Ammonia is not only capable of combining with acids, but also with many salts, as was seen from its forming definite compounds, AgCl,3NH3 and 2AgCl,3NH3, with silver chloride. Just as ammonia is absorbed by various oxygen salts of the metals, so also is it absorbed by the chlorine, iodine, and bromine compounds of many metals, and in so doing evolves heat. Certain of these compounds part with their ammonia even when left exposed to the air, but others only do so at a red heat; many give up their ammonia when dissolved, whilst others dissolve without decomposition, and when evaporated separate from their solutions unchanged. All these facts only indicate that ammoniacal, like aqueous, compounds dissociate with greater or lesser facility.[19] Certain metallic oxides also absorb ammonia and are dissolved in ammonia water. Such are, for instance, the oxides of zinc, nickel, copper, and many others; the majority of such compounds are unstable. The property of ammonia of combining with certain oxides explains its action on certain metals.[20] By reason of such action, copper vessels are not suitable for holding liquids containing ammonia. Iron is not acted on by such liquids.
The similarity between the relation of ammonia and water to salts and other substances is more especially marked in those cases in which the salt is capable of combining with both ammonia and water. Take, for example, copper sulphate, CuSO4. As we saw in Chapter [I]., it gives with water blue crystals, CuSO4,5H2O; but it also absorbs ammonia in the same molecular proportion, forming a blue substance, CuSO4,5NH3, and therefore the ammonia combining with salts may be termed ammonia of crystallisation.
Such are the reactions of combination proper to ammonia. Let us now turn our attention to the reactions of substitution proper to this substance. If ammonia be passed through a heated tube containing metallic sodium, hydrogen is evolved, and a compound is obtained containing ammonia in which one atom of hydrogen is replaced by an atom of sodium, NH2Na (according to the equation NH3 + Na = NH2Na + H). This body is termed sodium amide. We shall afterwards see that iodine and chlorine are also capable of directly displacing hydrogen from ammonia, and of replacing it. In fact, the hydrogen of ammonia may be replaced in many ways by different elements. If in this replacement NH2 remains, the resultant substances NH2R are called amides, whilst the substitution products, NHR2, in which only NH remains, are called imides,[20 bis] and those in which none of the ammoniacal hydrogen remains, NR3, are known as nitrides. Free amidogen, N2H4, is now known in a state of hydration under the name of hydrazine;[21] it combines with acids and resembles ammonia in this respect. In the action of different substances on ammonia it is the hydrogen that is substituted, whilst the nitrogen remains in the resultant compound, so to say, untouched. The same phenomenon is to be observed in the action of various substances on water. In the majority of cases the reactions of water consist in the hydrogen being evolved, and in its being replaced by different elements. This also takes place, as we have seen, in acids in which the hydrogen is easily displaced by metals. This chemical mobility of hydrogen is perhaps connected with the great lightness of the atoms of this element.
In practical chemistry[21 bis] ammonia is often employed, not only for saturating acids, but also for effecting reactions of double decomposition with salts, and especially for separating insoluble basic hydroxides from soluble salts. Let MHO stand for an insoluble basic hydroxide and HX for an acid. The salt formed by them will have a composition MHO + XH - H2O = MX. If aqueous ammonia, NH4OH, be added to a solution of this salt, the ammonia will change places with the metal M, and thus form the insoluble basic hydroxide, or, as it is said, give a precipitate.
| MX | + | NH4(OH) | = | NH4X | + | MHO |
| Salt of the metal. In solution | Aqueous ammonia. In solution | Ammonium salt. In solution | Basic hydrate. As precipitate |
Thus, for instance, if aqueous ammonia is added to a solution of a salt of aluminium, then alumina hydrate is separated out as a colourless gelatinous precipitate.[22]
In order to grasp the relation between ammonia and the oxygen compounds of nitrogen it is necessary to recognise the general law of substitution, applicable to all cases of substitution between elements,[23] and therefore showing what may be the cases of substitution between oxygen and hydrogen as component parts of water. The law of substitution may be deduced from mechanical principles if the molecule be conceived as a system of elementary atoms occurring in a certain chemical and mechanical equilibrium. By likening the molecule to a system of bodies in a state of motion—for instance, to the sum total of the sun, planets, and satellites, existing in conditions of mobile equilibrium—then we should expect the action of one part, in this system, to be equal and opposite to the other, according to Newton's third law of mechanics. Hence, given a molecule of a compound, for instance, H2O, NH3, NaCl, HCl, &c., its every two parts must in a chemical sense represent two things somewhat alike in force and properties, and therefore every two parts into which a molecule of a compound may be divided are capable of replacing each other. In order that the application of the law should become clear it is evident that among compounds the most stable should be chosen. We will therefore take hydrochloric acid and water as the most stable compounds of hydrogen.[24] According to the above law of substitution, if the elements H and Cl are able to form a molecule, HCl, and a stable one, they are able to replace each other. And, indeed, we shall afterwards see (Chapter [XI].) that in a number of instances a substitution between hydrogen and chlorine can take place. Given RH, then RCl is possible, because HCl exists and is stable. The molecule of water, H2O, may be divided in two ways, because it contains 3 atoms: into H and (HO) on the one hand, and into H2 and O on the other. Consequently, being given RH, its substitution products will be R(HO) according to the first form, and R2O according to the second; being given RH2, its corresponding substitution products will be RH(OH), R(OH)2, RO, (RH)2O, &c. The group (OH) is the same hydroxyl or aqueous radicle which we have already mentioned in the [third chapter] as a component part of hydroxides and alkalis—for instance, Na(OH), Ca(OH)2, &c. It is evident, judging from H(HO) and HCl, that (OH) can be substituted by Cl, because both are replaceable by H; and this is of common occurrence in chemistry, because metallic chlorides—for example, NaCl and NH4Cl—correspond with hydroxides of the alkalis Na(OH) or NH4(OH). In hydrocarbons—for instance, C2H6—the hydrogen is replaceable by chlorine and by hydroxyl. Thus ordinary alcohol is C2H6, in which one atom of H is replaced by (OH); that is, C2H5(OH). It is evident that the replacement of hydrogen by hydroxyl essentially forms the phenomenon of oxidation, because RH gives R(OH), or RHO. Hydrogen peroxide may in this sense be regarded as water in which the hydrogen is replaced by hydroxyl; H(OH) gives (OH)2 or H2O2. The other form of substitution—namely, that of O in the place of H2—is also a common chemical phenomenon. Thus alcohol, C2H6O, or C2H5(OH), when oxidising in the air, gives acetic acid, C2H4O2, or C2H3O(OH), in which H2 is replaced by O.
In the further course of this work we shall have occasion to refer to the law of substitution for explaining many chemical phenomena and relations.
We will now apply these conceptions to ammonia in order to see its relation to the oxygen compounds of nitrogen. It is evident that many substances should be obtainable from ammonia, NH3, or aqueous ammonia, NH4(OH), by substituting their hydrogen by hydroxyl, or H2 by oxygen. And such is the case. The two extreme cases of such substitution will be as follows: (1) One atom of H in NH3 is substituted by (OH), and NH2(OH) is produced. Such a substance, still containing much hydrogen, should have many of the properties of ammonia. It is known under the name of hydroxylamine,[25] and, in fact, is capable, like ammonia, of giving salts with acids; for example, with hydrochloric acid, NH3(OH)Cl—which is a substance corresponding to sal-ammoniac, in which one atom of hydrogen is replaced by hydroxyl.[25 bis] (2) The other extreme case of substitution is that given by ammonium hydroxide, NH4(OH), when the whole of the hydrogen of the ammonium is replaced by oxygen; and, as ammonium contains 4 atoms of hydrogen, the highest oxygen compound should be NO2(OH), or NHO3, as we find to be really the case, for NHO3 is nitric acid, exhibiting the highest degree of oxidation of nitrogen.[26] If instead of the two extreme aspects of substitution we take an intermediate one, we obtain the intermediate oxygen compounds of nitrogen. For instance, N(OH)3 is orthonitrous acid,[27] to which corresponds nitrous acid, NO(OH), or NHO2, equal to N(OH)3 - H2O, and nitrous anhydride, N2O3 = 2N(OH)3 - 3H2O. Thus nitrogen gives a series of oxygen compounds, which we will proceed to describe. We will, however, first show by two examples that in the first place the passage of ammonia into the oxygen compounds of nitrogen up to nitric acid, as well as the converse preparation of ammonia (and consequently of the intermediate compounds also) from nitric acid, are reactions which proceed directly and easily under many circumstances, and in the second place that the above general principle of substitution gives the possibility of understanding many, at first sight unexpected and complex, relations and transformations, such as the preparation of hydronitrous acid, HN3. In nature the matter is complicated by a number of influences and circumstances, but in the law the relations are presented in their simplest aspect.