Figure 10.9 summarizes the key features and equations pertinent to the thermodynamic aspects of electrochemistry. The upper 7 levels of the diagram refer to the overall reaction of acid dissolution of a metal (level 1) in the type of electrochemical cell shown as 1(c) in Fig. 10.4. This reaction is chosen for illustration; the procedure represented in the figure applies to any overall reaction. Level 2 gives the chemical potential difference between the product and reactant sides of the overall reaction. The two half-cells are not in equilibrium unless A|x = 0. Using the relation in the ellipse, A|x is converted to the cell electric potential s, or EMF, shown in level 3.

Note the distinction between A^ and s: the former refers to a chemical potential difference between the left and right sides of the overall reaction; the latter represents an electric potential difference between the left and right half-cells.

In level 4, the overall reaction is broken into its constituent half-cell reactions. Levels 5, 6 and 7 give the cell potential difference with different combinations of conditions in the two half-cells. In level 5, the species in both half cells are in their standard states, so the cell EMF is the standard electrochemical potential of the M/Mz+ couple. In level 6, the left half-cell is no longer in its standard state, but the right half cell remains a standard hydrogen electrode (SHE). The EMF is not the Nernst potential of the M/Mz+ couple. In level 7, both half-cells are nonstandard, and the cell delivers an EMF that is the difference between the Nernst potentials of the two half-cells.

Levels 8a and 8b refer to a simple chemical reaction involving the metal M, its ion Mz+, the acid concentration and the H2 pressure in equilibrium with the solution. In this case A^ and s are both zero and the equilibrium constant K is given by level 8a, where A^" = ssM - sH — sM . The concentrations in solution are related to the equilibrium constant via the law of mass action shown in level 8b. Note the close connection between levels 2 and 3 and levels 8a and 8b.

References j. Handbook of physics and Chemistry.

2. P. W. Atkins, Physical Chemistry, Freeman, pp 236, Sect. 12.1 (1978)

3. D. R. Gaskell, Introduction to Metallurgical Thermodynamics, 2nd Ed., Hemisphere, Sect. j4.8 (j98j)

Overall Reaction


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