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The simplest form of the chemical equation for the reduction of ferric iron would be:

(3.102)

Figure 3.18 Electrode reactions in the Daniell cell.

where the subscript aq denotes the aqueous species. This form suggests that the energy involved might be most conveniently measured in an electrochemical cell.

The Daniell cell pictured in Figure 3.18 can be used to measure the energy involved in the exchange of electrons between elements, for example, zinc and copper:

(3.103)

where the subscript s denotes the solid. Such a cell provides a measure of the relative preference of Zn and Cu for electrons. In practice, such measurements are made by applying a voltage to the system that is just sufficient to halt the flow of electrons from the zinc plate to the copper one. What is actually measured, then, is a potential energy, denoted E, and referred to as the electrode potential, or simply the potential of the reaction.

If we could measure the potential of two separate half-cell reactions:

we could determine the energy gain/loss in the transfer of an electron from an individual element. Unfortunately, such measurements are not possible (nor would these reactions occur in the natural environment: electrons are not given up except to another element or species§). This requires the establishment of an arbitrary reference value. Once such a reference value is established, the potential involved in reactions such as 3.102 can be established.