Chemical Properties of Cobalt

Chemical Properties of Cobalt are similar to many metals of this group. Cobalt is not sensibly altered by exposure to air or water at ordinary temperatures, but becomes superficially oxidised at red heat. After being maintained at red heat for some time in an atmosphere of pure nitrogen, the metal becomes less susceptible to attack by nitric acid, possibly because of the formation of a "passive" nitride.

Chemical Properties are very simalar to Iron. Cobalt thus resembles iron in exhibiting passivity, although to a less extent. Concentrated nitric acid attacks it, but acid in the proportion of one part water with two parts concentrated acid dissolves cobalt with extreme slowness.

The metal dissolves slowly in hydrochloric and in dilute sulphuric acid, evolving hydrogen. It combines with chlorine and bromine under the influence of heat, yielding the anhydrous chloride and bromide respectively. Neither cold nor hot solutions of sodium or potassium hydroxide attack cobalt unless the metal is simultaneously made an anode.

Heated in ammonia to about 470° C. cobalt yields a nitride which decomposes at 600° C. Heated in nitrogen peroxide it burns with incandescence, yielding cobalto-cobaltic oxide, Co₃O₄.

Electrolytic cobalt does not contain nitrogen, and yields no ammonia on heating in a current of pure hydrogen, thereby differing from its congeners, iron and nickel.

When obtained in a finely divided state by reduction of its oxides at 250° C. cobalt is pyrophoric. In this condition it decomposes acetylene in the cold, the metal becoming incandescent. A portion of the acetylene polymerises to benzene, whilst the remainder yields carbon and hydrogen.

Reduced at 400° C. the metal is not pyrophoric, but occludes a considerable volume of hydrogen. The reduced metal decomposes carbon monoxide at 350° to 450° C., depositing solid carbon. Thus:

Co + 2CO = Co + CO₂ + C.

In this respect, again, cobalt closely resembles iron.

The metal also combines under special conditions with nitrogen peroxide to form nitro-cobalt, Co₂(NO₂). When heated with silicon in the electric furnace, silicides are produced, whilst warming with carbon monoxide under pressure effects the production of cobalt tetracarbonyl, Co(CO)₄. Heated in nitric oxide to 150° C. the metal burns, yielding the monoxide; at red heat it decomposes steam.