The Standard Hydrogen Electrode
In order to calculate the potential for an electrochemical cell, without having to run all the thousands of possible combinations, some sort of standard electrode is needed to provide a reference point.
Originally a calomel electrode involving mercury was used. Now the standard hydrogen electrode is used. This involves hydrogen gas at one atmosphere pressure and 298K being turned into hydrogen ions:
H2(g) 2H+(aq) + 2e-
A platinum-black electrode is needed to provide a surface on which the hydrogen gas can be in contact with the hydrogen ions(aq)
The hydrogen electrode is always placed as the negative electrode of a cell and is written as:
The potential of this cell is assigned the value of 0 V. The potentials for other cells are then tabulated relative to this value. These are the standard electrode potentials, E,
A positive value for the E value means a favourable reaction is occurring and therefore the metal is less reactive than hydrogen.
A negative value for E means an unfavourable reaction and therefore the metal is more reactive than the standard hydrogen electrode.