Page images
PDF
EPUB

If we now turn to Group VI. we shall find that the 410 first member of the group, oxygen, to some extent summarises in itself, or is typical of, the properties of all the other members; and that the other elements placed in this group fall into two well marked families, one of which contains the even-series members and the other contains the odd-series members of the group.

M. E. C.

19

[blocks in formation]

Atom. weights

spec. gravs. Occurrence and preparation.

Colour, appearance, &c.

General chemical properties.

•10 (?) 7.7

The molecular weights of these elements are unknown.

Chrome-ironstone, (FeOCr2O3) and lead chromate, &c. occur in a few rocks; not widely distributed. Obtained by deoxidising Cr2Og by C, or by action of K on Cr2Cl, or by electrolysis of CrCl4 containing Cr2Cl6. Very hard, brittle, powder composed of minute brilliant tinwhite crystals. Descriptions differ much; probably the metal has not yet been obtained approximately pure. Burns in stream of O; heated in air is superficially oxidised; oxidised by molten KNOg or KClO3. Easily dissolved by dilute HCIAq or H2SO4Aq, but does not react with hot conc. HNO3. Combines readily with Cl and I when heated. Decomposes steam slowly at a red-heat. Replaces H of acids forming two series of salts. CrO3 is an anhydride; Cr2O3 seems to form a few salts by heating with basic oxides. Atom of Cr is perhaps hexavalent in gaseous molecules.

Infusible at full white-heat.

⚫072 (?)

11.3

Occurs in small quantities as oxide and sulphide, also as lead or cobalt molybdate. Obtained by reducing oxide or chloride by H, or the oxide by C or KCN.

Ashen-grey powder; when compressed, is a silver-white, lustrous, hard, brittle, infusible, metal.

Not oxidised in air at ordinary temperature but burns at low red-heat. No reaction with HCIAq, HFAq, or H2SO4Aq; oxidised by conc. HNO3 to MoO3; oxidised by molten KOH, but no reaction with KOHAq. Combines with Cl and Br, but not directly with I. Salts formed by replacing H of acids by Mo are scarcely known.

18.5 Softens and agglomerates at white-heat.

⚫0334

9.7

Occurs very sparingly, as tungstate of Ca, of Fe and Mn, and of Pb; also as oxide. Obtained by reducing the oxide or chloride in H.

Resembles iron in colour and lustre; hard and brittle; also obtained as a brown amorphous powder.

Burns in air at redheat; unchanged at ordinary temperatures. Oxidised to WO3 by hot HNO3Aq, HCIAq or H2SO4 Dissolves in hot KOHAq to form K tungstate and H. Combines with Cl at high temperatures. Does not seem to form salts by replacing H of acids. WOg is an anhydride, and it also combines with other more negative anhyMoO3 is an anhydrides, e.g. SO dride, and also combines with many more negative anhydrides, e.g. P205. Atom of Mo is pentavalent in gaseous molecules.

Atom of W is pentaand hexavalent in gaseous molecules.

12-9

Sparingly distributed, as oxide in pitchblende, as uranite of Ca and of Cu, as carbonate of I and Ca, &c. Obtained by reduc ing the chloride by Na.

White, lustrous, metal; softer than steel; malleable, but cannot be beaten into thin plates; also obtained as a greyblack powder.

Slowly tarnishes in air; oxidised rapidly in air at 150°-200. Combines with (1 and Br when heated, and very slowly with hot I vapour. Dissolves in most aqueous acids with evolution of H and formation of salts. Forms two series of salts, members of one of which always contain O in addition to U and the acid radicle.

UOg is an anhydride. Atom of U is tetravalent in gaseous molecules.

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 WC, only.

Oxides. MO, M2O, MO, MO,: hydrates of some of

these are known.

2 39

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

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

Acids. H,MO, H2M ̧O,, &c., H,MS.

2 79

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.

2

The sesquioxides MO, are stable compounds when M = Cr or Mo (chromic and molybdic oxides); no sesquioxide of W or U is known. Cr2O, 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(Cr20.3H,O) dissolves readily in acids forming chromic salts, e.g. Cr,3SŎ 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

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 WO, 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 UCl, 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, WỖ, 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 am monium 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

(UO2)(NO3)2

CrO, is very soluble in water forming a markedly acid liquid; under special conditions the hydrate Cro2. H2O—i.e. HCrO4, chromic acid can be obtained from this liquid.

3

2

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

2

2CrO ̧ + 3H2SO ̧Aq = Cr ̧(SO, ̧),Aq + 3H ̧0 + 30. This oxide readily parts with part of its oxygen and therefore acts as an oxidiser, e.g. when it is heated, Cr ̧ ̧ are produced.

and oxygen

MOO, is much less soluble in water than CrO,; 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, P2O, B ̧О„, &c. The oxide UO, interacts with a few acids to form salts (s. Salts, par. 417).

3

2

2

3

The oxides M,O, 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.

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 KS, &c. to form thio- (or sulpho-) salts, e.g. K,MOS.

2

2

When potassium thiomolybdate, K,MOS, is heated with MOS, a salt having the composition K MoS 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. KWS, 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.

2'

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; MoCl, MoCl, MoCl, MoCl, MoBr,, MoBr1, MoBr1; wci, wc, wc, WC, WBr, WBr,, WI,; UCI, UCI, UCI, UBr, UF,. The following have been gasified; MoCl, WC, wol 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.

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

2

« PreviousContinue »