Formal cell reaction

Subtract the reaction at the left hand electrode (in the cell as written down) from the reaction at the right hand electrode to find a formal cell reaction ... 

It should be noted that the formal cell reaction, as introduced in step (iii), depends upon how the cell is written down in step (i). For example, for the cell... 

It is thus helpful to distinguish the formal cell reaction from the spontaneous cell reaction. The latter is the reaction that would occur if the cell were shortcircuited. That is, the two electrodes were directly connected to each other, for example using a conducting wire. The nature of the spontaneous cell reaction can be readily deduced since, in reality, electrons will flow from a negative electrode to a positive electrode through an external circuit as is illustrated in the scheme below ... 

Knowledge of the standard electrode potential of a cell allows us to establish, thermodynamically, the direction of the corresponding cell reaction. Suppose that analysis of the cell using the protocol in Section 1.13 leads to the following formal cell reaction. 

The strategy commended in Section (1.13) allows us to associate a formal cell reaction with the above cell as follows. The potential determining equilibrium at the right hand electrode is ... 

As a general strategy for problem solving as a start always write down a cell diagram and workout the assodaled (formal) cell reaction. 

This enables the formal cell reaction to be deduced (Section 1.13) ... 

The formal cell reaction in this example was established above. The The potentials generated in spontaneous cell reaction—that occurring when the cell is short concentrationceUsaretiny.typicaMyof circuited—can be seen from the above to depend on the cell concentra- o te > tensotmiHivoits,asinthi8 tions. When Pi = / 2 = U wj = 0.1 m = 0.5 m2 then the spontaneous reaction is the same as the formal cell reaction since... 

On subtracting the following formal cell reaction is generated,... 

Notice that the reaction (6.7) involves the pure solid metal chlorides and pure elements in their standard states so that the free energies evaluated above are standard free energies regardless of the concentration of HCl in the cell— the latter does not enter the net formal cell reaction, or influence the cell EMF. It does however play the vital role of establishing the potentials on the two electrodes through the potential determining equilibria given above. 

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