CHAPTER VI THE COMPOUNDS OF NITROGEN WITH HYDROGEN AND OXYGEN

It is this gas which is of a brown colour. At a lower temperature it forms nitrogen peroxide, N 2 O 4 , whose molecular weight, and therefore density, is twice that of the dioxide. This substance, which is isomeric with nitrogen dioxide, as ozone is isomeric with oxygen, and has twice as great a vapour density (46 referred to hydrogen), is formed in greater quantity the lower the temperature, and crystallises at -10°. The reasons both of the variation of the colour of the gas (N 2 O 4 gives colourless and transparent vapours, whilst those of NO 2 are brown and opaque) and the variation of the vapour density with the variation of temperature are thus made quite clear; and as at the boiling point a density 38 was obtained, therefore at that temperature the vapours consist of a mixture of 79 parts by weight of N 2 O 4 with 21 parts by weight of NO 2 . It is evident that a decomposition here takes place the peculiarity of which consists in the fact that the product of decomposition, NO 2 , is polymerised ( i.e. becomes denser, combines with itself) at a lower temperature; that is, the reaction

N₂O₄ = NO₂ + NO₂

is a reversible reaction, and consequently the whole phenomenon represents a dissociation in a homogeneous gaseous medium, where the original substance, N₂O₄, and the resultant, NO₂, are both gases. The measure of dissociation will be expressed if we find the proportion of the quantity of the substance decomposed to the whole amount of the substance.

At the boiling point, therefore, the measure of the decomposition of nitrogen peroxide will be 21 p.c.; at 135° it = 1, and at 10° it = 0; that is, the N₂O₄ is not then decomposable. Consequently the limits of dissociation here are -10° and 135° at the atmospheric pressure.⁵³

Within the limits of these temperatures the vapours of nitrogen peroxide have not a constant density, but, on the other hand, above and below these limits definite substances exist. Thus above 135° N₂O₄ has ceased to exist and NO₂ alone remains. It is evident that at the ordinary temperature there is a partially dissociated system or mixture of nitrogen peroxide, N₂O₄, and nitrogen dioxide, NO₂. In the brown liquid boiling at 22° probably a portion of the N₂O₄ has already passed into NO₂, and it is only the colourless liquid and crystalline substance at -10° that can be considered as pure nitrogen peroxide.⁵⁴

The above explains the action of nitrogen peroxide on water at low temperatures. N₂O₄ then acts on water like a mixture of the anhydrides of nitrous and nitric acids. The first, N₂O₃, may be looked on as water in which each of the two atoms of hydrogen is replaced by the radicle NO, while in the second each hydrogen is replaced by the radicle NO₂, proper to nitric acid; and in nitrogen peroxide one atom of the hydrogen of water is replaced by NO and the other by NO₂, as is seen from the formulæ—

In fact, nitrogen peroxide at low temperatures gives with water (ice) both nitric, HNO₃, and nitrous, HNO₂, acids. The latter, as we shall afterwards see, splits up into water and the anhydride, N₂O₃.

If, however, warm water act on nitrogen peroxide, only nitric acid and monoxide of nitrogen are formed: 3NO₂ + H₂O = NO + 2NHO₃.