Electrochemical Series

Electrochemical Series

(1) The standard reduction potentials of a large number of electrodes have been measured using standard hydrogen electrode as the reference electrode. These various electrodes can be arranged in increasing or decreasing order of their reduction potentials. The arrangement of elements in order of increasing reduction potential values is called electrochemical series. It is also called activity series, of some typical electrodes.

(2) Characteristics of Electrochemical series

(i) The negative sign of standard reduction potential indicates that an electrode when joined with SHE acts as anode and oxidation occurs on this electrode. For example, standard reduction potential of zinc is –0.76 volt, When zinc electrode is joined with SHE, it acts as anode (–ve electrode) i.e., oxidation occurs on this electrode. Similarly, the +ve sign of standard reduction potential indicates that the electrode when joined with SHE acts as cathode and reduction occurs on this electrode.

(ii) The substances, which are stronger reducing agents than hydrogen are placed above hydrogen in the series and have negative values of standard reduction potentials. All those substances which have positive values of reduction potentials and placed below hydrogen in the series are weaker reducing agents than hydrogen.

(iii) The substances, which are stronger oxidising agents than ion are placed below hydrogen in the series.

(iv) The metals on the top (having high negative value of standard reduction potentials) have the tendency to lose electrons readily. These are active metals. The activity of metals decreases from top to bottom. The non-metals on the bottom (having high positive values of standard reduction potentials) have the tendency to accept electrons readily. These are active non-metals. The activity of non-metals increases from top to bottom.

Table : Standard reduction electrode potentials at 298K

Element

Electrode Reaction (Reduction)

Standard Electrode Reduction potential E0, volt

Li

Li++ e = Li

–3.05

K

K++ e = K

–2.925

Ba

Ba+++ 2e = Ba

–2.90

Sr

Sr+++ 2e = Sr

–2.89

Ca

–2.87

Na

–2.714

Mg

–2.37

Al

–1.66

Mn

Mn+++ 2e = Mn

–1.18

Zn

Zn2+ +2e=Zn

–0.7628

Cr

Cr3++3 e = Cr

–0.74

Fe

Fe2++ 2e = Fe

–0.44

Cd

Cd2++2e = Cd

–0.403

Co

Co+++ 2e = Co

–0.27

Ni

Ni2++2e = Ni

–0.25

Sn

Sn2++2e = Sn

–0.14

Pb

Pb+++ 2e = Pb

–0.12

H2

2H++2e = H2

0.00

Cu

Cu2++ 2e = Cu

+0.337

I2

I2+2e = 2I

+0.535

Hg

Hg2++2e = Hg

+0.885

Ag

Ag++ e = Ag

+0.799

Br2

Br2+2e = 2Br

+1.08

Pt

Pt+++ 2e = Pt

+1.20

Cl2

Cl2+2e = 2Cl

+1.36

Au

Au 3++3e = Au

+1.50

F2

F2+2e= 2F

+2.87

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