Oxygen as an Oxidizing Agent and the Chemistry of

Oxides

Oxygen is a powerful oxidizing agent; after all, one can say that the process of oxidation is named
after it! The reason why this is so is because of its high electronegativity. Upon reacting with other
species, it will undergo reduction and take on a negative oxidation state, oxidizing its partner in the
process. The only exception is in its reaction with fluorine, which is more electronegative than
oxygen is (see rules for assigning oxidation numbers above). Because of the relative abundance of
oxygen relative to fluorine, however, the “electron-grabbing” effect of oxygen is much more evident
in everyday life. Rusting, for example, occurs when iron is oxidized to ferric oxide (Fe 2 O 3 ) and
complexes with water molecules to form a hydrate (Fe 2 O 3 • xH 2 O where x is the number of water
molecules to which it is complexed and may vary).

Oxide is the general name usually given to binary compounds in which the oxygen is in the −2
oxidation state (as distinguished from peroxides and superoxides, for example). Certain oxides
dissolve in water to give acidic solutions; such oxides are called acidic anhydrides. Other oxides may
dissolve in water to give basic solutions; such oxides are called basic anhydrides. Acidic anhydrides
are mostly oxides of nonmetals, such as SO 3 , which dissolves in water to give sulfuric acid, H 2 SO 4.
Oxides of Groups IA and IIA metals, on the other hand, tend to be basic anhydrides, such as BaO and
CaO.

Instead of thinking of anhydrides in terms of what they would do in water, one can also think in the
opposite direction: Anhydrides are obtained by the removal of water from acidic and basic
compounds. CaO, for example, can be obtained by removing water from calcium hydroxide,
Ca(OH) 2 :

Ca(OH) 2    (s) →   CaO (s) +   H 2 O   (l)

Acidic and basic anhydrides are Lewis acids and bases.