Electrochemical Series

Standard single electrode potentials are always measured relative to the Standard Hydrogen Electrode, exactly as demonstrated earlier in the worked zinc example. When the standard reduction potentials of many different metal–metal-ion electrodes at 298 K are collected together and arranged in decreasing order, the result is a single reference table called the electrochemical series.

The standard reduction potential, $E^{o}$, is a direct measure of a species' oxidising tendency: the greater (more positive) the $E^{o}$ value, the greater the tendency that species shows to accept electrons and undergo reduction — in other words, the stronger it is as an oxidising agent. Reading the series from top to bottom, therefore, moves from the strongest oxidising agents (species most eager to be reduced, at the top, with the most positive $E^{o}$) to the strongest reducing agents (species most eager to be oxidised, at the bottom, with the most negative $E^{o}$).

This single ordering is genuinely one of the most useful tools introduced in the entire chapter, because it lets several different questions be answered at a glance, without any calculation:

• Which half-reaction oxidises the other? A species positioned higher in the table (more positive $E^{o}$) will spontaneously oxidise the reduced form of any species positioned lower in the table.
• Is a given cell reaction spontaneous? The standard cell emf for any pairing of two half-reactions can be read directly as the difference between their two tabulated potentials; if that difference is positive, the reaction is spontaneous. …