The standard reduction potential data at 25 ° C is given below. E ⁡ ο Fe 3 + Fe 2 + = + 0…

The standard reduction potential data at 25°C is given below.
E⁡ ο Fe 3 + Fe 2 + = + 0 · 7 7 V⁡ ; E⁡ ο Fe⁡ 2 + Fe⁡ = - 0 · 4 4 V⁡ E⁡ ο Cu⁡ 2 + Cu = + 0 · 3 4 V⁡ ; E⁡ ο Cu + Cu = + 0 · 5 2 V E⁡ ο O 2 g⁡ + 4 H⁡ + + 4 e⁡ - → 2 H⁡ 2 O = + 1 · 2 3 V⁡ ; E⁡ ο O 2 g + 2 H 2 O + 4 e - ⟶ 4 OH - = + 0 · 4 0 V E⁡ ο Cr 3 + Cr = - 0 · 7 4 V ; E⁡ ο Cr 2 + Cr = - 0 · 9 1 V
Match Eo of the redox pair in List I with the values given in List II and select the correct answer using the code given below the lists :
 
  List I   List II
A. E⁡ ο Fe 3 + Fe P. -0.18V
B. E⁡ ο 4 H 2 O ⇌ 4 H⁡ + + 4 OH - Q. -0.4V
C E⁡ ο Cu 2 + + Cu → 2 Cu⁡ + R. -0.04V
D. E⁡ ο Cr 3 + Cr 2 + S. -0.83V
  1. a-r;b-q;c-s;d-p;
  2. a-q;b-p;c-r;d-s;
  3. a-s;b-p;c-r;d-q;
  4. a-r;b-s;c-p;d-q;

Solution

P.)  
Fe 3 + + e - ⟶ Fe 2 + ; Δ G I o = - 1 F 0 . 7 7 Fe 2 + + 2 e - ⟶ Fe ; Δ G II o = - 2 F - 0 . 4 4 ---------------------------------- Add Fe 3 + + 3 e - ⟶ Fe ; Δ G III o = - 3 F E o Fe 3 + / Fe
- 3 F E o Fe 3 + / Fe = - 0 . 7 7 F + 0 . 8 8  F
                          =0.11F 
E Fe 3 + / Fe o = - 0 . 1 1 3 = - 0 . 0 3 7 ≅ - 0 . 0 4  V
Q.)   2 H 2 O ⟶ O 2 g + 4 H (aq) + + 4 e -   (Oxidation half reaction)
                                                      E°=-1.23 V
O 2 g + 2 H 2 O + 4 e - ⟶ 4 OH (aq) -  (reduction half reaction)
                                                   E°= +0.40V
It's an electrochemical cell, adding
4 H 2 O ⇌ 4 H + + 4 0 H -   at equilibrium
                                     E cell o = - 0 . 8 3  V
R.)  
                   2 Cu s ⟶ 2 Cu + + 2 e - E 0 = - 0 . 5 2 V Cu 2 + + 2 e - ⟶ Cu s E 0 = 0 . 3 4 V
Adding    Cu 2 + + Cu s ⟶ 2 Cu + E cell 0 = - 0 . 1 8  V
S
Cr 3 + + 3 e - ⟶ Cr ; Δ G I 0 = - 3 F - 0 . 7 4 Cr 2 + + 2 e - ⟶ Cr ; Δ G II 0 = - 2 F - 0 . 9 1

Subtracting
Cr 3 + + e - - Cr 2 + ⟶ 0 ; Cr 3 + + e - ⟶ Cr 2 + ; Δ G 0 = - 1 F E Cr 3 + / Cr 2 + 0
- F E Cr 3 + / Cr 2 + 0 = F 3 × 0 . 7 4 - 2 × 0 . 9 1
- E Cr 3 + / Cr 2 + 0 = F 2 . 2 2 - 1 . 8 2
E Cr 3 + / Cr 2 + 0 = - 0 . 4 0  V

Asked in: JEE Advanced 2013 (Paper 2)

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