General formulae and chemical characters of compounds. 412 The compositions of the more important compounds of these four metals are expressed by the following formulae; but representatives of each formula are not known for all the elements, thus sesquioxides, M,O,, of tungsten or uranium have not been obtained, and the formula MX, is represented by WCl, only.

Oxides. MO, MO, MO, MO, hydrates of some of these are known.

Sulphides. MS, M,S,, MS,, MS, MS.

Haloid compounds; chiefly chlorides. MX,, MX, MX,, MX,, MX,.

Acids. HMO,, H2M ̧O,, &c., H2MS.

Salts. MSO, M2NO, &c.; M.(SO), M.(NO), &c. when M=Cr. M(SO), M(NO), MO (SO), &c. when M-U. Salts of Mo and W are scarcely known.

The oxides MO are scarcely known; hydrates of Cro and 413 MOO are obtained by adding a solution of potash in air-free water to solutions of Cr,Cl, and Mo,Cl, respectively. These hydrates are rapidly oxidised in the air; neither yields corresponding salts by its reactions with acids.

ვა

2 3

2 3

The sesquioxides MO, are stable compounds when M = Cr or Mo (chromic and molybdic oxides); no sesquioxide of W or U is known. Cr.O, is prepared by precipitating a solution of a chromic salt by ammonia, washing, drying, and heating; in the case of Mo,O,, the hydrated oxide preciptated by potash is heated in hydrogen. Both oxides form dark coloured solids, insoluble or nearly insoluble in acids. Hydrated Cr2O (Cr ̧ ̧.3H2O) dissolves readily in acids forming chromic salts, e.g. Cr.380. This oxide also seems to combine with a few basic oxides, e.g. with CaO. It is therefore basic but also slightly acidic.

2

2

2

The dioxides MO, are dark coloured solids, obtained by reducing the oxides MO, directly or indirectly. CrO, is more easily formed by passing nitric oxide into an aqueous solution of potassium dichromate (K.Cr.O,); MoO, and WO̟, by heating MOO, and WO, in hydrogen to low redness, or by digesting a solution of MOO,, or WO,, in hydrochloric acid with copper or zinc and then precipitating by ammonia. UO, may be prepared by digesting UCI, with water.

2

CrO, is decomposed by heat at 300° with evolution of oxygen and production of Cr,O,; the other oxides MO, are oxidised to MO,, MoO, by heating with nitric acid, WỎ, by

2

414

heating in air, and UO, by the action of air at ordinary temperatures. These oxides MO, are slightly soluble in acids; UO, gives salts, e.g. U(SO), but no definite salts have yet been certainly obtained corresponding to any of the other oxides MO, although such salts seem to exist.

2

The trioxides MO, are anhydrides. CrO, is prepared by adding a sufficient quantity of concentrated sulphuric acid to a solution of potassium dichromate, MoO, and WO, are obtained by oxidising the lower oxides, or better from ammonium molybdate and tungstate, respectively, by heating with nitric acid and then washing out the ammonium nitrate formed. UO, is obtained by heating uranyl nitrate

3

(UO,)(NO3)2

CrO, is very soluble in water forming a markedly acid liquid under special conditions the hydrate Cro ̧. H2O—i.e. HCrO4, chromic acid can be obtained from this liquid.

3

CrO, interacts with acids to form oxygen and salts corresponding with the oxide Cr,O,; thus

2CrO ̧ + 3H2SO̟ ̧Aq = Cr ̧(SO ̧)„Aq + 3H ̧0 + 30.

3

2

This oxide readily parts with part of its oxygen and therefore acts as an oxidiser, e.g. when it is heated, Cr,O, and oxygen are produced.

3

2 3

3

MOO, is much less soluble in water than CrO1; WO, is only very slightly soluble in water; and UO, is insoluble. Hydrates of these oxides exist and exhibit acidic properties (s. Acids, par. 416), but none of them is obtained by the direct addition of water to the oxide. The oxides MoO, and WO, form various complex compounds with several anhydrides such as SO,, PO, B2O2, &c. The oxide UO, interacts with a few acids to form salts (s. Salts, par. 417).

2

2 39

3

The oxides MO and MO, are on the whole basic; the oxides MO, are acidic, but their acidic character is less marked as the atomic weight of M increases. The change from MO, to MO, is effected the more easily and directly the greater the atomic weight of M. Of the oxides MO,, UO, shews the most clearly marked basic character. Of the oxides MO. UO, is the most stable towards heat and reducing agents, and CrO, is the least stable.

3

2 3

39

The most important sulphides are Cr,S,; MOS,, MOS, MOS, WS,, WS,; and US,. Chromic sulphide, Cr,S, is prepared by passing sulphuretted hydrogen over hot chromic oxide (Cr,O); it is not obtainable by reactions between compounds

in solution. This sulphide is feebly acidic; it combines with certain more basic sulphides, e.g. with ZnS, CaS, &c.

2

When excess of sulphuretted hydrogen is passed into the solution of an alkaline molybdate, e.g. K,MOO,, and an acid is then added, molybdenum trisulphide MOS,, is precipitated. This sulphide is distinctly acidic; it interacts with K,S, &c. to form thio- (or sulpho-) salts, e.g. K,MOS.

2

When potassium thiomolybdate, K MOS, is heated with MOS, a salt having the composition KMOS is formed; this salt interacts with acids to produce molybdenum tetrasulphide MOS, Molybdenum disulphide, MoS,, is obtained by heating together MoO, and sulphur.

Tungsten trisulphide WS, is obtained similarly to MoS. The disulphide WS, is formed by heating together tungsten and sulphur. WS, is distinctly acidic, forming thiotungstates, e.g. K,WS, BaWS, &c.

Uranium disulphide, US,, is formed similarly to WS; it exhibits no acidic functions.

As CrS, has not not yet been prepared it is difficult to compare corresponding sulphides of the four elements; but on the whole it appears that the sulphides become more acidic as the atomic weight of the metals increases.

The haloid compounds of the four elements we are 415 considering are important. Their compositions are shewn by the following formulae; CrCl,, CrCl, CrBr,, CrBr,, CrI, CrI ̧, CrF; MoCI,, MoCl,, MoCl, MoCl, MoBr,, MoBr„, MoBr; wci, wc, WC, WC, WBr, WBr, WI; UCI, UCI, UCI, UBr, UF. The following have been gasified; MoCl, WCI, WC, UCI, UBr; these formulae are therefore molecular. The formulae of the other compounds are the simplest that can be given, but they are not necessarily molecular.

2 3

69

Chromic chloride, CrCl,, is obtained by heating an intimate mixture of chromic oxide and carbon in chlorine; chromous chloride, CrCl,, is formed by heating CrCl, in hydrogen. The higher chloride is stable in the air, but when strongly heated it gives Cr,O,; solutions of this chloride when heated give precipitates of various oxychlorides Cr,O,Cl,; the most important oxychloride of chromium is CrO,Cl. Chromic chloride forms either violet crystals by subliming in chlorine or hydrochloric acid gas, or a greenish solid by dissolving chromic hydrate in hydrochloric acid, evaporating nearly to dryness, and heating in chlorine. The violet form is almost insoluble

416

in water; the green form readily dissolves in water. Chromous chloride, CrCl,, is very unstable; it removes chlorine readily from various chlorides, e.g. HgCl, and absorbs oxygen rapidly from the air.

When molybdenum is strongly heated in chlorine the pentachloride, MoCl, is formed. By heating this chloride in carbon dioxide the tetra- and di-chlorides, MoCl, and MoCl,, are obtained the trichloride is also got from the pentachloride, by heating in hydrogen. Various oxychlorides of molybdenum are known; the more important are MoO,Cl, and MoOCI,.

2 2

Tungsten hexachloride, WC, is produced by strongly heating tungsten in a stream of dry chlorine; hot water decomposes it to WO, and hydrochloric acid; heated in air it yields WOCI. The penta- and di-chlorides, WCI, and WCI,, are obtained from WC, by heating in hydrogen; the tetrachloride WCI, is produced by heating a mixture of WC, and WC, in hydrogen or carbon dioxide. The oxychlorides WO,Cl, and WOCI, are known.

2

3

When uranium dioxide, UO,, is mixed with carbon and heated in chlorine the tetrachloride UCI is formed ; this chloride is decomposed by hot water to UO, and hydrochloric acid; it is an energetic reducing agent, e.g. it reduces ferric chloride to ferrous chloride. The pentachloride UCl, is formed by the direct addition of chlorine to UCl; but when UCl, is heated to 230° in hydrogen or carbon dioxide it is again separated into the tetrachloride and chlorine. Only a few oxyhaloid compounds of uranium have been prepared; the chief are UO,X, where X = Cl, Br, or F.

2

Acids and salts derived therefrom. Many of the hydroxides of chromium, molybdenum, tungsten, and uranium, are acidic.

The precipitate obtained by adding ammonia to a solution of a chromic salt varies in composition according to the conditions, but it is always a hydrate of the oxide Cr,O,(Cr ̧ ̧.xH ̧O). This compound is basic, as it interacts with acids to form salts. By dissolving CrO, in a little cold water, warming, and again cooling, crystals of the hydrate CrO,. H2O(= H2Cro ̧) are said to be formed. This compound is distinctly acidic, from it is derived a well marked series of salts, the chromates, MCrO1, M = K2, Ba, &c.

2

Ammonia ppts. UO, H2O from solutions of the tetrachloride UCI,; this hydrate dissolves in acids to form salts, e.g. U(SO4)2;

it is therefore basic. The hydrate UO.H2O(= HUO,) is obtained indirectly from UO,(NO),; this hydrate interacts with some acids to form salts, e.g. UO,(SO) &c. and it also interacts with alkalis to form salts of the form MUO (M = Na,, Ba, &c.), it is therefore both basic and acidic.

3

Hydrates of the dioxide and sesquioxide of molybdenum MOO,.xH2O and Mo,O,.H.O are known, but their interactions with acids have been little examined; they appear however to possess only basic properties. Various hydrates of MoO, are prepared indirectly (that is not by addition of water to the oxide); the chief are MQO,. H,O and MoO,.2H2O= H ̧MoO and HMOO,; these compounds are acidic; they may however also shew basic functions; their interactions with acids have not been much investigated.

3

2

Two hydrates of tungsten trioxide are known, WO,.H2O and WO,. 2H ̧O = H ̧WO̟ and H ̧WO̟,; these compounds are acidic, and possibly also basic. Another compound H,W,O,,.7H2O has been obtained; it is acidic.

2 4 13

The chief salts derived from the acidic hydroxides of the elements under consideration, by replacing hydrogen by metals, are the chromates and dichromates, the molybdates, di- tri- tetra&c. molybdates, the di- tri- &c. tungstates, and the uranates.

The chromates MCrO, where M = Ba, Ag,, K,, &c. &c. are prepared by double decomposition from potassium chromate which may be obtained by neutralising a solution of chromium trioxide with potash. The alkaline chromates are also formed by fusing chromic oxide, Cr,O,, or a chromic salt, with potash and a little potassium nitrate or chlorate. When potassium chromate (K,CrO,) is treated with dilute sulphuric acid potassium dichromate, K,Cr,O,, is formed; from this salt a series of dichromates MCr,O, is obtained. Trichromates MCr2O10 and tetrachromates MCr ̧O, are also known; but the best marked

2 2

7

4 13'

salts are the chromates and dichromates.

4

2

The molybdates MMOO, are obtained by double decomposition from the alkali salts; most of these salts form non-crystalline masses. Dimolybdates MMO,O,, trimolybdates MMo 0109 and tetramolybdates MMO,O,,, are obtained by boiling various metallic carbonates with molybdenum trioxide under various conditions: these salts crystallise well and are more stable and definite than the molybdates.

139

A few tungstates MWO, are obtained by heating WO with alkali or alkaline carbonates; several series of condensed tungstates or polytungstates exist belonging to the