Chemical Properties of Molybdenum

Chemical Properties of Molybdenum are related with its oxidation ability. Molybdenum is not appreciably oxidised in air at ordinary temperatures, but at a dull red heat the trioxide, MoO₃, is slowly formed, the oxidation taking place more rapidly at 600° C. In oxygen alone vigorous combustion takes place at 500° to 600° C. Oxidation can be effected also by fusion with potassium chlorate, or less violently with potassium nitrate. The metal is attacked by fused, but not by aqueous, caustic alkali. When heated in steam, it is converted first into the dioxide and then into the trioxide; in a mixture of hydrogen and water-vapour, under suitable conditions, the dioxide is formed. By fluorine it is attacked at ordinary temperatures, by chlorine at a dull red heat, by bromine at a red heat, but in the case of iodine there is no reaction. With hydrogen sulphide at 1200° C. the sulphide is formed. Molybdenum does not combine directly with hydrogen, nitrogen, or phosphorus, but with boron, carbon, and silicon, compounds are formed. For this reason crystalline carbides are always formed when molybdenum is heated in carbon crucibles in the electric furnace. Molybdenum may be oxidised to the trioxide by means of carbon dioxide,

Mo + 3CO₂ = MoO₃ + 3CO,

but the reaction is reversible, and under suitable conditions the trioxide may be reduced to the metal by carbon monoxide; the metal is volatile in carbonyl chloride.

Molybdenum is, generally speaking, somewhat resistant to the action of acids, but is less so than tungsten. It is untouched by hydrofluoric acid, hot or cold, and concentrated hydrochloric and sulphuric acids attack it only very slowly, the latter at elevated temperatures evolving sulphur dioxide, with the production of a green solution. Hot dilute hydrochloric acid slowly dissolves the metal, but sulphuric acid under similar conditions does not do so. Moderately dilute nitric acid attacks the metal rapidly; the concentrated acid induces a condition of passivity, the action being slow owing to the deposition of a film of molybdic anhydride, MoO₃, upon the surface of the metal. Aqua regia attacks the metal rapidly, particularly on heating.

An especially active form of molybdenum has been obtained by electrolysis of a solution of molybdenum trioxide in hydrochloric acid, using a mercury cathode, the mercury being removed from the amalgam formed by distillation.