2Fe++, 2SO4- + 2H+, SO4- + O = 2Fe+++, 3SO4- + H2O.
Hydrogen ions have been oxidized to water, while the charge of each iron ion has been increased from 2 to 3.
In a similar way the conversion of ferrous chloride into ferric chloride may be written
Fe++, 2Cl- + Cl = Fe+++, + 3Cl-.
Here again the valence of the iron and the charge on the iron ion has been increased from 2 to 3, though no oxygen has entered into the reaction. As a rule, however, changes of this kind are brought about by the use of an oxidizing agent, and are called oxidations.
The term "oxidation" is applied to all reactions in which the valence of the metal of a compound is increased, or, in other words, to all reactions in which the charge of a cation is increased.
Reduction of ferric salts. The changes which take place when a ferric salt is converted into a ferrous salt are the reverse of the ones just described. This is seen in the equation
FeCl3 + H = FeCl2 + HCl
In this reaction the valence of the iron has been changed from 3 to 2. The same equation may be written
Fe+++, 3Cl- + H = Fe++, + H+ + 3Cl-