T. A. Pinfold University of Witwatersrand Johannesburg, southAfrica
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I
The Presentation of Electrode Potentials Using an Energy Level Diagram
The tabular form in which standard electrode potentials are usually presented often leads to confusion regarding the polarity of electrochemical cells and the chemical reactions occurring in them. This probably arises because the relation between the electrode potential and the free energy change of the corresponding half-cell reaction is not emphasized. The difficulties can be diminished by representing the electrochemical series on an energy diagram of the kind shown in the figure. Energy levels corresponding to the oxidized states in the half-cell reactions amear on the left-hand branch while those of all the re&ced states are represented by one level (the reduction level) on the righehand side. The oxidized and reduced species involved in each reaction appear to the left and right, the values of the standard potentials at 25°C being interposed in the center. The curve, which is a plot of the free energy change per faraday against reaction co-ordinate, has the usual shape for a chemical reaction, emphasizing that charge transfer processes also involve activation energies and transition states. Such a graphical representation has a number of advantages: 1) It is clear in which direction any selected pair of electrode processes must occur to lower the total free energy of the system. As both reduction and oxidation must occur, the cell reaction involves a change from an upper level on the left-hand branch to reduction d
oiiddion t
the reduction level and finally to a lower level on the left-hand branch. The relevant levels are recognized by comparing the chemical species appearing in the representation of the cell with those appearing in the figure. For example, for I&, AgIII-/AgCIAg/ it is clear that levels corresponding to Ag+ and AgI are involved. As the former species is at the higher energy, reduction occurs by
accompanied by an oxidation which, from the figure, is A~ + I- AgI e -
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+
The silver-silver iodide electrode, therefore, is the more negative of the two, the emfof thecellis0.80 - (-0.15) = 0.95 V, and the cell reaction is A g + + I- AgI
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2) Changes occur in the energy levels with changes in the activity of the species, by an amount predicted from the expression
where p is the partial molar free energy of the species and a is its activity. For an electrode of the first kind, e.g. IZnlZn2+1,in which the activity of the solid remains unity and hence for which the reduction level does not change, the electrode potential becomes more negative with decreasing activity of the ion. For an electrode of the second kind, e.g. IAg, AgIII-1, the reverse is true. On decreasing the activity of Iions, the reduction level becomes negative while that corresponding to solid AgI remains unchanged, the overall result being a less negative potential. With redox electrodes, e.g. PtlSnz+, Sn4+l, an increase in the ratio of the activities of Sn4+to Sn2+ions results in a more positive electrode potential, and vice versa. In concentration cells of the kind (Zn(ZnP+(al)(Zn2+(an)lZn1
the emf is the difference in the potentials corresponding to the energy levels of Zn2+ions at the two activities. The left-hand electrode has the more negative potential if a1 < a2 In a concentration cell such as Reprerentdion of the electrochemical series on on energy diogram,
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where p~ and p2 are the partial pressures, the energy
level for the oxidized state, H,+O, will be the same for both electrodes. In this case a reduction level for each electrode should be considered. If PI < PZ, and hence a1 < @, for the gas, the reduction level of the left-hand electrode will be a t a higher energy than that for the right-hand electrode. Oxidation will occur at the former, therefore, making it the more negative of the two electrodes. 3) Although equilibrium conditions do not exist during electrolysis, and cognizance needs to be taken of the presence of overpotentials, the figure can be used
to anticipate which cathodic or anodic process will occur preferentially. For example, if a solution of CU~+ and Cd2+ ions a t the same activity is electrolyzed, copper will be deposited on the cathode, as the of Cu2+ ions less energy. An anodic process may involve the dissolution of the electrode or the discharge of an ion to form gas, The discharge of OH- ions to form oxygen, for example, is more difficult than dissolution of a cadmium anode to form Cd2+ions.
Volume 49, Number 7, July 1972
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