Prediction of Spontaneous Redox Reactions

To determine whether or not a combination of two electrode potentials will generate electricity (i.e. spontaneous) by the direction in which they are applied, take the sum of their standard electrode potentials, i.e. oxidation potential + reduction potential, to obtain the e.m.f. of the cell.

A positive value of the. e.m.f. indicates that the combination will generate electricity (i.e. spontaneous). This is because, by the expression, ∆G^Ɵ = - n FE^Ɵ, ∆G^Ɵ will be –ve since E^Ɵ (standard. e.m.f. of the cell) is +ve.

F is Faraday, n is the number of moles of electrons transferred in the half reactions as written. If the e.m.f. value is –ve, then ∆G^Ɵ will be +ve – this indicates that the combination will not produce electricity (non- spontaneous).

Note: when E^Ɵ value is +ve, ∆G^Ɵ value will be –ve – spontaneity. When E^Ɵ value is –ve, ∆G^Ɵ value will be +ve – non-spontaneity.

Examples: 1. Consider a cell of Fe in its salt solution as the anode and Ni in its salt solution as the cathode. Will this cell be spontaneous?

Solution:

Anode reaction: Fe → Fe²⁺ + 2e^- ; E^Ɵ = +0.44 volt – oxidation potential

Cathode reaction: 2e^- + Ni²⁺ → Ni; E^Ɵ = -0.25 volt – reduction potential

Net reaction: Fe + Ni²⁺ → Fe²⁺ + Ni ; E^Ɵ = +0.19 volt.

The value of e.m.f. (E^Ɵ) is +ve, then the process is spontaneous (i.e., electricity will be generated).

2. Consider a cell of hydrogen electrode as cathode, and the anode Ag in solution of silver salt. Determine if its spontaneous.

Solution:

Anode reaction: 2Ag → 2Ag⁺ + 2e^- ; E^Ɵ = -0.80 volt – oxidation potential

Cathode reaction: 2e^- + 2H⁺ → H₂ ; E^Ɵ = 0.00 volt – reduction potential

Net reaction: 2Ag + 2H⁺ → 2Ag⁺ + H₂ ; E^Ɵ = -0.80 volt.

Here, the value of e.m.f. (E^Ɵ) of the cell is –ve.

This indicates that the cell as arranged is not spontaneous. I.e., electricity will not be generated.

However, to make the system spontaneous, the direction of arrangement must be reversed. I.e., the cathode should be Ag in the solution of its salt, and hydrogen electrode the anode. This goes to confirm the order of reduction as given by the electrochemical series.

I.e. Ag is lower than H, hence its ions are more reduced, and will make the cathode for this system to be spontaneous. Thus, the standard electrode potential value of Ag as the cathode changes to +ve (+0.80 volt) Note: when you change the direction of the electrodes, their E^Ɵ values also change to the opposite sign.

General note:

Electrolytic cells and voltaic cells are types of electrochemical cells; i.e., electric current and chemical reactions are involved in the operation of both of them.

In electrolytic cells, a direct electric current from an outside source causes non-spontaneous oxidation-reduction reactions to take place, but in voltaic cells, oxidation- reduction reactions take place simultaneously, producing electric current.

In either types of electrochemical cell, oxidation occurs at the anode, and reduction occurs at the cathode.