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When the acid H,PO, is heated to about 200°, it loses water and forms the acid HP,0,, when this is heated to about 400° it loses water and forms the acid HPO,; thus, (1) 2H,PO, -H,0=H.P,0,; (2) H.P.0,-H,O=2HPOZ.

None of the acids corresponding to arsenic or antimony pentoxide is obtained by the interaction of the oxide with water. The acid H, Aso, is formed by oxidising arsenious oxide in presence of water (thus As, Aq + 5H.0 + 401 = 2H, AsO, Aq + 4HCIA); when this acid is heated it yields H.As.0, and then HĀsog. The acid H,Sbo, is produced

,, by the interaction of antimonic chloride with a little cold water; thus SbCl:+4H,0=H SbO, +5HCl; the acid H Sb 0, is obtained by heating H Sbo, to about 100', and the acid HSbO, by heating H,Sb,0, to about 200°.

Solutions of either ÉPO, or HP,0, when heated give a solution of H PO,; solutions of HÀso, and H As,, even at the ordinary temperature change rapidly to a solution of H_AsO, ; but solutions of HSb0, and H,Sb,0, seem to be more stable than a solution of H_SbO

To the acids corresponding to the oxides M,0; are given 216 names ending in -ic; phosphoric, arsenic, antimonic, acid. The prefix ortho- is employed to distinguish the acid of the form H,MO, the acid of the form HMO, is called meta-, and the remaining acid, H,M,On, is called pyro. The acids, especially the phosphoric acids, are also distinguished as tribasic phosphoric acid H,PO, monobasic HPO,, and tetrabasic phosphoric acid H.P.O., Nitrogen forms a meta-acid only.

All the oxyacids of nitrogen are more or less easily de- 217 composed, by heat, or by reactions with other substances; hyponitrous acid (HNO Aq), and nitrous acid (HNO, Aq), combine with oxygen at ordinary temperatures to form nitric acid (HNO.); these acids therefore act as reducing agents. Nitric acid we know is an energetic oxidising agent. The lower acids of phosphorus, H,PO, and H PO3, are reducing agents; but they do not combine with oxygen so rapidly or at such low temperatures as the lower acids of nitrogen do. An aqueous solution of arsenious oxide may possibly contain the acid H,AsO;; this solution, like that of antimonious oxide, is a weak reducing agent. The highest acids of phosphorus can scarcely be classed as oxidising agents; they are fully oxidised, but they do not easily part with oxygen: these acids are not separated into oxide and water by heat alone. The highest oxides of arsenic and antimony (M,O) are reduced by heat

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to lower oxides (M,0z) and oxygen; these oxides therefore sometimes react as oxidising agents : inasmuch as the highest acids of arsenic and antimony can be separated by heat alone into oxide (M203) and water, it follows that these acids will sometimes react as oxidisers.

The preceding sketch of the oxides and oxyacids of the elements of the nitrogen groups shews how closely related these elements are to each other; but it also shews a gradation of properties from nitrogen to bismuth. Bismuth is evidently more widely separated from the other members of the group than these are from each other. Nitrogen and phosphorus are distinctly non-metallic, negative, elements; bismuth is metallic; arsenic and antimony stand midway between the metals and the non-metals; these elements are sometimes classed, with one or two others, as metalloids.

Our examination of some metals and non-metals shewed that non-metals sometimes exhibit allotropy. We might reasonably expect to find nitrogen and phosphorus existing each in more than one form, and it would certainly be incumbent on us to inquire whether arsenic and antimony exhibit allotropy or not. We should scarcely expect to find more than a single form of bismuth.

The existence of more than one form of nitrogen has not been proved; experimental results are however on record which point to the possibility of nitrogen undergoing allotropic change.

At least two distinct modifications of phosphorus are known. Ordinary phosphorus is a yellowish-white, semitransparent, crystalline, solid ; spec. gravity = 1.8; melting point = 44"; easily soluble in carbon disulphide, ether, and various oils. It combines with oxygen, the halogens, and sulphur, very rapidly and at low temperatures. It is extremely poisonous. When ordinary phosphorus is heated to about 240° in an atmosphere of carbon dioxide it is changed into a red amorphous solid. This change is more quickly and completely accomplished by heating ordinary phosphorus in a closed vessel to about 300°: a portion of the phosphorus is oxidised, and the rest is transformed into red phosphorus. Red, or amorphous, phosphorus is heavier than ordinary phosphorus; spec. gravity = 2.1; it is insoluble in carbon disulphide and ether; it combines with oxygen, the halogens, and sulphur only at fairly high temperatures. Red phosphorus is not poisonous. When heated in a stream of carbon dioxide

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to about 260° red phosphorus is changed to ordinary phosphorus; but it may be heated in closed tubes considerably above 300° without undergoing this change. To prove that ordinary and red phosphorus are forms of the same element it suffices to heat a weighed quantity of ordinary phosphorus in an atmosphere of dry nitrogen or carbon dioxide to 230°—240", until the change into red phosphorus is completed, and to weigh the product; no change of mass has occurred. The red phosphorus is then heated to 260°—280° in a very slow current of dry nitrogen, when ordinary phosphorus is produced; the mass of this is the same as the mass of the phosphorus at the beginning of the experiment. To make the proof more complete, equal masses of ordinary and red phosphorus are burnt in oxygen so that none of the products escape; the product of each combination is dissolved

the solutions are heated for some time and the quantity of phosphoric acid (H,PO) in each is determined : it is found that each gram of either phosphorus has produced 3.16 grams of phosphoric acid (H PO.). If equal masses of the two varieties of phosphorus interact with chlorine, the same mass of the same compound is obtained in each case.

If however the quantity of heat produced during the burning 221 of equal masses of the two varieties of phosphorus is measured, different values are obtained. Let 1 gram of ordinary phosphorus be wholly burnt to phosphorus pentoxide, about 5900 gram-units of heat are produced ; let 1 gram of red phosphorus be wholly burnt to phosphorus pentoxide, about 5500 gram-units of heat are produced. In each case the change which proceeds is represented thus :

2P+50=P,0
If the symbol po represents 31 grams of ordinary, and
PB 31 grams of red, phosphorus (combining weight of phos-
phorus = 31), then we have these statements :

2P" + 50 = P.03; 365,800 gram-units of heat produced.
2P% +50=P,05; 341,000

Hence it follows that in the change from 62 grams of ordinary, to 62 grams of red, phosphorus, 24,800 gram-units of heat are produced. In other words energy is degraded in the change from ordinary to red phosphorus; the quantity of energy degraded, or rendered less available for doing work, is equal to nearly 25,000 gram-units of heat-energy per 62 grams M. E. C.

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of phosphorus changed from ordinary to red phosphorus. Equal masses of the two forms of phosphorus do not contain equal quantities of energy: red phosphorus contains less energy than an equal mass of ordinary phosphorus. The red variety of phosphorus is more stable, and less easily undergoes chemical change, than the ordinary variety.

Arsenic is known in two, or perhaps, three modifications. The change from one to the other form is attended by change from crystals to an amorphous powder, or vice versa, change of specific gravity and some other physical properties, and production or disappearance of heat. It is not quite certain whether antimony does or does not undergo allotropic change. Only one form of bismuth is known.

The haloid compounds of the elements of the nitrogen group have the compositions MX, and MX;; P. I, also exists.

The haloid compounds of nitrogen have not been fully studied; they are extremely explosive, and it is therefore very dangerous to work with them.

Phosphorus forms compounds of both compositions PX and PX, ; e.g. PCI, PCI, PBr,, PF,; the known haloid compounds of arsenic and bismuth all belong to the type, or general form, MX,; antimony combines with halogens in both ratios M :X, and M :X, All the compound MX, except PF, are decomposed by heat into MX, and X, ; most of the compounds MX, can be gasified without decomposition. By the interactions of the haloid compounds MX, with water either oxyacids (or oxides) and haloid acids, or oxyhaloid compounds, are produced. Thus PC1, + 3H2O + Aq = H,PO, Aq + 3HCIAq. 2 AsCl2 + 3H,0 + Aq = As,o, Aq + 6HCIAq. SbCl, † 1,0+ Aq =Sboci + 2HCIAq; if there is only a

little water. 2SbC1, +3H,0 + Aq = Sb,O,Aq + 6HClAq; if there is much

water, and especially if the water is warmed. BiCl, + H,0+ Aq = BIOCI + 2HC1Aq; whatever quantity of

water is used. An oxychloride of phosphorus, POCI,, is obtained by the interaction of ortho-phosphoric acid (H,PO) with phosphoric chloride (PCI). An oxychloride of arsenic, Asoci, can be obtained by heating together arsenious oxide and chloride (As,, + AsCl, = 3 AsOCI).

We have already learned (par. 156) that the haloid com

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pounds of non-metallic elements, as a class, interact with water; that the compounds of the more distinctly negative elements generally produce oxides (or oxyacids) and haloid acids; and that the compounds of the less negative non-metals generally produce oxyhaloid compounds and haloid acids. The oxyhaloid compounds of metals are usually very complex, and are produced either by heating the oxides with haloid compounds, or by passing halogen over a heated mixture of oxide and carbon.

A classification of the elements of the nitrogen group into 224 metals and non-metals based on the behaviour of their haloid compounds with water would place phosphorus and arsenic with the non-metals, antimony with the metalloids, and bismuth either with the metalloids or the metals. A classification of the same elements based on the properties of their oxides would lead to the same, or about the same, arrangement; bismuth would certainly be placed with the metals. A classification based on the existence and properties of hydrides would result in all the elements except bismuth being placed among the non-metals. A classification based on the occurrence or non-occurrence of allotropy would result in phosphorus and arsenic being placed with the non-metals, antimony with the metalloids, and bismuth with the metals.

The properties of the sulphides of a group of elements 225 sometimes afford class-marks by using which the elements may be subdivided into families.

All the elements of the nitrogen group form sulphides : the following are certainly known :-NS, PS, PS, and several other compounds of phosphorus and sulphur; As, Sa, As Ss; Sb Sy, Sb Ss; Bi Sy, and probably other sulphides of bismuth. Nitrogen sulphide differs considerably from the others : it is very explosive; it is decomposed by water or aqueous alkalis giving ammonia and ammonium salts of sulphur oxyacids. The sulphides of phosphorus react with aqueous solutions of potash to form, potassium salts of sulpho(or thio-) acids of phosphorus; but these salts are unstable and easily decomposed. The sulphides of arsenic and antimony dissolve in aqueous solutions of potash or potassium sulphide (KS) to form potassium sulpho- or thio-arsenite and antimonite, respectively (K, Ass, ; ÉSbS,); these solutions are decomposed by boiling, or by interacting with dilute acids, giving arsenious, or antimonious, sulphide, and hydrogen sulphide. Bismuth sulphide does not interact either with caustic potash or potassium sulphide.

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