Question

An electrochemical cell, composed of an iron electrode (anode) immersed in 0.10 M Fe (NO3)2 and silver electrode (cathode) immersed in 0.11M AgNO3, was found to have a cell potential of 1.21V. The standard reduction potential for Ag+ / Ag half-cell is +0.80V. The standard reduction potential for the Fe+2/ Fe half – cell is -0.45V.

1. Write the half-cell reaction at the anode.

2.Write the half –cell reaction at the Cathode

3.Write over all cell reaction

4. Determine the standard cell potential using the standard reduction potential given above.

5. Determine the standard cell potential using the Nernst equation

Answer 1

Ans 1.

Oxidation reaction takes place at anode. Thus, Fe loses two electrons and gets converted into Fe²⁺.

The half-cell reaction can be written as follows:

Ans 2.

Reduction reaction takes place at cathode. Thus, Ag⁺ accepts an electron to form Ag(s). The half-cell reaction can be written as follows:

Ans 3.

At anode Fe loses two electrons whereas at cathode Ag accepts one electron. Thus, to balance the number of electrons half-cell reaction at cathode should be multiplied by 2. The overall cell reaction can be written by adding two half-cell reactions as follows:

Anode half cell reaction:              

Cathode half-cell reaction:          

Net cell reaction:                        

Ans 4.

Standard cell potential can be calculated by addition of standard reduction potentials of two half-cells as shown below:

Anode half cell reaction:                                           -(-0.45V)

Cathode half-cell reaction:                                +0.80V

The standard reduction potential for the Fe²⁺/ Fe half – cell is -0.45V. Therefore for half-cell Fe/Fe²⁺ sign of potential is reversed.

Thus, standard cell potential = standard reduction potential of anode + standard reduction potential of cathode

Standard cell potential = -(-0.45V) + 0.80V = 1.25V

Ans 5.

Nernst equation can be written as follows:

Ecell = E^ocell – 0.0592/n log [Fe2+] / [Ag+]²

Where, Ecell = cell potential

E^ocell = standard cell potential

Therefore, E^ocell = Ecell + 0.0592/n log [Fe2+] / [Ag+]²

E^ocell = 1.21 V + 0.0592/2 log (0.10) /(0.11)²

E^ocell = 1.21 V + 0.0592/2 log (8.2645)

E^ocell = 1.21 V + (0.0592/2) (0.9172)

E^ocell = 1.24 V

Therefore, according to Nernst equation standard cell potential of given cell is 1.24 V