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is the only member of the group which forms a haloid compound of the form MX, stable in the state of gas. Niobium and tantalum pentachlorides can be gasified; the pentachlorides of phosphorus and antimony are dissociated to MCI, + Cl, when heated.

The haloid compounds are all decomposed by water, generally forming solutions of haloid acid and the hydrated oxide of the element; thus PCI, gives H_PO,Aq, AsCl, gives As, Aq, Vcl, gives V,0, Aq, NbCl, gives Nb,05 xH,O, Tači; gives Ta, Os. xH,O, Sbči, gives Sb.O,Aq if much warm water is used but solid Sbo CI if less water is added,in each case HCIAq is also formed ; BiCl, gives BioCl and HCIAq whatever be the quantity, or the temperature, of the water employed.

Many oxyhaloid compounds of Group V. exist; the more important belong to the forms MOCI and MOCI..

The sulphides, M Sg, of the lower members of the group 430 generally interact with alkaline sulphides to form thio- (or sulpho-) salts; thus As, S, dissolves in ammonium sulphide solution to form à solution of ammonium thio-arsenite, (NH),AsS,; Sb Sg under similar conditions forms a solution of NË SbsThe best studied and apparently most distinctly acidic sulphides are As S, and Sb,S,; P.s, and V S, also interact with alkaline sulphides to form thiosalts. Ta, S, is not known, but Ta s, is said not to react with alkaline sulphides. Bi,S, shews no acidic properties. Nitrogen forms the sulphide NS,; it is prepared by passing ammonia into sulphur dichloride (SCI); it is very easily decomposed by heat. Phosphorus forms several sulphides, P S, P, S, P,S3, P,S3, &c.; they are generally obtained by direct union of phosphorus and sulphur. The highest sulphide of vanadium VS, is formed by passing sulphuretted hydrogen over hot V,03; Ta s, is produced by passing carbon disulphide (CS,) over hot Ta Os. The sulphides As S, Sb Sy, and Bis, are produced as solid precipitates when sulphuretted hydrogen is passed into acidulated solutions of the oxides MOX

The acids formed by the combination of elements of 431 Group V. with hydrogen and oxygen are numerous and important. The following table shews the compositions of the best marked of these acids, and exhibits the relations of composition between them and their corresponding oxides.

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Acid. Corresponding

Remarks.
Oxide.
HNOAq*

N,0 Acid not formed from oxide; but

aqueous solution of acid boiled

gives the oxide. HNO.AZ

N,O2 Oxide dissolves in water to form a

solution of the acid. HNO,

N207 Oxide reacts with water to form the

acid; oxide also obtained from the acid by withdrawing water,

but not by action of heat. H.H,PO,AQ

No oxide known corresponding to

acid. HZ.HPO,Aq P,03l Oxides interact with water to form HPO3, H3PO, H,P,0, P205)

the acids; oxides not obtained

by heating the acids. HVO3, H.V,0, (also salts V,05 Acids not obtained directly from of form M,VO)

oxide; oxide obtained by heating

the acids. Salts of form MAsO, and As,O2 Solution of oxide in water interacts M,As

with alkalis &c. to form salts. HASO3, H.AsO4, H As,O, As,Og Acids not obtained from the oxide

by action of water; oxide is

obtained by heating the acids. No definite acid of Nb, only hydrates

Nb,05.XH,0; Nb,0; Oxide obtained by heating the hy. most salts are complex,

drates; hydrates not obtained may be represented as

directly from the oxide. xNb,05.yRO where

RO=KO, CaO, &c. H2Sb03 (also salts of form Sb,02 Acid not obtained directly from MSb0)

oxide; oxide formed by beating

the acid. HSbO3, H2SbO4, H,Sb,O, Sb,Og Acids not obtained directly from

oxide; oxide formed by heating

the acids. H.Ta,Oz (also salts of Ta,OAcid not obtained directly from form Msa), and various

oxide; oxide produced by heating complex salts

the acid. «Ta,O5.YRO)

No acids, or salts derived from acids, of didymium, erbium, or bismuth, have been isolated.

* The symbol Aq here signifies that the acid to the formula of which it is added exists only in aqueous solution, and has not been isolated as a solid.

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Hyponitrous acid (HNO) and nitrous acid (HNO.) act as reducing agents; they readily combine with oxygen to produce nitric acid. Nitric acid on the other hand is very frequently used as an oxidiser; when heated it is decomposed to water, oxygen, and nitrogen dioxide (2HNO, = H,0 + 2NO, + O). All the nitrogen acids are monobasic.

When aqueous solutions of hypophosphorous acid (H PO,) and phosphorous acid (H PO) are boiled, phosphine (PX) is evolved, and phosphoric acid (H,PO.) remains in solution. The three phosphoric acids, ortho- H PO, meta- HPO,, and pyro-HP,0,, may be formed by adding water to phosphoric anhydride P,0, (8. Chap. XI., par. 215). Hypophosphorous acid is monobasic, and phosphorous acid is dibasic. Of the three phosphoric acids, orthophosphoric H PO, forms the largest number of definite salts: sodium pyrophosphate, Na P,0, is obtained by heating ordinary sodium phosphate (2Na HPO, = H,0 + Na, P.02); sodium metaphosphate, NaPO, may be obtained by heating sodium-ammonium phosphate (Na(NH2HPO = H2O + NH4 + NaPO). When orthophosphoric acid is heated to 230° or so pyrophosphoric acid is obtained (2H_PO, = H,O+H P,0,), at a red heat metaphosphoric acid is produced (H.PO, = 8,0+ HPO). When metaor pyro-phosphoric acid is boiled with water orthophosphoric acid is produced.

Metavanadic acid HVO,, and pyrovanadic acid H,V,0, are prepared, indirectly, from salts of these acids. Besides the salts of these acids, numerous polyvanadates (or condensed vanadates) exist; the following are given as examples, Na V.0.1, SrVO,

,
No arsenious acid has been isolated. An

aqueous

solution of arsenious oxide (As,0) may contain arsenious acid; when this solution is neutralised with soda the salt NaAso, is obtained; by adding silver nitrate to an aqueous solution of Aso, Ag, Aso, is precipitated. The arsenites are unstable salts, their composition seems to change with small variations in the conditions of their formation. Arsenic acid, H, Ason, is formed by oxidising As,o, in presence of water, either by nitric acid or by chlorine, and crystallising. This acid loses water. at 150° or so with for nation of pyro-arsenic acid (2H, As0,= H,O+H As, 0,), and at about 210° water is again evolved and meta-arsenic acid remains

(H,As,O, = H,0 + 2HAsOz).

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Both meta- and pyro-arsenic acids are at once changed to the ortho-acid by solution in water.

Antimonious oxide dissolves in hot caustic soda solution ; from this solution the salt NaSb0, is obtained on cooling. Ortho-antimonic acid H Sb0, is produced, indirectly, from tartar emetic, KSC,H,Om. The antimonites are easily oxidised; they are unstable and easily undergo change.

Antimonic acid, H Sb0, is obtained by adding a little water to SbCl, and drying the solid thus obtained over sulphuric acid; at 100° water is evolved and pyro-antimonic acid, H,Sb,07, remains, and at 200° this acid again loses water with production of meta-antimonic acid, HSbOz.

These acids seem all to exist in aqueous solution ; salts derived from HSb0z and H Sb On, but not from H_SbO, are known.

Salts are obtained by replacing the hydrogen of various acids by the elements vanadium, didymium, erbium, or bismuth. The salts of didymium and erbium have not been much studied; they seem to belong to the form M 3X, where X = SO, 2NO, &c. Vanadium pentoxide, V,0 interacts

, with alkalis to produce salts of the form M VO; but it also interacts with sulphuric acid, and with a few other acids, to produce basic salts, e.g. (VO),(50) Vanadium tetroxide, V,,, also interacts with sulphuric acid to form the salt VO

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Bismuthous oxide, Bi,Oz, forms a series of salts by interacting with acids; these salts belong to the form Bi 3X (X=SO, 2NO,, PO, &c.), e.g. Bi 380, Bi,6NO,, Biro,

BiBO BiAso, Most of these salts are decomposed by water with formation of basic salts; the composition of some of these basic salts is represented by the general formula BiOX where X=NO, SO.

4, &c.; but 2

others are more complex, and their com

many position can be expressed only by such a formula as &Bi,O,.yR. H.0 where R=an acidic oxide, e.g. NO, SO, &c.

The elements of Group V. shew a gradual change of properties from the decidedly negative nitrogen to the metallic bismuth. Various small sub-classes appear in the group ; thus arsenic and antimony are very closely related; so are niobium and tantalum ; nitrogen and phosphorus also are very similar in many chemical properties. Vanadium appears to be more distinctly, metallic in its chemical properties than the

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elements which succeed it in the even-series, but it is to be remembered that the properties of these elements have been very imperfectly investigated. The group cannot be divided into two families comprising the even-series and odd-series elements, respectively. Although it cannot be said that all the elements shew marked similarities, yet the group-character is impressed on them all. All the elements of this group are distinctly more like each other than they are like the elements other

group. Group I. presents a number of elements some of which are very similar in their chemical properties, while others are so different from these and from one another that it seems at first sight quite a mistake to place them in the same group.

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