Engineering Chemistry S Datta

ELECTROCHEMICAL CELLS 235
Table 10.2: Normal potentials (∈°) of some redox system (Hydrogen scale)
Electrode Electrode reaction Electrode potential
∈°(volts)
(Pt) Cr +2 /Cr +2 Cr 2+ + e Cr 2+ 0.41
(Pt) V + 2 /V +2 V +3 + e V +2 – 0.21 – 0.25
(Pt) H + / 1 2 H 2
H + 1
+ e
2 H 2
0.00
(Pt) Sn +4 /Sn +2 Sn +4 + 2e Sn +2 + 0.15
(Pt) Cu +2 /Cu +1 Cu +2 + e Cu +1 + 0.16
(Pt) [Fe(CN) 6
] 2– /[Fe(CN) 4
] 4– Fe(CN) 3– 6
+ e Fe(CN)
4–
6
+ 0.36
(Pt) MnO – 4
/MnO
2–
4
MnO
–
4
+ e MnO
2–
4
+ 0.54
(Pt) 1 2 I 2 /I– 1 2 I 2 + e I– + 0.535
(Pt) Fe +2 /Fe +2 Fe +3 + e Fe +2 + 0.77
(Pt) 1 2 Br 2 /Br– 1 2 Br 2 + e Br– + 1.065
(Pt) IO 2 – / 1 2 I 2
IO 3
–
+ 6H + + 5e
1
2 I 2 + 3H 2 O + 1.20
(Pt) MnO 2
/Mn +2 MnO 2
+ 4H + 2e Mn +2 + 2H 2
O + 1.23
Tl +3 /Tl +2 Tl +3 + 2e Tl +1 + 1.25
1
2 Cl 2 /Cl– 1 2 Cl 2 + e Cl– + 1.36
Cr 2
O 2– 7
/2Cr +3 Cr 2
O
2–
7
+ 14 H + + 6e 2Cr +3 + 7H 2
O + 1.36
MnO – 4
/Mn +2 MnO
–
4
+ 8H + + 5e Mn +2 + 4H 2
O + 1.52
BrO – 2
/ 1 2 Br 2
BrO
–
3
+ 6H + 1
+ 5e
2 Br 2 + 3H 2 O + 1.52
Ce +4 /Ce +3 Ce +4 + e Ce +2 + 1.61
MnO – 4
/MnO 2
MnO
–
4
+ 4H + + 3e MnO 2
+ 2H 2
O + 1.69
Co +2 /Co CO +2 + e CO +2 + 1.82
S 2
O 2– 4
/2SO
2–
4
S 2
O
2–
4
+ 2e 2SO
2–
4
+ 2.01
1
2 F 2 /F– 1 2 F 2 + e F– + 2.65
The pair which occupies lower position in the table can oxidise the pair which occupies
a higher position in the table.
Latimer Diagram
The redox potential of different oxidation states for a particular element can be
summarised with the help of EMF diagram and the simplest type of such diagram is the Latimer
diagram.
A Latimer diagram in a compact form explains a great deal of information and shows
the relationships between the various species in a simple and lucid form.
We know that Mn exists in different oxidation states and in acidic medium preferably at
zero pH, the different oxidation states are represented by the corresponding ∈° values as
follows:
Mn 2+ + 2e + Mn, ∈° = – 1.18 V
Mn +3 + e Mn 2+ , ∈° = 1.52 V
MnO 2
+ 4H + + e Mn 3+ + 2H 2
O, ∈° = .95 V
MnO
2–
4
+ 4H + + 2e MnO 2
+ 2 H 2
O, ∈° = 2.26 V
MnO 4
+ e MnO
2–
4
, ∈° = + .56 V
MnO 2
+ 4H + + 2e Mn 2+ + 2H 2
O, ∈° = 1.23 V