Oxidation

Oxidation
Points : Oxidation, Definition Definition Oxidation, however, is also a phenomenon which works to our disadvantage, particularly in so far as the metallurgist is concerned, since a large number of otherwise useful metals show a great affinity for oxygen and combine with it whenever they are able. This is particularly so at high temperatures so that the protection of metal surfaces by means of fluxes is often necessary during melting and welding operations. Although corrosion is generally a more complex phenomenon, oxidation is always involved and expensive processes such as painting, plating or galvanizing must be used to protect the metallic surface.

It should be noted that, to the chemist, the term oxidation has a much wider meaning, and in fact refers to a chemical process in which the electronegative (or non-metallic) constituent of the molecule is increased. For example, iron (II) chloride may be oxidized to iron (III) chloride.

2F_eCI₂ + Cl₂ = 2F_e CI₂ + Cl₃
Iron (II) Iron (III)
Chloride Chloride

The element oxygen is not involved in this reaction, yet we say that iron (II) chloride has been ‘oxidized’ by chlorine since the chlorine ion is electronegative and so the electronegative portion of the iron (III) chloride molecule is greater than that of the Iron (II) chloride molecule.

Since iron exhibits valences of both two and three this is indicated, in current chemical nomenclature, by writing either ‘iron (I)’ or ‘iron (III) as appropriate. Formerly the term ‘ferrous or ‘ferric’ were used to describe these two series of compounds. In fact any metal exhibiting a variable valence gave rise to ‘-ous’ and ‘ic’ series of compounds: -ous being used to describe that series in which the metal exhibits the lower valence and -ic that in which the metal exhibits the higher valence.

Whilst many metals exist in the Earth’s crust in combination with oxygen as oxides, other are combined with sulphur as sulphides. The latter form the basis of many of the non-ferrous (that is, containing no iron) metal ores. The separation and removal of the oxygen or sulphur contained in the ore from the metal itself is often a difficult and expensive process. Most of the sulphide ores are first heated in air to convert them to oxides, e.g., -

The oxide, whether occurring naturally or produced as indicated in the above equation, is then generally mixed with carbon in the form of coke or anthracite and heated in a furnace. In most cases some of the carbon is burned simultaneously in order to provide the necessary heat which will cause the reaction to proceed more quickly. Under these conditions carbon usually proves to hive a great affinity for oxygen than does the metal and so takes oxygen away form the metal, forming carbon dioxide and leaving the metal (often impure) behind, e.g., -

2ZnO + C = 2Zn + C0₂