu} = 0.36 V$

Hence the reaction is Zn + Cu2+ –> Cu + Zn2+ is spontaneous and not the reverse one.

5. An oxidising agent with higher E°Red value can oxidise a system with lower E°Red value. Thus the numerical magnitude of E°Red value is a measure of the strength of an oxidant. All oxidising agents are characterised by higher value of E°Red

E.g. since E°F2/2F− = 2.87 volt, F2 is one of the strongest oxidising agents and no common oxidants are known which can oxidise fluoride ion into fluorine gas.

Also Read :

→ Electrolysis
→ Faraday’s First Law of Electrolysis
→ Faraday’s second Law of Electrolysis
→ Electrode Potential
→ Galvanic Cells
→ IUPAC Cell Representation
→ The Nernst Equation
→ Standard Electrode Potential

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