Electrochemical Series (or) Electromotive Series

By
On:

Definition : When elements are arranged in increasing order of their standard reduction electrode potentials, the series so obtained is known as electrochemical (or) electromotive series.

or

A series in which standard reduction potentials of various electrodes are arranged in the order of increasing reduction potentials (largest reduction potential at the top) or decreasing oxidation potential is known as electrochemical series.

Important characteristics of electrochemical series

  • Active metals occupy the top of the series.
  • If the reduction potential is more negative, the metal will displace other metals from their salts more reactively.
  • Metals near the top of the series are strongly electropositive and lose electrons readily to give cations, i.e., they are readily oxidised. Hence, they are good oxidising agents.
  • Elements at the bottom of the series are highly electronegative and gain electron readily to form anions, i.e., they are readily reduced. Hence, they are good oxidising agents.
  • Metals above hydrogen in the series displace hydrogen from dilute acids.
Electrode Electrode reaction (reduction) E° Reduction (Volt)
Li+ / Li Li + e \rightleftharpoons Li(s) -3.045
Ca2+ / Ca Ca2+ + 2e \rightleftharpoons Ca(s) -2.87
Mg2+ / Mg Mg2+ + 2e \rightleftharpoons Mg(s) -2.37
Zn2+ / Zn Zn2+ + 2e \rightleftharpoons Zn(s) -0.763
Fe2+ / Fe Fe2+ + 2e \rightleftharpoons Fe(s) -0.440
Sn2+ / Sn Sn2+ + 2e \rightleftharpoons Sn(s) -0.136
2H+ / H2; Pt 2H+ + 2e \rightleftharpoons H2 (g) ±0.00
Sn4+ / Sn2+ Sn4+ + 2e \rightleftharpoons Sn2+ +0.15
Cu2+ / Cu Cu2+ + 2e \rightleftharpoons Cu(s) +0.337
Fe3+ / Fe2+ Fe3+ + e \rightleftharpoons Fe2+ +0.771
Ag+ / Ag Ag2 + e \rightleftharpoons Ag (s) +0.779
I2 / 2l I2 (s) + 2e \rightleftharpoons 2l +0.530
Br2 / 2Br Br2 (s) + 2e \rightleftharpoons 2Br +1.060
Cl2 / 2Cl Cl2 (g) + 2e \rightleftharpoons 2Cl +1.360
F2 / 2F F2 (g) + 2e \rightleftharpoons 2F +2.87

Applications of electrochemical series

Electrochemical series find many applications. A few of them is given below to appreciate the immense importance of this series.

(i) Calculation of emf of a cell

The electrochemical series is very helpful in determining the emf of a cell.

Ecell = E°cathode – E° anode

Eg.,

Consider a galvanic cell consisting of zinc and copper; zinc being anode. The standard reduction potentials of zinc and copper are −0.76°V and +0.34°V, respectively. therefore, emf of the cell is given by

Ecell = E°cathode -E°anode
= 0.34 – (-0.76)
= 1.10°V

(ii) Predicting the feasibility or spontaneity of a redox reaction

The electrochemical series enables us to predict whether a particular redox reaction is possible or not

Eg.,

In general, a redox reaction can occur if the species losing the electrons lies above that of species which gains the electrons in the electrochemical series.

In a reversible cell, when the cell yields electrical energy, there is a fall in free energy

-ΔG = nFE.

(a) If ΔG= negative; E= positive. Then the cell reaction is feasible or spontaneous.

(b) If ΔG= positive; E= negative. Then the cell reaction is nonspontaneous or not feasible.

(c) If ΔG=0; E=0. Then, the cell reaction has the equilibrium.

(iii) Displacement of metal or element

If a metal or element lies higher in electrochemical series, it can displace another metal from its solution lying below it in electrochemical series.

Eg.,

Iron lies above copper in electrochemical series. So iron can displace copper from copper sulphate solution.

Fe +CuSO4 → Cu + FeSO4

(iv) Displacement of hydrogen from acids

Only those metals which lie above hydrogen in electrochemical series can displace hydrogen from dilute acids.

Eg.,

Zinc lies above hydrogen in electrochemical series. Therefore, it can displace hydrogen from dilute acids.

Zn+ H2SO4 → ZnSO4 + H2      Ezn = 0.76V

Metals like copper and silver lie below hydrogen in electrochemical series and cannot displace hydrogen from acids.

Ag+ H2SO4 → no reaction      EAg =+0.80V

(v) Oxidising and reducing agents

In electrochemical series, the elements are arranged in the order of increasing reduction potentials.

Therefore, the elements at the top of series lose their electrons easily and are, therefore, strong reducing agents.

It easily undergoes reduction while it is difficult to oxidise. The elements at the bottom of the series gain electrons easily and are strong oxidising agents.

Eg.,

Lithium has the largest reduction potential. It means that it is difficult to reduce Li+ ions to Li. On the other hand, it will be most easy for Li to get oxidised to Li+ ions.

(vi) Determination of equilibrium constant for the reaction

Standard electrode potential can be used to determine equilibrium constant for the reaction.

We know that,

-ΔG° = RT lnK = 2.303 RT logK
logK = -ΔG°

So by knowing  for a cell reaction, its equilibrium constant can be calculated.

Read More Topics
Nernst Equation
Measurement of single electrode potential
Definition and origin of electrode potential

Santhakumar Raja

Hello The goal of this blog is to keep students informed about developments in the field of education. encourages pupils to improve as writers and readers.

For Feedback - techactive6@gmail.com

Leave a Comment