COMPOUNDS OF NITROGEN WITH OXYGEN
Nitrogen combines with oxygen to form five different oxides. The formulas and names of these are as follows:
| N2O | nitrous oxide. |
| NO | nitric oxide. |
| NO2 | nitrogen peroxide. |
| N2O3 | nitrogen trioxide, or nitrous anhydride. |
| N2O5 | nitrogen pentoxide, or nitric anhydride. |
These will now be briefly discussed.
Nitrous oxide (laughing gas) (N2O). Ammonium nitrate, like all nitrates, undergoes decomposition when heated; and owing to the fact that it contains no metal, but does contain both oxygen and hydrogen, the reaction is a peculiar one. It is represented by the equation
NH4NO3 = 2H2O + N2O.
The oxide of nitrogen so formed is called nitrous oxide or laughing gas. It is a colorless gas having a slight odor. It is somewhat soluble in water, and in solution has a slightly sweetish taste. It is easily converted into a liquid and can be purchased in this form. When inhaled it produces a kind of hysteria (hence the name "laughing gas"), and even unconsciousness and insensibility to pain if taken in large amounts. It has long been used as an anæsthetic for minor surgical operations, such as those of dentistry, but owing to its unpleasant after effects it is not so much in use now as formerly.
Chemically, nitrous oxide is remarkable for the fact that it is a very energetic oxidizing agent. Substances such as carbon, sulphur, iron, and phosphorus burn in it almost as brilliantly as in oxygen, forming oxides and setting free nitrogen. Evidently the oxygen in nitrous oxide cannot be held in very firm combination by the nitrogen.
Fig. 39
Nitric oxide (NO). We have seen that when nitric acid acts upon metals, such as copper, the reaction represented by the following equation takes place:
3Cu + 8HNO3 = 3Cu(NO3)3 + 2NO + 4H2O.
Nitric oxide is most conveniently prepared in this way. The metal is placed in the flask A (Fig. 39) and the acid added slowly through the funnel tube B. The gas escapes through C and is collected over water.
Pure nitric oxide is a colorless gas, slightly heavier than air, and is practically insoluble in water. It is a difficult gas to liquefy. Unlike nitrous oxide, nitric oxide does not part with its oxygen easily, and burning substances introduced into this gas are usually extinguished. A few substances like phosphorus, which have a very strong affinity for oxygen and which are burning energetically in the air, will continue to burn in an atmosphere of nitric oxide. In this case the nitric oxide loses all of its oxygen and the nitrogen is set free as gas.
Action of nitric oxide with oxygen. When nitric oxide comes into contact with oxygen or with the air, it at once combines with the oxygen even at ordinary temperatures, forming a reddish-yellow gas of the formula NO2, which is called nitrogen peroxide. This action is not energetic enough to produce a flame, though considerable heat is set free.
Nitrogen peroxide (NO2). This gas, as we have just seen, is formed by allowing nitric oxide to come into contact with oxygen. It can also be made by heating certain nitrates, such as lead nitrate:
Pb(NO3)2 = PbO + 2NO2 + O.
It is a reddish-yellow gas of unpleasant odor, which is quite poisonous when inhaled. It is heavier than air and is easily condensed to a liquid. It dissolves in water, but this solution is not a mere physical solution; the nitrogen peroxide is decomposed, forming a mixture of nitric and nitrous acids:
2NO2 + H2O = HNO2 + HNO3.
Nitrogen peroxide will not combine with more oxygen; it will, however, give up a part of its oxygen to burning substances, acting as an oxidizing agent:
NO2 = NO + O.
Acid anhydrides. The oxides N2O3 (nitrogen trioxide) and N2O5 (nitrogen pentoxide) are rarely prepared and need not be separately described. They bear a very interesting relation to the acids of nitrogen. When dissolved in water they combine with the water, forming acids:
N2O3 + H2O = 2HNO2,
N2O5 + H2O = 2HNO3.
On the other hand, nitrous acid very easily decomposes, yielding water and nitrogen trioxide, and by suitable means nitric acid likewise may be decomposed into water and nitrogen pentoxide:
2HNO2 = H2O + N2O3,
2HNO3 = H2O + N2O5.
In view of the close relation between these oxides and the corresponding acids, they are called anhydrides of the acids, N2O3 being nitrous anhydride and N2O5 nitric anhydride.
DEFINITION: Any oxide which will combine with water to form an acid, or which together with water is formed by the decomposition of an acid, is called an anhydride of that acid.