Electron Transfer
In an electrolytic cell, electron transfer between a metal electrode and species in solution near the electrode depend on the energy of electrons in the electrode and the energy of the lowest unoccupied orbitals or the highest occupied orbitals or the species in solution. The energy level of electrons in the metal is referred to as the Fermi level. As the charge on the electrode becomes more negative the Fermi level increases. When the energy of electrons in the metal equals or exceeds that of the lowest unoccupied orbital, electrons can transfer from the electrode to the species in solution. The species is reduced in the process. For example Cu+2 could be reduced to Cu+1. If the potential of the electrode is made more positive it will reach the point where the energy of electrons in the metal is lower than that of electrons in the highest occupied orbitals of the dissolved species. In this case electrons can transfer from the species in solution to the metal. These processes are shown in Figure 1.
Figure 1.
In an electrolytic cell the electrode potential is controlled. In the illustration below (Figure 2.) the relation between the energy of electrons in the electrode and lowest unoccupied orbital of the species in solution are shown. If they were at the same energy, electron transfer would be taking place but at equal rates in both directions. The greater the difference the faster the rate of reduction of the dissolved species.
Figure 2.
The figure below (Figure 3.) shows the situation where the Fermi level falls below the energy of electrons in the highest occupied orbitals in the solute. Electrons are passed to the electrode and the solute is oxidized in the process. If the potential of the electrode were not being controlled this process would cause the Fermi level to increase. In the case of the controlled potential of an electrolytic cell the electrons passed to the electrode are drained off as current through the cell. The opposite process is taking place at the counter electrode.
Figure 3.
If both the oxidized and reduced species are in solution in the presence of an electrode in a galvanic cell where the potential of the electrode is not controlled, the Fermi level is determined by the relative concentrations of oxidized and reduced species in solution. This is the so-called redox electrode. This situation is illustrated in Figure 4.
Figure 4.
In the case of a redox electrode there is no net current beyond the initial charging. The redox potential of the electrode is measured against a reference electrode. There is no net chemical change involved either. If the two species in solution are ferric and ferrous ions there is a constant exchange of electrons between the two but there is no change in the concentration of either. The metal electrode functions as a sort of middle-man for electron exchange.
In the case of an electrolytic cell with an applied potential sufficient to cause reduction or oxidation of solute species, a current flows through the cell and can be measured. The current is a measure of the rate of the reaction. Obviously the current is a function of the surface area of the electrode-solution interface. At the same potential a larger electrode will produce more current. This is a linear relationship. The current is also a function of the concentration of the species being reduced or oxidized.