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SRC'sLECTURES Lecture-2
REDOX IMPOSSIBLE IS NOTHING

INORGANIC CHEMISTRY by SRC

COURSE CURRICULUM UNDER CHOICE

BASED CREDIT SYSTEM
SYLLABUS
FOR
BACHELOR
UNIVERSITY OF CALCUTTA

SEMESTER – 1
Calcutta University
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SRC'sLECTURES Lecture-2
REDOX IMPOSSIBLE IS NOTHING
Standard redox potential
For a redox couple involving n moles of electrons, such as,

Ox + ne ⇌ Red
The redox potential (E) may be expressed by Nernst equation,
𝑹𝑻 𝒂𝒐𝒙
E = E0 + ln …………. (1)
𝒏𝑭 𝒂𝒓𝒆𝒅

 Where, aox and ared are the activities of the Ox and Red forms respectively at
equilibrium, n = number moles of electrons involved, F = Faraday's constant, 96500
coulomb.mol-1 of electron, R = Universal gas constant, 8.3538 volt. coulomb.K-1, T =
absolute temperature (K) and E0 = standard electrode potential of the redox couple.
 When the Ox and the Red forms are present in equilibrium at their unit activities
(i.e. aox = ared = 1 mol.lit-1), gaseous reactants are present at a partial pressure of 1
atm. and the solid reactants are present in their pure forms, the logarithmic term of
the r.h.s of equation (1) vanishes and the potential E becomes equal to E0, the
standard redox potential of the redox couple.
 Substituting the values of R, F and T (298K, i.e. 250) and transforming the natural
logarithm to logarithm to base 10 by multiplying with 2.303 the equation (1) is
transformed to equation (2)
𝟎.𝟎𝟓𝟗 𝒂𝒐𝒙
E = E0 + log ……. (2)
𝒏 𝒂𝒓𝒆𝒅

 Since the activities (a's) are difficult to measure, for practical purpose, activities of
the respective species (say, i) may be replaced by their equilibrium concentrations
(represented by [ ]i's), which are measureable and are related to the activities
according to: ai = [ ]i x fi, where fi = activity coefficient of the ith species and equation
(2)is then transformed to:
𝟎.𝟎𝟓𝟗 [𝑶𝒙]
E ≈ E0 + log ………. (3)
𝒏 [𝑹𝒆𝒅]

Formal potential:
When either or both the components of a redox couple simultaneously participate in
reactions other than the desired redox reaction, such as complex formation, precipitation,
the equilibrium concentrations of the Ox and the Red forms depend upon the extent of to
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SRC'sLECTURES Lecture-2
REDOX IMPOSSIBLE IS NOTHING
which they contribute to such side reactions. Under these conditions, it is appropriate to
write the Nernst equation (3) in terms of analytical concentrations of the Ox and the Red
forms replacing the standard redox potential (E0) with formal potential (E0') of the redox
couple:

E = E0' +
𝟎.𝟎𝟓𝟗 [𝑶𝒙]
log ………… (4)
𝒏 [𝑹𝒆𝒅]

Where, [Ox] and [Red] are the analytical concentrations of the Ox and the Red forms
respectively. The formal potential (E0') is related to E0 depending upon the nature of the
species involved in the redox reaction.

CLASS EXERCISE
Problems on Formal Potential
(Solutions to be discussed in my next Lecture)

1
❶ The standard potential of E0 [Fe(CN)6]3-/Fe(CN)6]4-& 2 I2/I- are 0.36V & 0.54 V are
respectively. From the E0 values it is evident that Iodine should oxidize ferrocyanide to
ferricyanide.

2[Fe(CN)6]4- + I2 = 2[Fe(CN)6]3- + 2I-

But, in a fairly concentrated acidic medium, the reverse reaction takes places. Explain.

❷ A solution of K3[Fe(CN)6] cannot oxidize iodide to iodine. But, it can do so in the

presence of Zn ions. Explain.

❸ Though Cu2+/Cu+ system cannot be oxidize iodide to iodine, but in presence of excess
0 0
KI, the oxidation becomes possible, Given ECu 2+ /Cu + = 0.15 V; E1
I /I − = 0.54 V
2 2

❹ At, pH = 8, arsenate cannot oxidize iodide to iodine, but, iodine can oxidize arsenite to
0 0
arsenate. Given: EAs O 3− /As O 3− = +0.56 V; E1 − = +0.54 V
4 3 I /I
2 2
𝐀𝐬𝐎𝟑−
𝟑 + I2 + H2O = 𝐀𝐬𝐎𝟑−
𝟒 + 2H + 2H + 2I
+ + -

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SRC'sLECTURES Lecture-2
REDOX IMPOSSIBLE IS NOTHING
❺ Redox potential of MnO4-/Mn2+ system largely depend on the pH of the system –
explain.

or
Selective oxidation of the halides can be achieved by the control of pH, when titration with
KMnO4.
0
(EMn O−
4 /Mn
2+ = 1.51 V) E10Cl − = 1.36 V
2 2 /Cl

E10Br − = 1.07 V E10I − = 0.54 V

2 2 /Br 2 2
/I

Problems on Latimer Diagram to

be discussed in my Next Lecture

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