Aqueous electrochemical cells consist of half-cells containing aqueous solutions of ionized species. This branch of electrochemistry exhibits an additional dimension not found in the solid-state counterpart. The added degree of complexity is illustrated in the schematic 3x3
matrix of Fig. 10.4. The columns contain the three operational modes: galvanic, electrolytic and equilibrium. The rows show the three types of devices: two half-cells separated by a bridge; two electrodes with a common electrolyte; and electrolyte only.
In the first row, the compartments are separated by a bridge that prevents intermixing of the contents of the half-cells. This is the sole mode of operation available to solid-state electrochemical devices of the sort described up until now. Combination 1(c) is particularly useful in determining the thermodynamic aspects of electrochemical systems.
In the second row labeled "whole cell", half-cells no longer exist, although half-cell reactions remain; two different electrodes are immersed in a common electrolyte. Configuration 2(b) is how nearly all practically applications of electrochemistry are conducted (e.g., electrolysis of water, plating of metals, the lead-acid battery).
In the third variant in the matrix, called "no cell", all that remains is the electrolyte and perhaps a solid that takes part in the reaction. Arguably, this mode should not even deserve to be termed electrochemistry. However, it represents ionic equilibria, and because its thermodynamic description relies on data obtained by configuration 1(c), it is properly considered as part of electrochemistry.
typeX^ galvanic electrolysis equilibrium
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