Calcium Nitride, Ca₃N₂

Calcium Nitride, Ca₃N₂, in an impure state was first obtained by Maquenne by heating calcium amalgam in a current of nitrogen.

Moissan prepared it as small, transparent, yellow-brown crystals, of density 2.63 at 17° C., by heating calcium in a nickel boat in a current of nitrogen. " Active" calcium, produced by melting and then cooling slowly to form a coarse-grained crystalline structure, begins to absorb nitrogen at 300° C. The maximum action is at 440° C., above which temperature the rate slows down until it ceases at 800° C. The absorption is favoured by the presence of a superficial layer of nitride. By quickly chilling metallic calcium from 840° C. an " inactive " form is produced which only begins to combine with nitrogen at 800° C. " Active " calcium produces a brown nitride, and " inactive " black. The velocity is expressed by the formula where v is the velocity, P the pressure of nitrogen, and K a constant.

Pure calcium is almost passive towards nitrogen, but the presence of more electropositive metals, for example, potassium and barium, accelerates the absorption, arsenic and antimony retard it, and bismuth, copper, and zinc apparently protect it completely. Calcium nitride always acts as an accelerator. With a suitable alloy, containing also 5 per cent, of nitride, pure argon can be obtained in a few minutes from atmospheric nitrogen, even below 320° C.

The melting-point of calcium nitride is about 1200° C. It reacts with chlorine and bromine vapour in the cold, or with only a slight rise in temperature, with iodine vapour at dull red heat, with phosphorus and sulphur at red heat, and with carbon in the electric furnace, forming carbide, the cyanide being an intermediate product. Water attacks it immediately, giving calcium hydroxide and ammonia. With dilute acids it forms calcium and ammonium salts, but is not affected by concentrated sulphuric and nitric acids. Alcohol reacts with it at red heat, forming the ethylate and ammonia. Sodium, potassium, and magnesium have no effect upon it at red heat. When heated strongly in hydrogen, calcium hydride, nitrogen, and some ammonia are formed. The reaction is reversible, according to Haber and van Oordt. Ammonia is not formed in the reverse reaction.

Ca₃N₂ + 3H₂ ⇔ 3CaH₂ + N₂.

Moissan proposed the use of calcium nitride as an intermediary in the fixation of atmospheric nitrogen.

The heat of formation of calcium nitride is 111.2 Cal.