While processes taking place in galvanic cells are spontaneous and follow the principle of free energy minimization, electrolysis involves the separation of the constituents of a molecule with a consequent free energy increase obtained from electric power.

Figure 9.9 is a schematic drawing of an electrolysis cell. It describes the electrolysis of an MA salt. The reactions taking place at the electrodes are

M+ + e— ) M — AGM at the cathode (9.57)

A ) A + e — AGa at the anode (9.58)

MA ) M + A — (AGm + AGa). (9.59)

The voltage applied between the electrodes has to exceed a threshold VTh for the separation between A and M to take place. This threshold is obtained when the work done in transporting an electron mole from the cathode to the anode equals the molar free energy of the separation reaction:

96 500 Vt = AGm + AGa = AG. (9.60)

For multiply charged n+ negative ions one gets:

96 500nVt = AGm + AGa = AG. (9.61)