Question

Write a balanced equation and determine Eº for each of the following cells: a) Cr Cr3+||Ni2+|Ni b) (CF|C1,||MnO | Mn?)

Answer 1

Answer:

Explanation:

Step 1: Write both half reactions

Cr / Cr3+ : oxidation  Cr(s) → Cr3+ + 3e-  

Ni2+ / Ni : reduction Ni2+ +2e- → Ni

Step 2: Balance reactions and look up the standard potential for the  half-reactions

2(Cr → Cr3+ + 3e-)    E° ox = 0.74 V

3(Ni2+ +2e- → Ni)     E° red = -0.25 V

2Cr + 3Ni2+ +6e- → 2Cr3+ +6e- + 3Ni

E°cell = E° red + E° ox  = -0.25 + 0.74  = 0.49

E = E ° − 0.0257 V /n * ln Q  = E ° − 0.0257 V /n  *l n [ C r 3 + ]/ [ N i 2 + ]

With E° = 0.49 V

b)

Step 1: Write both half reactions

MnO4-/ Mn2+ (redution)   MnO4-  +8H+ +5e- ⇔ Mn2+ +4H2O

Cf ⇔ Cf2+ +2e-   (oxidation)   Cf ⇔ Cf2+ +2e-

Step 2: Balance reactions and look up the standard potential for the  half-reactions

MnO4-  +8H+ +10-e- ⇔ Mn2+ +4H2O    E° = 1.51 V

5Cf ⇔ 5 Cf2+ +10e-        E° =2.12 V

2 MnO4- + 16H+ + 5Cf ⇔ 2Mn2+ + 8H2O + 5Cf2+

E°cell = E° red + E° ox  = 1.51 + 2.12  = 3.63 V

E = E ° − 0.0257 V /n * ln Q  = E ° − 0.0257 V /n  *l n [ C f2 + ]/ [ Mno4- ]

With E° =3.63 V