Drawing a pictorial diagram, like the figure below, to define a voltaic cell takes a lot of time. Cell notation is an abbreviation that summarizes the important information about a voltaic cell. In cell notation, a vertical line, |, denotes a phase boundary and a double line, ||, a salt bridge. The anode electrode is written to the left, followed by the anode solution, then the salt bridge, then the cathode solution, and, finally, the cathode electrode to the right. Figure below shows how the cell notation for a voltaic cell relates to various components of the cell.
Note that spectator ions, such as NO3−, are not included in the cell notation, and if there are coefficients in a half-reaction, the coefficients are not included (that is, the coefficients of 2 in the silver half-reaction do not appear in the cell notation). When known, the initial concentrations of ions are usually included in the cell notation, so a more complete cell notation for the cell above is Cu(s) | Cu2+(aq, 1 M) || Ag+(aq, 1 M) | Ag(s).
Some redox reactions involve species that are poor conductors of electricity, such as gases or ionic solids. For such substances, an inert electrode that does not participate in the reactions is used. An example of such a voltaic cell is shown below.
The redox reaction involved is:
|Oxidation (anode):||Mg(s)||⟶||Mg2+(aq) + 2e−|
|Reduction (cathode):||2H+(aq) + 2e−||⟶||H2(g)|
|overall:||Mg(s) + 2H+(aq)||⟶||Mg2+(aq) + H2(g)|
This voltaic cell uses an inert platinum wire for the cathode, so the cell notation is:
The magnesium electrode is an active electrode because it participates in the redox reaction. Inert electrodes, like the platinum electrode, do not participate in the redox reaction but must be present so that there is a complete electrical circuit. Platinum and gold are among the least reactive metals so they are good choices for inert electrodes. Graphite, also inert to many chemical reactions, is another common option.
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